Category Archives: LASzip

Swiss add liberal “Open LiDAR” and break with conservative stereotypes like Bank Secrecy, Yodeling and Punctuality

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My new favorite Swiss Miss, Viola Amherd, said today “Geodaten gehören heute zur Infrastruktur wie die Straßen und die Eisenbahn”, which says that “today, geodata are part of the infrastructure like roads and railways”. The federal councilor of Switzerland announced that programmers and planners, whether private or professional, can now download the data free of charge and use it for their projects. There are almost no limits to innovation for information projects.

Hello Germany? Hello Bavaria? Hello Austria? Where is your Open-Data-Autobahn? Whatever happened to unlimited speeds and endless Fahrvergnügen … (-;

This means in Switzerland large amounts of LiDAR are now available for download from this portal here. There is amazing data there, including high-resolution ortho-photos and land cover data. However. we went straight for the LAS files and got ourselves a few tiles near the Sazmartinshorn in the example below.

The tiles can be selected in a number of ways via an interactive map and come as individually zipped LAS files. We downloaded the nine tiles indicated above (2.16 GB), unzipped the bulky LAS files (4.78 GB) and compressed them to the compact LAZ format (638 MB) with laszip. Using LAZ instead of zipped LAS would lower storage size and transmission bandwidth by a factor of 3.5. Something the stereotypically frugal people of Switzerland may want to consider … (-;

Then we process the data with a few typical command lines with the result shown below. The first uses blast2dem to create a hill shaded 1 meter DTM from the points classified as ground.

blast2dem ^
-i swisssurface3d_laz\*.laz -merged ^
-keep_class 2 -thin_with_grid 0.5 ^
-step 1.0 -hillshade ^
-o swiss_dtm_1m_hillshade.jpg

hillshade of 1 meter DTM computed with BLAST

We use lasgrid to visualize the varying last return density per 2 meter by 2 meter area across the surveyed area with a false coloring that maps 10 or fewer pulses per square meter to blue and 20 or more pulses per square meters to red.

lasgrid ^
-i swisssurface3d_laz\*.laz -merged ^
-keep_last ^
-step 2.0 ^
-density ^
-false -set_min_max 10 20 ^
-o swiss_dendity_2m_10_20.jpg

last return density per 2 meter area. blue = 10 or less, red = 20 or more

A lasinfo report reveals that the scanner used was a RIEGL and the returning pulse width was quantized in tenths of a nanosecond into the user data field. We use lasgrid to visualize the range of the pulse width between 4.0 and 6.0 nanoseconds with a false coloring. Make sure to drop the points for which no pulse width was recorded (i.e. user data is zero) to avoid artifacts in the visualization.

lasgrid ^
-i swisssurface3d_laz\*.laz -merged ^
-drop_user_data 0 -keep_last ^
-step 2.0 ^
-user_data -lowest ^
-false -set_min_max 40 60 ^
-o swiss_pulsewidth_40_60.jpg

shortest last return pulse width per 2 meter area. blue = 4.0 ns or less, red = 6.0 ns or more

Finally we created a portal with laspublish to visualize the point cloud data interactively with Potree. The four screenshots below highlight only a few of the abilities for visualizing and measuring the point cloud.

laspublish ^
-i swisssurface3d_laz\*.laz ^
-elevation ^
-odir swisssurface3d_portal ^
-title Sazmartinshorn ^
-o sazmartinshorn.html ^
-olaz -overwrite

Colored by elevation with a distance and two height measurements.
The intensity coloring reveals some scanner artifact drawn across the mountain flank.
No surprise that the return type here is predominantly yellow single returns.
Mountains scream for a coloring by elevation, here mapped from 1700 to 2600 meters.

The open data license can be found here and we are hereby naming the source.

Philippines use Taal Vulcano Eruption as Opportunity to become Very First Asian Country with Open LiDAR

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UPDATE: As of January 30th also orthophotos and classified LAZ tiles are available for download.

It took just a few years of nagging, a vulcanic eruption, and then a few more weeks of nagging but now it has happened. The Philippines have become the first country in Asia to offering LiDAR as open data for free and unencumbered download. The portal created by the UP Training Center for Applied Geodesy and Photogrammetry (UP TCAGP) and their DREAM and PHIL LiDAR program already offers LiDAR-derived 1 meter DTM and DSM data flown between 2013 and 2017 as part of a national mission to aquire flood mapping data for a certain area around the Taal Vulcano. In the coming days orthophotos and the classified LiDAR point cloud will be added (at the moment the data is still undergoing another quality assurance review process).

As a quick test we went to the new online portal and downloaded the 34 DTM raster tiles that cover the Taal Vulcano Lake as seen in the screenshot below.

taal_vulcano_open_lidar_download_portal

Downloading the area-of-interest is easy with LiPAD’s nice download portal.

The downloaded 1 meter DTM tiles are in TIF format and each cover an area of 1000 by 1000 meter. However, they are overlapping because they have a 50 meter buffer, so that each raster contains elevation samples organized in 1100 columns by 1100 rows plus “no data” values. We use two LAStools commands to remove the buffers. First we use our new demzip to turn the TIF to RasterLAZ format. Use demzip from version 200131 of LAStools (or newer) as older releases did not handle “no data” values correctly.

demzip -i Taal\DTM\*.tif ^
       -olaz

The conversion from TIF to RasterLAZ also reduces the total file size for the 34 files from 157 MB to 27 MB. Next we remove the buffers using a new functionality in lasgrid (make sure you have the latest LAStools version 200112 or newer).

lasgrid -i Taal\DTM\*.laz ^
        -step 1 ^
        -use_tile_size 1000 ^
        -odir Taal\DTM_unbuffered ^
        -olaz

Without buffers the total file size in RasterLAZ format shrinks to 22 MB. Now we have the data in a format that can either be treated as a raster or as a point cloud. Hence we can use laspublish and quickly create a visualization of the Taal Vulcano Island with Potree which we then copied onto our university Web space for you to play with.  This was he are able to instantly create an 3D visualization portal that lets anyone do various simple and also more complex measurements.

laspublish -i Taal\DTM_unbuffered ^
           -elevation ^
           -odir Taal\DTM_portal ^
           -o TaalVulcanoIsland.html ^
           -title "DTM of Taal Vulcano Island" ^
           -description "DTM of Taal Vulcano Island" ^
           -olaz -overwrite

Below we see the result visualized with the Desktop version of Potree. You can access the interactive portal we have created here with any Web browser.

taal_vulcano_open_lidar_dtm

Visualizing the 1 meter DTM of Taal Vulcano Island as RasterLAZ point cloud with Potree to instantly create interactive portal allowing simple measurements that give an intuition about the height and the size of the vulcanic formation that makes up Taal Vulcano Island.

We would like to acknowledge the UP Training Center for Applied Geodesy and Photogrammetry (UP TCAGP) and their DREAM and PHIL LiDAR program for providing easy and unencumbered open access to this data with a license that encourages data reuse and repurposing. Kudos for being first in Asia to make open LiDAR happen!!!

Another European Country Opens LiDAR: Welcome to the Party, Slovakia!

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We got a little note from Vítězslav Moudrý from CULS pointing out that the Geodesy, Cartography and Cadastre Authority of the Slovak Republic has started releasing LiDAR as open data on their interactive Web portal. Congratulations, Slovakia!!! Welcome to the Open Data Party!!! We managed to download some data starting from this Web portal link and describe the process of obtaining LiDAR data from the Low Tatras mountain range in central Slovakia with pictures below.

open_data_portal_slovakia_01

(1) click the new “data export” link

open_data_portal_slovakia_02

(2) change the export selection to “Shape”

open_data_portal_slovakia_03

(3) change the file format to “LAZ”

open_data_portal_slovakia_04

(4) zoom to a colored area-of-interest

open_data_portal_slovakia_05

(5) zoom further and draw a nice polygon

open_data_portal_slovakia_06

(6) edit polygon into nice shape and realize heart is red because area is too big

open_data_portal_slovakia_07

(7) zoom further and draw polygon smaller than 2 square kilometer

open_data_portal_slovakia_08

(8) when polygon turns green, accept license, enter email address and export

open_data_portal_slovakia_09

(9) short wait and you get download link to such an archive

open_data_portal_slovakia_10

(10) license conditions: PDF auto-translated from Slovak to English

 

open_data_portal_slovakia_11

(11) LiDAR are spatially indexed flight lines clipped to area-of-interest

open_data_portal_slovakia_12_density_all_returns_20_50

(12) all return density: blue = 20 and red = 50 returns per square meter

lasgrid -i LowTatras\*.laz -merged ^
        -step 2 -point_density_16bit ^
        -false -set_min_max 20 50 ^
        -o LowTatras\density_all_returns_20_50.png

open_data_portal_slovakia_13_density_last_returns_4_40

(13) last return density: blue = 4 and red = 40 last returns per square meter

lasgrid -i LowTatras\*.laz -merged ^
        -keep_last ^
        -step 2 -point_density_16bit ^
        -false -set_min_max 4 40 ^
        -o LowTatras\density_last_returns_4_40.png

open_data_portal_slovakia_14_density_ground_returns_4_40

(14) ground return density: blue = 4 and red = 40 ground returns per square meter

lasgrid -i LowTatras\*.laz -merged ^
        -keep_classification 2 ^
        -step 2 -point_density_16bit ^
        -false -set_min_max 4 40 ^
        -o LowTatras\density_ground_returns_4_40.png

open_data_portal_slovakia_14_overlap_10cm_20cm_diff

(15) flight line difference image: white <= +/- 10 cm and red/blue >= +/- 20 cm

lasoverlap -i LowTatras\*.laz -faf ^
           -drop_classification 7 18 ^
           -min_diff 0.1 -max_diff 0.2 ^
           -o LowTatras\overlap_10cm_20cm.png

Finally we compute a DSM and a corresponding DTM using the already existing ground classification with BLAST using the command sequence shown below.

hillshaded 1m DSM
hillshaded 1m DTM

 

lasthin -i LowTatras\*.laz -merged ^
        -drop_classification 7 18 ^
        -step 0.5 -highest ^
        -o LowTatras\highest_50cm.laz

blast2dem -i LowTatras\highest_50cm.laz ^
          -hillshade ^
          -o LowTatras -o dsm_1m_hillshaded.png

blast2dem -i LowTatras\*.laz -merged ^
          -keep_classification 2 ^
          -thin_with_grid 0.5 ^
          -hillshade ^
          -o LowTatras\dtm_1m_hillshaded.png

We thank the Geodesy, Cartography and Cadastre Authority of the Slovak Republic for providing their LiDAR as open data for both commercial and non-commercial purposes and name the source of the data used above (as the license requires) as the Office of Geodesy, Cartography and Cadastre of the Slovak Republic (GCCA SR) or – in Slovak – the Úrad geodézie, kartografie a katastra Slovenskej republiky (ÚGKK SR).

Which European country goes next? Czech Republic? Poland? Hungary? Switzerland?

 

 

Another German State Goes Open LiDAR: Saxony

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Finally some really good news out of Saxony. 😊 After North Rhine-Westphalia and Thuringia released the first significant amounts of open geospatial data in Germany in a one-two punch in January 2017, we now have a third German state opening their entire tax-payer-funded geospatial data holdings to the tax-paying public via a simple and very easy-to-use online download portal. Welcome to the open data party, Saxony!!!

Currently available via the online portal are the LiDAR-derived raster Digital Terrain Model (DTM) at 1 meter resolution (DGM 1m) for everything flown since 2015 and and at 2 meter resolution (DGM 2m) or 20 meter resolution (DGM 20m) for the entire state. The horizontal coordinates use UTM zone 33 with ETRS89 (aka EPSG code 25833) and the vertical coordinate uses the “Deutsche Haupthöhennetz 2016” or “DHHN2016” (aka EPSG code 7837). Also available are orthophotos at 20 cm (!!!) resolution (DOP 20cm).

dgm_1000_rdax_87

Overview of current LiDAR holdings. Areas flown 2015 or later have LAS files and 1 meter rasters. Others have LiDAR as ASCII files and lower resolution rasters.

Offline – by ordering through either this online form or that online form – you can also get the 5 meter DTM and the 10 meter DTM, the raw LiDAR point clouds, LiDAR intensity rasters, hill-shaded DTM rasters, as well as the 1 meter and the 2 meter Digital Surface Model (DSM) for a small administrative fee that ranges between 25 EUR and 500 EUR depending on the effort involved.

Our immediate thought is to get a copy on the entire raw LiDAR points clouds (available as LAS 1.2 files for all  data acquired since 2015 and as ASCII text for earlier acquisitions) and find some portal willing to hosts this data online. We are already in contact with the land survey of Saxony to discuss this option and/or alternate plans.

Products available in Saxony’s new open data portal.
Downloading the 1m DTM raster.
Downloading the 20 cm ortophoto.

Let’s have a look at the data. First we download four 2 km by 2 km tiles of the 1 meter DTM raster for an area surrounding the so called “Greifensteine” using the interactive map of the download portal, which are provided as simple XYZ text. Here a look at the contents of one ot these tiles:

more Greifensteine\333525612_dgm1.xyz
352000 5613999 636.26
352001 5613999 636.27
352002 5613999 636.28
352003 5613999 636.27
352004 5613999 636.24
[...]

Note that the elevation are not sampled in the center of every 1 meter by 1 meter cell but exactly on the full meter coordinate pair, which seems especially common  in German-speaking countries. Using txt2las we convert these XYZ rasters to LAZ format and add geo-referencing information for more efficient subsequent processing.

txt2las -i greifensteine\333*_dgm1.xyz ^
        -set_scale 1 1 0.01 ^
        -epsg 25833 ^
        -olaz

Below you see that going from XYZ to LAZ reduces the amount of  data from 366 MB to 10.4 MB, meaning that the data on disk becomes over 35 times smaller. The ability of LASzip to compress elevation rasters was first noted during the search for missing airliner MH370 and resulted in our new LAZ-based compressor for height grid called DEMzip.  The resulting LAZ files now also include geo-referencing information.

96,000,000 333525610_dgm1.xyz
96,000,000 333525612_dgm1.xyz
96,000,000 333545610_dgm1.xyz
96,000,000 333545612_dgm1.xyz
384,000,000 bytes

2,684,820 333525610_dgm1.laz
2,590,516 333525612_dgm1.laz
2,853,851 333545610_dgm1.laz
2,795,430 333545612_dgm1.laz
10,924,617 bytes

Using blast2dem we then create a hill-shaded version of the 1 meter DTM in order to overlay a visual representation of the DTM onto Google Earth.

blast2dem -i greifensteine\333*_dgm1.laz ^
          -merged ^
          -step 1 ^
          -hillshade ^
          -o greifensteine.png

Below the result that nicely shows how the penetrating laser of the LiDAR allows us to strip away the forest to see interesting geological features in the bare-earth terrain.

In a second exercise we use the available RGB orthophoto images to color one of the DTM tiles and explore it using lasview. For this we download the image for the top left of the four tiles that covers the area containing the “Greifensteine” from the interactive download portal for orthophotos. As the resolution of the TIF image is 20 cm and that of the DTM is only 1 meter, we first down-sample the TIF using gdalwarp of GDAL.

gdalwarp -tr 1 1 ^
         -r cubic ^
         greifensteine\dop20c_33352_5612.tif ^
         greifensteine\dop1m_33352_5612.tif

If you are not yet using GDAL today is a good day to start. It nicely complements the point cloud processing functionality of LAStools for raster inputs. Next we use lascolor to give each elevation pixel of the DTM stored in LAZ format its corresponding color from the orthophoto.

lascolor -i greifensteine\333525612_dgm1.laz ^
         -image greifensteine\dop1m_33352_5612.tif ^
         -odix _rgb -olaz

Now we can view the colored DTM in LAZ format interactively with lasview or any other LiDAR viewing software and turn on the RGB colors from the orthophoto as needed to understand the scene.

lasview -i greifensteine\333525612_dgm1_rgb.laz

We thank the “Staatsbetrieb Geobasisinformation und Vermessung Sachsen (GeoSN)” for giving us easy access to the 1 meter DTM and the 20 cm orthophoto that we have used in this article through their new open geodata portal as open data under the user-friendly license “Datenlizenz Deutschland – Namensnennung – Version 2.0.

National Open LiDAR Strategy of Latvia humiliates Germany, Austria, and other European “Closed Data” States

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Latvia, officially the Republic of Latvia, is a country in the Baltic region of Northern Europe has around 2 million inhabitants, a territory of 65 thousand square kilometers and – since recently – also a fabulous open LiDAR policy. Here is a list of 65939 tiles in LAS format available for free download that cover the entire country with airborne LiDAR with a density from 4 to 6 pulses per square meters. The data is classified into ground, building, vegetation, water, low noise, and a few other classifications. It is licensed Creative Commons CC0 1.0 – meaning that you can copy, modify, and distribute the data, even for commercial purposes, all without asking permission. And there is a simple and  functional interactive download portal where you can easily download individual tiles.

latvia_open_data_portal_01

Interactive open LiDAR download portal of Latvia.

We downloaded the 5 by 5 block of square kilometer tiles matching “4311-32-XX.las” for checking the quality and creating a 1m DTM and a 1m DSM raster. You can follow along after downloading the latest version of LAStools.

Quality Checking

We first run lasvalidate and lasinfo on the downloaded LAS files and then immediately compress them with laszip because multi-core processing of uncompressed LAS files will quickly overwhelm our file system, make processing I/O bound, and result in overall longer processing times with CPUs waiting idly for data to be loaded from the drives.

lasinfo -i 00_tiles_raw\*.las ^
        -compute_density ^
        -histo z 5 ^
        -histo intensity 256 ^
        -histo user_data 1 ^
        -histo scan_angle 1 ^
        -histo point_source 1 ^
        -histo gps_time 10 ^
        -odir 01_quality -odix _info -otxt ^
        -cores 3
lasvalidate -i 00_tiles_raw\*.las ^
            -no_CRS_fail ^
            -o 01_quality\report.xml

Despite already excluding a missing Coordinate Reference System (CRS) from being a reason to fail (the lasinfo reports show that the downloaded LAS files do not have any geo-referencing information) lasvalidate still reports a few failing files, but scrutinizing the resulting XML file ‘report.xml’ shows only minor issues.

Usually during laszip compression we do not alter the contents of a file, but here we also add the EPSG code 3059 for CRS “LKS92 / Latvia TM” as we turn bulky LAS files into slim LAZ files so we don’t have to specify it in all future processing steps.

laszip -i 00_tiles_raw\*.las ^
       -epsg 3059 ^
       -cores 2

Compression reduces the total size of the 25 tiles from over 4.1 GB to below 0.6 GB.

Next we use lasgrid to visualize the last return density which corresponds to the pulse density of the LiDAR survey. We map each 2 by 2 meter pixel where the last return density is 2 or less to blue and each 2 by 2 meter pixel it is 8 or more to red.

lasgrid -i 00_tiles_raw\*.laz ^
        -keep_last ^
        -step 2 ^
        -density_16bit ^
        -false -set_min_max 2 8 ^
        -odir 01_quality -odix _d_2_8 -opng ^
        -cores 3

This we follow by the mandatory lasoverlap check for flight line overlap and alignment where we map the number of overlapping swaths as well as the worst vertical difference between overlapping swaths to a color that allows for quick visual quality checking.

lasoverlap -i 00_tiles_raw\*.laz ^
           -step 2 ^
           -min_diff 0.1 -max_diff 0.2 ^
           -odir 01_quality -opng ^
           -cores 3

The results of the quality checks with lasgrid and lasoverlap are shown below.

last return density: blue = 2 per square meter, red = 8 per square meter
flight line overlap: blue – 1 flight line, aqua – 2 flight lines, yellow – 3 flight lines, orange – 4 flight lines, red – 5 or more flight lines
flight line alignment: white – error less than 10 cm, red or blue – error more than 20 cm

Raster Derivative Generation

Now we use first las2dem to create a Digital Terrain Model (DTM) and a Digital Surface Model (DSM) in RasterLAZ format and then use blast2dem to create merged and hill-shaded versions of both. Because we will use on-the-fly buffering to avoid edge effects along tile boundaries we first spatially index the data using lasindex for more efficient access to the points from neighboring tiles.

Resulting hill-shaded DTM.
Resulting hill-shaded DSM. 

lasindex -i 00_tiles_raw\*.laz ^
         -cores 3

las2dem -i 00_tiles_raw\*.laz ^
        -keep_class 2 9 ^
        -buffered 25 ^
        -step 1 ^
        -use_orig_bb ^
        -odir Latvia\02_dtm_1m -olaz ^
        -cores 3

blast2dem -i 02_dtm_1m\*.laz ^
          -merged ^
          -hillshade ^
          -step 1 ^
          -o dtm_1m.png

las2dem -i 00_tiles_raw\*.laz ^
        -drop_class 1 7 ^
        -buffered 10 ^
        -spike_free 1.5 ^
        -step 1 ^
        -use_orig_bb ^
        -odir 03_dsm_1m -olaz ^
        -cores 3

blast2dem -i 03_dsm_1m\*.laz ^
          -merged ^
          -hillshade ^
          -step 1 ^
          -o dsm_1m.png

Because the overlaid imagery does not look as nice in our new Google Earth installation, below are the DTM and DSM at versions down-sampled to 25% of their original size.

hill-shaded DTM down-sampled to 25%
hill-shaded DSM down-sampled to 25%

Many thanks to SunGIS from Latvia who tweeted us about the Open LiDAR after we chatted about it during the Foss4G 2019 gala dinner. Kudos to the Latvian Geospatial Information Agency (LGIA) for implementing a modern national geospatial policy that created opportunity for maximal return of investment by opening the expensive tax-payer funded LiDAR data for re-purposing and innovation without barriers. Kudos!

Clean DTM from Agisoft Photogrammetric Points of Urban Scene

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We occasionally get permission to distribute a nice data sets and blog about how to best process it with LAStools because this gets around having to pay our “outrageous” consulting fees. (-: This time we received a nice photogrammetric point cloud of the Tafawa Balewa Square in Lagos Island, Lagos, Nigeria. This area is part of the central business district of Lagos and characterized by high-rise buildings. The Tafawa Balewa Square was constructed in 1972 over the site of a defunct track for horse racing and is bounded by Awolowo road, Cable street, Force road, Catholic Mission street and the 26-story Independence House. We want to create a nice Digital Terrain Model from the dense-matching point cloud that was generated with Photoscan by AgiSoft and – as always with photogrammetry – we have to take special care of low noise points. The final result is shown below. All processing commands used are here.

After downloading the data it is useful to familiarize yourself with the file, which can be done with lasview, lasinfo, and lasgrid using the command lines shown below. According to the lasinfo report there are around 47 million points points with RGB colors in the file and their average density is around 100 points per square meter.

lasview -i 0_raw\TafawaBalewa.laz

lasinfo -i 0_raw\TafawaBalewa.laz ^
        -cd -histo intensity 256 ^
        -histo z 1 ^
        -odir 1_quality -odix _info -otxt

lasgrid -i 0_raw\TafawaBalewa.laz ^
        -step 1 ^
        -density ^
        -false -set_min_max 50 150 ^
        -odir 1_quality -odix _d_50_150 -opng

The average point density value of 100 from the lasinfo report suggests that 50 as the minimum and 150 as the maximum are good false color ramp values for a map showing how the point density per square meter is distributed.

Color-coded point density: blue equals 50 or less and red means 150 or more points per square meter.

We use lastile to create a buffered tiling for the 47 million points. We use a tile size of 200 meters and request a large buffer of 50 meters around every tile because there are large buildings in the survey areas. We also flag buffer points as withheld so they can be easily be dropped later.

lastile -i 0_raw\TafawaBalewa.laz ^
        -tile_size 200 -buffer 50 -flag_as_withheld ^
        -odir 2_tiles_raw -o tafawa.laz

If you inspect the resulting tiles – such as ‘tafawa_544000_712600.laz’ as shown here – with lasview you will see the kind of low noise that is shown below and that may cause a ground classification algorithm. While our lasground software is able to deal with a certain amount of low noise – if there are too many it will likely latch onto them. Therefore we will first generate a subset of points that has as few as possible of such low noise points.

Typical low noise in dense-matching photogrammetry points in urban scene.

Next we use a sequence of three LAStools modules, namely lasthin, lasground, and lasheight to classify this photogrammtric point cloud into ground and non-ground points. All processing commands used are here. First we use lasthin to give the point the classification code 8 that is closest to the 50th percentile in elevation within every 50 centimeter by 50 centimeter cell (but only if the cells containing at least 20 points).

lasthin -i 2_tiles_raw\tafawa*.laz ^
        -step 0.5 ^
        -percentile 50 20 ^
        -classify_as 8 ^
        -odir 3_tiles_median_50cm -olaz ^
        -cores 3

Next we use lasground to ground-classify only the points that have classification code 8 (i.e. by ignoring those with classification codes 0) and set their classification code to ground (2) or non-ground (1). Because of the large buildings in this urban scene we use ‘-metro’ which uses a large step size of 50 meters for the pre-processing. This also sets the internally used bulge parameter to 5.0 which you can see if you run the tool in verbose ‘-v’ mode. In three different trial runs we determined that forcing the bulge parameter to be 0.5 instead gave better results. The bulge and the spike parameters can be useful to vary in order to improve ground classification results (also see the README file).

lasground -i 3_tiles_median_50cm\tafawa*.laz ^
          -ignore_class 0 ^
          -metro -bulge 0.5 ^
          -odir 4_tiles_ground_50cm -olaz ^
          -cores 3

The resulting ground points are a subset with a resolution of 50 centimeter that is good enough to create a DTM with meter resolution, which we do with las2dem command line shown below. We really like storing DTM elevation rasters to the LAZ point format because it is a more compressed way of storing elevation rasters compared to ASC, BIL, TIF, or IMG. It also makes the raster output a natural input to subsequent LAStools processing steps.

las2dem -i 4_tiles_ground_50cm\tafawa*.laz ^
        -keep_class 2 ^
        -step 1 -kill 100 ^
        -use_tile_bb ^
        -odir 5_tiles_dtm_1m -olaz ^
        -cores 3

Finally we use blast2dem to create a seamless hill-shaded version of our 1 meter DTM from on-the-fly merged elevation rasters. This is the DTM pictured at the beginning of this article.

blast2dem -i 5_tiles_dtm_1m\tafawa*.laz -merged ^
          -step 1 ^
          -hillshade ^
          -o dtm_1m.png

The corresponding DSM pictured at the beginning of this article was generated with the two command lines below by first keeping only the 95th percentile highest elevation of every 50 cm by 50 cm cell with lasthin (which remove spurious high noise points) and then by triangulating the surviving points with blast2dem into a seamless TIN that is also hill-shaded and rasterized with 1 meter resolution. Running the 64 bit version of lasthin (note the ‘-cpu64‘ switch) allows us to work on the entire data set (rather than its tiles version) at once, where the standard 32 bit version may run out of memory.

lasthin -i 0_raw\TafawaBalewa.laz ^
        -cpu64 ^
        -step 0.5 ^
        -percentile 95 20 ^
        -o 0_raw\TafawaBalewa_p95_50.laz

blast2dem -i 0_raw\TafawaBalewa_p95_50.laz ^
          -step 1 ^
          -hillshade ^
          -o dsm_1m.png

In order to generate the final DTM at higher resolution we use lasheight to pull all points into the ground class that lie within a 5 cm distance vertically below or a 15 cm distance vertically above the triangulated surface of ground points computed in the previous step. You could experiment with other values here to be less or more conservative about pulling detail into the ground class.

lasheight -i 4_tiles_ground_50cm\tafawa*.laz ^
          -classify_between -0.05 0.15 2 ^
          -odir 6_tiles_ground -olaz ^
          -cores 3

We repeat the same processing step as before las2dem to create the raster DTM tiles, but this time with a resolution of 25 cm.

las2dem -i 6_tiles_ground\tafawa*.laz ^
        -keep_class 2 ^
        -step 0.25 -kill 100 ^
        -use_tile_bb ^
        -odir 7_tiles_dtm_25cm -olaz ^
        -cores 3

And we again use blast2dem to create a seamless hill-shaded version of the DTM from on-the-fly merged elevation rasters, but this time with a resolution of 25 cm. This is the DTM shown below. All processing commands used are here.

blast2dem -i 7_tiles_dtm_25cm\tafawa*.laz -merged ^
          -step 0.25 ^
          -hillshade ^
          -o dtm_25cm.png

Hill-shade of final DTM with resolution of 25 cm.

Digital Pothole Removal: Clean Road Surface from Noisy Pix4D Point Cloud

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How to generate a clean Digital Terrain Model (DTM) from point clouds that were generated with the image matching techniques implemented in various photogrammetry software packages like those from Pix4D, AgiSoft, nframes, DroneDeploy and others has become an ever more frequent inquiry. There are many other blog posts on the topic that you can peruse as well [1,2,3,4,5,6,7,8]. In the following we go step by step through the process of removing low noise from a high-density point cloud that was generated with Pix4D software. A composite of the resulting DTM and DSM is shown below.

Final DSM and DTM created with LAStools for a photogrammetric point cloud of a road generated by Pix4D.

After downloading the data it is useful to familiarize oneself with the number of points, the density of points and their geo-location. This can be done with lasview, lasinfo, and lasgrid using the command lines shown below. There are around 19 million points in the file and their density averages around 2300 points per square meter. Because the RGB values have a 16 bit range (as evident in the lasinfo report) we need to add the option ‘-scale_rgb_down’ to the command line when producing the RGB raster with lasgrid.

lasview -i 0_photogrammetry\densified_point_cloud.laz

lasinfo -i 0_photogrammetry\densified_point_cloud.laz ^
        -cd ^
        -o 1_quality\densified_point_cloud.txt

lasgrid -i 0_photogrammetry\densified_point_cloud.laz ^
        -scale_rgb_down ^
        -step 0.10 ^
        -rgb ^
        -fill 1 ^
        -o 1_quality\densified_point_cloud.png

The first step is to use lastile and create smaller and buffered tiles for these 19 million photogrammetry points. We use a tile size of 100 meters, request a buffer of 10 meters around every tile, and flag buffer points as withheld so they can be easily be dropped later. We also make sure that all classification codes are reset to 0.

lastile -i 0_photogrammetry\points.laz ^
        -set_classification 0 ^
        -tile_size 100 -buffer 10 -flag_as_withheld ^
        -o 2_tiles_raw\seoul.laz -olaz

We start with lassort as a pre-processing step that rearranges the points into a more coherent spatial order which often accelerates subsequent processing steps.

lassort -i 2_tiles_raw\seoul_*.laz ^
        -odir 3_tiles_temp0 -olaz ^
        -cores 4

Next we use a sequence of four modules, namely lasthin, lasnoise, lasground, and lasheight with fine-tuned parameters to remove the low noise points that are typical for point clouds generated from imagery by photogrammetry software. A typical example for such noise points are shown in the image below generated with this call to lasview:

lasview -i 3_tiles_temp0\seoul_210400_542900.laz ^
        -inside 210406 542894 210421 542921 ^
        -points 20000000 ^
        -kamera 0 -95 90 0 -0.3 1.6 ^
        -point_size 4

Ground surface noise (exaggerated by pressing <]> in lasview which doubles the scale in z).

As always, the idea is to construct a surface that is close to the ground but always above the noise so that it can be used to declare all points beneath it as noise. Below is a processing pipeline whose parameters work well for this data and that you can fine tune for the point density and the noise profile of your own data.

First we use lasthin to give those points the classification code 8 that are closest to the 70th percentile in elevation within every 20 cm by 20 cm cell. As statistics like percentiles are only stable for a sufficient number of points we only do this for cells that contain 25 points or more. Given that we have an average of 2300 points per square meter this should easily be the case for all relevant cells.

lasthin -i 3_tiles_temp0\seoul_*.laz ^
        -step 0.20 ^
        -percentile 70 25 ^
        -classify_as 8 ^
        -odir 3_tiles_temp1 -olaz ^
        -cores 4

The we run lasnoise only points on the points with classification code 8 and reclassify all “overly isolated” points with code 9. The check for isolation uses cells of size 20 cm by 20 cm by 5 cm and reclassifies the points in the center cell when the surrounding neighborhood of 27 cells has only 3 or fewer points in total. Changing the parameters for ‘-step_xy 0.20 -step_z 0.05 -isolated 3’ will remove isolated points more or less aggressive.

lasnoise -i 3_tiles_temp1\seoul_*.laz ^
         -ignore_class 0 ^
         -step_xy 0.20 -step_z 0.05 -isolated 3 ^
         -classify_as 9 ^
         -odir 3_tiles_temp2 -olaz ^
         -cores 4

Next we use lasground to ground-classify only the surviving points (that still have classification code 8) by ignoring those with classification codes 0 or 9. This sets their classification code to either ground (2) or non-ground (1). The temporary surface defined by the resulting ground points will be used to classify low points as noise in the next step.

lasground -i 3_tiles_temp2\seoul_*.laz ^
          -ignore_class 0 9 ^
          -town -ultra_fine -bulge 0.1 ^
          -odir 3_tiles_temp3 -olaz ^
          -cores 4

Then we use lasheight to classify all points that are 2.5 cm or more below the triangulated surface of temporary ground points as points as noise (7) and all others as unclassified (1).

lasheight -i 3_tiles_temp3\seoul_*.laz ^
          -classify_below -0.025 7 ^
          -classify_above -0.225 1 ^
          -odir 4_tiles_denoised -olaz ^
          -cores 4

The progress of each step is illustrated visually in the two image sequences shown below.

all points.
after lasthin: all 70th elevation percentile points per 20 cm by 20 cm cell.
same. all 70th elevation percentile points per 20 cm by 20 cm cell.
after lasnoise with isolated points in blue.
after lasnoise without isolated points.
after lasground.
after lasheight: using surface defined by ground points to classify low noise.
low noise only.
points without low noise.

all points.
temporary ground surface for classifying low noise.
low noise points.
points without low noise.

Now that all noise points are classified we start a standard processing pipeline, but always ignore the low noise points that are now classified with classification code 7.

The processing steps below create a 10 cm DTM raster. We first use lasthin to classify the lowest non-noise point per 10 cm by 10 cm cell. Considering only those lowest points we use lasground with options ‘-town’, ‘-extra_fine’, ‘-bulge 0.05’, and ‘-spike 0.05’. Using las2dem the resulting ground points are interpolated into a TIN and rasterized into a 10 cm DTM cutting out only the center 100 meter by 100 meter tile. We store the DTM raster as a gridded LAZ for maximal compression and finally merge these gridded LAZ files to create a hillshaded raster in PNG format with blast2dem.

lasthin -i 4_tiles_denoised\seoul_*.laz ^
        -ignore_class 7 ^
        -step 0.10 ^
        -lowest ^
        -classify_as 8 ^
        -odir 5_tiles_thinned_lowest -olaz ^
        -cores 4

lasground -i 5_tiles_thinned_lowest\seoul_*.laz ^
          -ignore_class 1 7 ^
          -town -extra_fine ^
          -bulge 0.05 -spike 0.05 ^
          -odir 6_tiles_ground -olaz ^
          -cores 4

las2dem -i 6_tiles_ground\seoul_*.laz ^
        -keep_class 2 ^
        -step 0.10 ^
        -use_tile_bb ^
        -odir 7_tiles_dtm -olaz ^
        -cores 4

blast2dem -i 7_tiles_dtm\seoul_*.laz -merged ^
          -hillshade ^
          -step 0.10 ^
          -o dtm_hillshaded.png

The processing steps below create a 10 cm DSM raster. We first use lasthin to classify the highest non-noise point per 10 cm by 10 cm cell. With las2dem the highest points are interpolated into a TIN and rasterized into a 10 cm DSM cutting out only the center 100 meter by 100 meter tile. Again we store the raster as gridded LAZ for maximal compression and merge these files to create a hillshaded raster in PNG format with blast2dem.

lasthin -i 4_tiles_denoised\seoul_*.laz ^
        -ignore_class 7 ^
        -step 0.10 ^
        -highest ^
        -classify_as 8 ^
        -odir 8_tiles_thinned_highest -olaz ^
        -cores 4

las2dem -i 8_tiles_thinned_highest\seoul_*.laz ^
        -keep_class 8 ^
        -step 0.10 ^
        -use_tile_bb ^
        -odir 9_tiles_dsm -olaz ^
        -cores 4

blast2dem -i 9_tiles_dsm\seoul_*.laz -merged ^
          -hillshade ^
          -step 0.10 ^
          -o dsm_hillshaded.png

The final result is below. The entire script is linked here. Simply download it, modify it as needed, and try it on this data or on your own data.

final DSM.
final DTM.

Scripting LAStools to Create a Clean DTM from Noisy Photogrammetric Point Cloud

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A recent inquiry by Drone Deploy in the LAStools user forum gave us access to a nice photogrammetric point cloud for the village of Fillongley in the North Warwickshire district of England. They voted “Leave” with a whopping 66.9% according to the EU referendum results by the BBC. Before we say “Good riddance, Fillongley!” we EU-abuse this little village one last time and remove all their low noise points to create a nice Digital Terrain Model (DTM). The final result is shown below.

Side by side comparison of DTM and DSM generated with LAStools from photogrammetric point cloud by Drone Deploy.

After downloading the data it is useful to familiarize yourself with the point number, the point density and their geo-location, which can be done with lasview, lasinfo, and lasgrid using the command lines shown below. There are around 50 million points and their density averages close to 70 points per square meter.

lasview -i 0_photogrammetry\points.laz

lasinfo -i 0_photogrammetry\points.laz ^
        -cd ^
        -o 1_quality\fillongley.txt

lasgrid -i 0_photogrammetry\points.laz ^
        -step 0.50 ^
        -rgb ^
        -fill 1 ^
        -o 1_quality\fillongley.png

The first step is to use lastile and create smaller and buffered tiles for these 50 million photogrammetry points. We use a tile size of 200 meters, request a buffer of 25 meters around every tile, and flag buffer points as withheld so they can be easily be dropped later.

lastile -i 0_photogrammetry\points.laz ^
        -tile_size 200 -buffer 25 -flag_as_withheld ^
        -o 2_tiles_raw\fillongley.laz -olaz

Next we use a sequence of four modules, namely lasthin, lasnoise, lasground, and lasheight with fine-tuned parameters to remove the low noise points that are typical for point clouds generated from imagery by photogrammetry software. A typical example for such noise points are shown in the image below.

lasview -i 2_tiles_raw\fillongley_596000_5815800.laz ^
        -inside 596050 5815775 596150 5815825 ^
        -kamera 0 -89 -1.75 0 0 1.5 ^
        -point_size 3

Clumps of low noise points typical for photogrammetry point clouds.

The idea to identify those clumps of noise is to construct a surface that is sufficiently close to the ground but always above the noise so that it can be used to classify all points beneath it as noise. However, preserving true ground features without latching onto low noise points often requires several iterations of fine-tuning the parameters. We did this interactively by repeatedly running the processing on only two representative tiles until a desired outcome was achieved.

First we use lasthin to give the point the classification code 8 that is closest to the 20th percentile in elevation within every 90 cm by 90 cm cell (but only if the cells containing at least 20 points). Choosing larger step sizes or higher percentiles resulted in missing ground features. Choosing smaller step sizes or lower percentiles resulted in low noise becoming part of the final ground model.

lasthin -i 2_tiles_raw\fillongley_*.laz ^
        -step 0.90 ^
        -percentile 20 20 ^
        -classify_as 8 ^
        -odir 3_tiles_temp1 -olaz ^
        -cores 4

The we run lasnoise only points on the points with classification code 8 (by ignoring those with classification code 0) and reclassify all “overly isolated” points with code 12. The check for isolation uses cells of size 200 cm by 200 cm by 50 cm and reclassifies the points in the center cell when the surrounding neighborhood of 27 cells has only 3 or fewer points in total. Changing the parameters for ‘-step_xy 2.00 -step_z 0.50 -isolated 3’ will remove noise more or less aggressive.

lasnoise -i 3_tiles_temp1\fillongley_*.laz ^
         -ignore_class 0 ^
         -step_xy 2.00 -step_z 0.50 -isolated 3 ^
         -classify_as 12 ^
         -odir 3_tiles_temp2 -olaz ^
         -cores 4

Next we use lasground to ground-classify only the surviving points (that still have classification code 8) by ignoring those with classification codes 0 or 12 and set their classification code to ground (2) or non-ground (1). The temporary surface defined by the resulting ground points will be used to classify low points as noise in the next step.

lasground -i 3_tiles_temp2\fillongley_*.laz ^
          -ignore_class 0 12 ^
          -town -ultra_fine ^
          -odir 3_tiles_temp3 -olaz ^
          -cores 4

Then we use lasheight to classify all points that are 20 cm or more below the triangulated surface of temporary ground points as points as noise (7) and all others as unclassified (1).

lasheight -i 3_tiles_temp3\fillongley_*.laz ^
          -classify_below -0.20 7 ^
          -classify_above -0.20 1 ^
          -odir 4_tiles_denoised -olaz ^
          -cores 4

The progress of each step is illustrated visually in the two image sequences shown below.

 

Clumps of low noise points typical for photogrammetry point clouds.
The thinned 20th elevation percentile points.
The thinned 20th elevation percentile points classified with code 8.
In yellow the points classified to code 12 by lasnoise.
The ground points used to create a temporary surface.
The pink points below the temporary ground surface are noise.
The points classified as noise,
The clean photogrammetric point clouds.

 

Clumps of low noise points typical for photogrammetry point clouds.
The thinned 20th elevation percentile points.
The thinned 20th elevation percentile points classified with code 8.
In yellow the points classified to code 12 by lasnoise.
The ground points used to create a temporary surface.
The pink points below the temporary ground surface are noise.
The points classified as noise,
The clean photogrammetric point clouds.

Now that all noise points are classified we start a standard processing pipeline, but always ignore the noise points that are now classified with classification code 7.

The processing steps below create a 25 cm DTM raster. We first use lasthin to classify the lowest non-noise point per 25 cm by 25 cm cell. Considering only those lowest points we use lasground with options ‘-town’, ‘-extra_fine’, or ‘-bulge 0.1’. Using las2dem the resulting ground points are interpolated into a TIN and rasterized into a 25 cm DTM cutting out only the center 200 meter by 200 meter tile. We store the DTM raster as a gridded LAZ for maximal compression and finally merge these gridded LAZ files to create a hillshaded raster in PNG format with blast2dem.

lasthin -i 4_tiles_denoised\fillongley_*.laz ^
        -ignore_class 7 ^
        -step 0.25 ^
        -lowest ^
        -classify_as 8 ^
        -odir 5_tiles_thinned_lowest -olaz ^
        -cores 4

lasground -i 5_tiles_thinned_lowest\fillongley_*.laz ^
          -ignore_class 1 7 ^
          -town -extra_fine -bulge 0.1 ^
          -odir 6_tiles_ground -olaz ^
          -cores 4

las2dem -i 6_tiles_ground\fillongley_*.laz ^
        -keep_class 2 ^
        -step 0.25 ^
        -use_tile_bb ^
        -odir 7_tiles_dtm -olaz ^
        -cores 4

blast2dem -i 7_tiles_dtm\fillongley_*.laz -merged ^
          -hillshade ^
          -step 0.25 ^
          -o dtm_hillshaded.png

The processing steps below create a 25 cm DSM raster. We first use lasthin to classify the highest non-noise point per 25 cm by 25 cm cell. With las2dem the highest points are interpolated into a TIN and rasterized into a 25 cm DSM cutting out only the center 200 meter by 200 meter tile. Again we store the raster as gridded LAZ for maximal compression and merge these files to create a hillshaded raster in PNG format with blast2dem.

lasthin -i 4_tiles_denoised\fillongley_*.laz ^
        -ignore_class 7 ^
        -step 0.25 ^
        -highest ^
        -classify_as 8 ^
        -odir 8_tiles_thinned_highest -olaz ^
        -cores 4

las2dem -i 8_tiles_thinned_highest\fillongley_*.laz ^
        -keep_class 8 ^
        -step 0.25 ^
        -use_tile_bb ^
        -odir 9_tiles_dsm -olaz ^
        -cores 4

blast2dem -i 9_tiles_dsm\fillongley_*.laz -merged ^
          -hillshade ^
          -step 0.25 ^
          -o dsm_hillshaded.png

The final result is below. The entire script is linked here. Simply download it, modify it as needed, and try it on your data.

 

final 25 cm Digital Surface Model
final 25 cm Digital Terrain Model

In Sweden, all they Wanted for Christmas was National LiDAR as Open Data

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Let’s heat up some sweet, warm and spicy Glögg in celebration! They must have been good boys and girls up there in Sweden. Because “Jultomten” or simply ”Tomten” – how Sweden’s Santa Clause is called – is assuring a “God Jul” for all the Swedish LiDAR lovers this Christmas season.

Only a few weeks ago this tweet of ours had (mistakenly) included Sweden in a list of European countries that had released their national LiDAR archives as open data for public reuse over the past six years.

Turns out we were correct after all. Sweden has just opened their LiDAR data for free and unencumbered download. To get the data simply create a user account and browse to the ftp site for download as shown in the image sequence below.

Open Geodata Landing Page.
Instructions for Accessing Open Geodata.
Browsing all Open Geodata Sets
Open LiDAR Data for 2018
Downloading selected LAZ tiles

The released LiDAR data was collected with a density of 1 to 2 pulses per square meter and is distributed in LASzip compressed LAZ tiles of 2500 by 2500 meters. The returns are classified into four classes: ground (2), water (9), low noise (7) and high noise (18). All items that can not be classified as any of the first four classes coded as left unclassified (1). The LAZ files do not contain CRS information, but this can easily be added with horizontal coordinates in SWERED99 TM (EPSG code 3006) and elevations in RH2000 height (EPSG code 5613).

Below a look with lasview at a 5 km by 5 km area that composed of the four tiles ‘18P001_67100_5800_25.laz‘, ‘18P001_67100_5825_25.laz‘, ‘18P001_67125_5800_25.laz‘ and ‘18P001_67125_5825_25.laz‘ with several of the different color modes available.

 

Colored by classification.
Colored by elevation 70 m to 140 m.
Colored by elevation 70 m to 140 m.
Colored by return type.
Colored by flightline.
Colored by intensity.

Some more details: The data was acquired at flying altitude of around 3000 meter with a maximum scan angle of ± 20º and a minimum side overlap of 10% between the flightlines. The laser footprint on ground is below 75 centimeters with slight variation based on the flying altitude. The laser scanning survey was performed with LiDAR instruments that can provide at least three returns from the same pulse. All LiDAR returns are preserved throughout the entire production chain.

The LiDAR data comes with the incredibly Creative Commons – CC0 license, which means that you can use, disseminate, modify and build on the data – even for commercial purposes – without any restrictions. You are free to acknowledge the source when you distribute the data further, but it is not required.

The LiDAR data will eventually cover approximately 75% of Sweden and new point clouds will continuously be added as additional scanning is performed according to the schedule shown below. The survey will be returning to scan every spot again after about 7 years.

2018-2022 LiDAR acquisition plan for Sweden

Below a lasinfo report for tile ‘18P001_67125_5825_25.laz‘. One noticeable oddity is the distribution of intensities. The histogram across all intensities with bins of size 256 shows two clearly distinct sets of intensities each with their own peak and a void of values between 3000 and 10000.

lasinfo -i 18P001_67125_5825_25.laz -cd -histo intensity 256
reporting all LAS header entries:
  file signature:             'LASF'
  file source ID:             0
  global_encoding:            1
  project ID GUID data 1-4:   00000000-0000-0000-0000-000000000000
  version major.minor:        1.2
  system identifier:          ''
  generating software:        'TerraScan'
  file creation day/year:     303/2018
  header size:                227
  offset to point data:       227
  number var. length records: 0
  point data format:          1
  point data record length:   28
  number of point records:    20670652
  number of points by return: 13947228 4610837 1712043 358397 42147
  scale factor x y z:         0.01 0.01 0.01
  offset x y z:               0 0 0
  min x y z:                  582500.00 6712500.00 64.56
  max x y z:                  584999.99 6714999.99 136.59
LASzip compression (version 3.2r2 c2 50000): POINT10 2 GPSTIME11 2
reporting minimum and maximum for all LAS point record entries ...
  X            58250000   58499999
  Y           671250000  671499999
  Z                6456      13659
  intensity          32      61406
  return_number       1          5
  number_of_returns   1          5
  edge_of_flight_line 0          1
  scan_direction_flag 0          1
  classification      1         18
  scan_angle_rank   -19         19
  user_data           0          1
  point_source_ID  1802       1804
  gps_time 222241082.251248 222676871.876191
number of first returns:        13947228
number of intermediate returns: 2110980
number of last returns:         13952166
number of single returns:       9339722
covered area in square units/kilounits: 5923232/5.92
point density: all returns 3.49 last only 2.36 (per square units)
      spacing: all returns 0.54 last only 0.65 (in units)
overview over number of returns of given pulse: 9339722 5797676 4058773 1263967 210514 0 0
histogram of classification of points:
        10888520  unclassified (1)
         9620725  ground (2)
           22695  noise (7)
          138147  water (9)
             565  Reserved for ASPRS Definition (18)
intensity histogram with bin size 256.000000
  bin [0,256) has 1753205
  bin [256,512) has 3009640
  bin [512,768) has 2240861
  bin [768,1024) has 1970696
  bin [1024,1280) has 1610647
  bin [1280,1536) has 1285858
  bin [1536,1792) has 974475
  bin [1792,2048) has 790480
  bin [2048,2304) has 996926
  bin [2304,2560) has 892755
  bin [2560,2816) has 164142
  bin [2816,3072) has 57367
  bin [3072,3328) has 18
         [void]
  bin [10752,11008) has 589317
  bin [11008,11264) has 3760
  bin [11264,11520) has 99653
  bin [11520,11776) has 778739
  bin [11776,12032) has 1393569
  bin [12032,12288) has 1356850
  bin [12288,12544) has 533202
  bin [12544,12800) has 140223
  bin [12800,13056) has 16195
  bin [13056,13312) has 2319
  bin [13312,13568) has 977
  bin [13568,13824) has 765
  bin [13824,14080) has 648
  bin [14080,14336) has 289
  bin [14336,14592) has 513
  bin [14592,14848) has 383
  bin [14848,15104) has 178
  bin [15104,15360) has 526
  bin [15360,15616) has 108
  bin [15616,15872) has 263
  bin [15872,16128) has 289
  bin [16128,16384) has 69
  bin [16384,16640) has 390
  bin [16640,16896) has 51
  bin [16896,17152) has 186
  bin [17152,17408) has 239
  bin [17408,17664) has 169
  bin [17664,17920) has 58
  bin [17920,18176) has 227
  bin [18176,18432) has 169
  bin [18432,18688) has 40
  bin [18688,18944) has 401
  bin [18944,19200) has 30
  bin [19200,19456) has 411
  bin [19456,19712) has 34
  bin [19712,19968) has 34
  bin [19968,20224) has 398
  bin [20224,20480) has 24
  bin [20480,20736) has 108
  bin [20736,20992) has 267
  bin [20992,21248) has 29
  bin [21248,21504) has 318
  bin [21504,21760) has 26
  bin [21760,22016) has 59
  bin [22016,22272) has 184
  bin [22272,22528) has 52
  bin [22528,22784) has 18
  bin [22784,23040) has 116
  bin [23040,23296) has 55
  bin [23296,23552) has 89
  bin [23552,23808) has 250
  bin [23808,24064) has 24
  bin [24064,24320) has 52
  bin [24320,24576) has 14
  bin [24576,24832) has 29
  bin [24832,25088) has 71
  bin [25088,25344) has 74
  bin [25344,25600) has 2
  bin [25600,25856) has 17
  bin [25856,26112) has 2
  bin [26368,26624) has 9
  bin [26624,26880) has 1
  bin [26880,27136) has 1
  bin [27136,27392) has 1
  bin [27392,27648) has 1
  bin [27648,27904) has 3
  bin [28416,28672) has 2
  bin [29184,29440) has 4
  bin [30720,30976) has 1
  bin [30976,31232) has 2
  bin [31232,31488) has 1
  bin [32512,32768) has 1
  bin [36864,37120) has 1
  bin [58368,58624) has 1
  bin [61184,61440) has 1
  average intensity 3625.2240208968733 for 20670652 element(s)

Scrutinizing LiDAR Data from Leica’s Single Photon Scanner SPL100 (aka SPL99)

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We show how simple reordering and clever remapping of single photon LiDAR data can reduce file size by a whopping 50%. We also show that there is at least one Leica’s SPL100 sensor out there that should be called SPL99 because one of its 100 beamlets (the one with beamlet ID 53) does not seem to produce any data … (-:

Closeup on the returns of two beamlet shots colored by beamlet ID from 1 (blue) to 100 (red). Beamlet ID 53 is missing.

Following up on a recent discussion in the LAStools user forum we take a closer look at some of the single photon LiDAR collected with Leica’s SPL100 sensor made available as open data by the USGS in form of LASzip-compressed tiles in LAS 1.4 format of point type 6. This investigation was sparked by the curiosity of what value was stored to the “scanner channel” field that was added to the new point types 6 to 10 in the LAS 1.4 specification.

lasview -i USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP120310_LAS_2018.laz ^
        -copy_scanner_channel_into_point_source ^
        -color_by_flightline

Visualizing this 2 bit number whose value can range from 0 to 3 for the first tile we downloaded resulted in this non-conclusive “magic eye” visualization. What do you see? A sailboat?

Visualizing the “scanner channel” field by mapping its four different values to different colors.

Jason Stoker from the USGS suggested that this is the truncated “beamlet” ID. Leica’s SPL100 sensor uses 100 beamlets rather than one or two laser beams to collect data. Storing the beamlet IDs between 1 and 100 to this 2 bit field that can only hold numbers between 0 and 3 is kind of pointless and should be avoided. LASzip switches prediction contexts based on this field resulting in slower compression speed and lower compression rates. The beamlet ID is also stored in the 8 bit “user data” field, so that we can simply zero the “scanner channel” field. To investigate this further we downloaded these nine tiles from this FTP site of the USGS:

Whenever we download LAZ files we first run laszip with the ‘-check’ option which performs a sanity check to make sure that the files are not truncated or otherwise corrupted. In our case we get nine solid reports of SUCCESS.

laszip -i USGS_LPC_SD_MORiver_Woolpert_B1_*_2018.laz -check

A visual inspection with lasview tells us that there are a number of flightlines in the data.

lasview -i USGS_LPC_SD_MORiver_Woolpert_B1_*_2018.laz ^
        -points 15000000 ^
        -color_by_flightline

nine tiles colored by flightline
showing only flightline 2003
extracting 0.25 seconds worth of points from flightline 2003

We use las2las to extract flightline 2003 and lasinfo to produce a histogram of GPS times which we use in turn to decide on which quarter second of GPS time worth of data we want to extract again with las2las.

las2las -i USGS_LPC_SD_MORiver_Woolpert_B1_2016_*_LAS_2018.laz ^
        -merged ^
        -keep_point_source 2003 ^
        -o USGS_LPC_SD_MORiver_Woolpert_B1_ps_2002.laz

lasinfo -i USGS_LPC_SD_MORiver_Woolpert_B1_ps_2002.laz ^
        -cd ^
        -histo gps_time 1 ^
        -odix _info -otxt

las2las -i USGS_LPC_SD_MORiver_Woolpert_B1_ps_2002.laz ^
        -keep_gps_time 176475495 176475495.25 ^
        -o USGS_LPC_SD_MORiver_Woolpert_B1_gps176475495_quarter.laz

It always helps to give your LAZ files meaningful names in case you find them again two years later or so. We can nicely see the circular scanning pattern Leica’s SPL100 sensor. With lasview we measure that this single flightline has an extent of about 2000 meters on the ground. The lasinfo report shows a pulse density of around 19 last returns per square meter. We then sort the points by GPS time using lassort. This groups together all the returns that are the result of one “shot” of the laser with 100 beamlets as we can nicely see in the las2txt output below. Each group of returns has slightly below 100 points and there is one group every 0.00002 seconds. This means the SPL100 is firing once every 20 microseconds.

lassort -i USGS_LPC_SD_MORiver_Woolpert_B1_gps176475495_quarter.laz ^
        -gps_time ^
        -odix _sorted -olaz

las2txt -i USGS_LPC_SD_MORiver_Woolpert_B1_gps176475495_quarter_sorted.laz ^
        -parse tuxyz ^
        -stdout | more
176475495.000008 4 514408.78 4830989.78 487.79
176475495.000008 9 514410.38 4830987.49 487.70
176475495.000008 47 514411.49 4830987.71 487.70
        [ ... 86 lines deleted ... ]
176475495.000008 39 514408.53 4830991.81 487.80
176475495.000008 50 514407.97 4830991.69 487.80
176475495.000008 16 514409.24 4830991.46 487.85
176475495.000028 55 514413.51 4830985.79 487.61
176475495.000028 97 514411.10 4830990.03 487.74
176475495.000028 72 514411.30 4830989.53 487.74
        [ ... 82 lines deleted ... ]
176475495.000028 45 514410.30 4830986.19 487.70
176475495.000028 3 514409.15 4830987.52 487.73
176475495.000028 96 514411.81 4830985.46 487.67
176475495.000048 66 514411.35 4830985.15 487.67
176475495.000048 83 514411.59 4830984.65 487.61
176475495.000048 64 514413.09 4830983.93 487.61
        [ ... 78 lines deleted ... ]
176475495.000048 4 514407.30 4830984.82 487.70
176475495.000048 34 514408.65 4830983.01 487.70
176475495.000048 21 514408.11 4830982.90 487.70
176475495.000068 13 514408.25 4830981.13 487.66
176475495.000068 92 514410.53 4830984.23 487.68
176475495.000068 44 514407.17 4830980.88 487.67
        [ ... 80 lines deleted ... ]
176475495.000068 76 514408.67 4830984.37 487.71
176475495.000068 47 514409.23 4830980.27 487.67
176475495.000068 87 514412.11 4830981.93 487.61
176475495.000088 97 514408.80 4830982.62 487.70
176475495.000088 33 514407.24 4830980.68 487.64
176475495.000088 30 514407.36 4830981.77 487.68
[ ... ]

Now we can “play back” the returns in acquisition order. We map returns from one group to the same color in lasview with the new ‘-bin_gps_time_into_point_source 0.00002’ option (that will be available in the next LAStools release). For a slower playback we add ‘-steps 5000’. Press the ‘c’ key to switch through the coloring options. Press the ‘s’ key repeatedly to incrementally show the points. To take a step back press <SHIFT>+’s’.

lasview -i USGS_LPC_SD_MORiver_Woolpert_B1_gps176475495_quarter_sorted.laz ^
        -bin_gps_time_into_point_source 0.00002 ^
        -scale_user_data 2.5 ^
        -steps 5000 ^
        -win 1024 384

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The last image colors the points by the values in the user data field (multiplied by 2.5), which essentially maps the beamlet IDs between 1 and 100 to a rainbow color ramp from blue to red. This tells us how the numbering of the beamlets from 1 to 100 corresponds to their layout in space. The next sequence of images takes a closer look at that.

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From a compression point of view it makes sense to (1) zero the meaningless scanner channel, (2) order the points by GPS time stamps to groups beamlet returns together, and (3) order the points with the same time stamp by the user data field. The compression gain is enormous with the 9 tiles going from over 3 GB to under 2 GB:

ORIGINAL:
337,156,981 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP120300_LAS_2018.laz
331,801,150 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP120310_LAS_2018.laz
358,928,274 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP120320_LAS_2018.laz
328,597,628 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP130300_LAS_2018.laz
355,997,013 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP130310_LAS_2018.laz
360,403,079 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP130320_LAS_2018.laz
355,399,781 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP140300_LAS_2018.laz
354,523,659 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP140310_LAS_2018.laz
357,248,968 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP140320_LAS_2018.laz
  3,140,056,533 Bytes

IMPROVED:
197,641,087 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP120300_LAS_2018_sorted.laz
194,750,096 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP120310_LAS_2018_sorted.laz
210,013,408 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP120320_LAS_2018_sorted.laz
190,687,275 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP130300_LAS_2018_sorted.laz
206,447,730 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP130310_LAS_2018_sorted.laz
209,580,551 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP130320_LAS_2018_sorted.laz
205,827,197 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP140300_LAS_2018_sorted.laz
203,808,113 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP140310_LAS_2018_sorted.laz
206,789,959 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP140320_LAS_2018_sorted.laz
  1,825,545,416 Bytes

Enumerating the 100 beamlets with a geometrically more coherent order would improve compression even more. Can anyone convince Leica to do this? The simple mapping of beamlet IDs shown below that arranges the beamlets into a zigzag order another huge compression gain of 15 percent. Altogether reordering and remapping lower the compressed file size by a whopping 50 percent.

Beamlet ID mapping table to improve spatial coherence.

168,876,666 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP120300_LAS_2018_mapped_sorted.laz
165,241,508 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP120310_LAS_2018_mapped_sorted.laz
176,524,959 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP120320_LAS_2018_mapped_sorted.laz
163,679,216 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP130300_LAS_2018_mapped_sorted.laz
176,086,559 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP130310_LAS_2018_mapped_sorted.laz
178,909,108 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP130320_LAS_2018_mapped_sorted.laz
174,735,634 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP140300_LAS_2018_mapped_sorted.laz
171,679,105 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP140310_LAS_2018_mapped_sorted.laz
174,997,090 USGS_LPC_SD_MORiver_Woolpert_B1_2016_14TNP140320_LAS_2018_mapped_sorted.laz
  1,550,729,845 Bytes

Once this is done a final space-filling sort into a Hilbert-curve or a Morton-order with lassort or lasoptimize would improve spatial coherence for efficient spatial indexing with lasindex.

Oh yes … the SPL100 was not firing on all cylinders. The beamlet ID 53 that would have mapped to 61 in our table was not present in any of the 9 tiles with 355,047,478 points that we had downloaded as the lasinfo histogram below shows.

lasinfo -i USGS_LPC_SD_MORiver_Woolpert_B1_2016_*_2018.laz -merged -histo user_data 1
lasinfo (180911) report for 9 merged files
reporting all LAS header entries:
  file signature:             'LASF'
  file source ID:             0
  global_encoding:            17
  project ID GUID data 1-4:   194774FA-35FE-4591-D484-010AFA13F6D9
  version major.minor:        1.4
  system identifier:          'Woolpert LAS'
  generating software:        'GeoCue LAS Updater'
  file creation day/year:     332/2017
  header size:                375
  offset to point data:       1376
  number var. length records: 1
  point data format:          6
  point data record length:   30
  number of point records:    0
  number of points by return: 0 0 0 0 0
  scale factor x y z:         0.01 0.01 0.01
  offset x y z:               0 0 0
  min x y z:                  512000.00 4830000.00 286.43
  max x y z:                  514999.99 4832999.99 866.81
  start of waveform data packet record: 0
  start of first extended variable length record: 0
  number of extended_variable length records: 0
  extended number of point records: 355047478
  extended number of points by return: 298476060 52480771 3929583 157365 3699 0 0 0 0 0 0 0 0 0 0
variable length header record 1 of 1:
  reserved             0
  user ID              'LASF_Projection'
  record ID            2112
  length after header  943
  description          'OGC WKT Coordinate System'
    WKT OGC COORDINATE SYSTEM:
    COMPD_CS["NAD83(2011) / UTM zone 14N + NAVD88 height - Geoid12B (metre)",PROJCS["NAD83(2011) / UTM zone 14N",GEOGCS["NAD83(2011)",DATUM["NAD83_National_Spat
ial_Reference_System_2011",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],AUTHORITY["EPSG","1116"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","
8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","6318"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0]
,PARAMETER["central_meridian",-99],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EP
SG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH],AUTHORITY["EPSG","6343"]],VERT_CS["NAVD88 height - Geoid12B (metre)",VERT_DATUM["North American Vertica
l Datum 1988",2005,AUTHORITY["EPSG","5103"]],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Gravity-related height",UP],AUTHORITY["EPSG","5703"]]]
the header is followed by 4 user-defined bytes
LASzip compression (version 3.1r0 c3 50000): POINT14 3
reporting minimum and maximum for all LAS point record entries ...
  X            51200000   51499999
  Y           483000000  483299999
  Z               28643      86681
  intensity        3139      12341
  return_number       1          5
  number_of_returns   1          5
  edge_of_flight_line 0          0
  scan_direction_flag 0          1
  classification      1         10
  scan_angle_rank  -127        127
  user_data           1        100
  point_source_ID  1061       2005
  gps_time 176475467.194000 176496233.636563
  extended_return_number          1      5
  extended_number_of_returns      1      5
  extended_classification         1     10
  extended_scan_angle        -21167  21167
  extended_scanner_channel        0      3
number of first returns:        298476060
number of intermediate returns: 6282
number of last returns:         355000765
number of single returns:       298435629
overview over extended number of returns of given pulse: 298435629 52515017 3935373 157750 3709 0 0 0 0 0 0 0 0 0 0
histogram of classification of points:
       138382030  unclassified (1)
       207116732  ground (2)
         9233160  noise (7)
          310324  water (9)
            5232  rail (10)
 +-> flagged as withheld:  9233160
 +-> flagged as extended overlap: 226520346
user data histogram with bin size 1.000000
  bin 1 has 3448849
  bin 2 has 3468566
  bin 3 has 3721848
  bin 4 has 3376990
  bin 5 has 3757996
  bin 6 has 3479546
  bin 7 has 3799930
  bin 8 has 3766887
  bin 9 has 3448383
  bin 10 has 3966036
  bin 11 has 3232086
  bin 12 has 3686789
  bin 13 has 3763869
  bin 14 has 3847765
  bin 15 has 3659059
  bin 16 has 3666918
  bin 17 has 3427468
  bin 18 has 3375320
  bin 19 has 3222116
  bin 20 has 3598643
  bin 21 has 3108323
  bin 22 has 3553625
  bin 23 has 3782185
  bin 24 has 3577792
  bin 25 has 3063871
  bin 26 has 3451800
  bin 27 has 3518763
  bin 28 has 3845852
  bin 29 has 3366980
  bin 30 has 3797986
  bin 31 has 3623477
  bin 32 has 3606798
  bin 33 has 3762737
  bin 34 has 3861023
  bin 35 has 3821228
  bin 36 has 3738173
  bin 37 has 3902190
  bin 38 has 3726752
  bin 39 has 3910989
  bin 40 has 3771132
  bin 41 has 3718437
  bin 42 has 3609113
  bin 43 has 3339941
  bin 44 has 3003191
  bin 45 has 3697140
  bin 46 has 2329171
  bin 47 has 3398836
  bin 48 has 3511882
  bin 49 has 3719592
  bin 50 has 2995275
  bin 51 has 3673925
  bin 52 has 3535992
  bin 54 has 3799430
  bin 55 has 3613345
  bin 56 has 3761436
  bin 57 has 3296831
  bin 58 has 3810146
  bin 59 has 3768464
  bin 60 has 3520871
  bin 61 has 3833149
  bin 62 has 3639778
  bin 63 has 3623008
  bin 64 has 3581480
  bin 65 has 3663180
  bin 66 has 3661434
  bin 67 has 3684374
  bin 68 has 3723125
  bin 69 has 3552397
  bin 70 has 3554207
  bin 71 has 3535494
  bin 72 has 3621334
  bin 73 has 3633928
  bin 74 has 3631845
  bin 75 has 3526502
  bin 76 has 3605631
  bin 77 has 3452006
  bin 78 has 3796382
  bin 79 has 3731841
  bin 80 has 3683314
  bin 81 has 3806024
  bin 82 has 3749709
  bin 83 has 3808218
  bin 84 has 3634032
  bin 85 has 3631015
  bin 86 has 3712206
  bin 87 has 3627775
  bin 88 has 3674966
  bin 89 has 3231151
  bin 90 has 3780037
  bin 91 has 3621958
  bin 92 has 3623264
  bin 93 has 3853536
  bin 94 has 3623380
  bin 95 has 3418309
  bin 96 has 3374827
  bin 97 has 3464734
  bin 98 has 3562560
  bin 99 has 3078686
  bin 100 has 3426924