LiDAR heights of burial mounds and cairns

[contributed by guest blogger Lars Forseth]

Archaeologists are increasingly finding ALS/LiDAR useful for making better surveys of archaeological sites and monuments. This is also done where these sites are in danger of being developed, and thus destroyed, see i.e (Risbøl 2011; Gustavsen et al. 2013). Norwegian archaeologists at several county councils and museums have detected unknown sites in woodland or areas previously not surveyed. LiDAR is now available for large areas, as the national mapping authority of Norway, Statens Kartverk, is using this data as a source for generating elevation and contour maps.

aerial view of a mound in North-Trøndelag

aerial view of a mound in North-Trøndelag

Working in North-Trøndelag county, my colleague Kristin Foosnæs at NTNU and I have embarked on a project to create a survey of the larger burial mounds and burial cairns of the county. North-Trøndelag seems to have an unusual large amount of such mounds larger than 20 meters (462 such are so far identified, far more than in any other county). We have gathered exact survey data for a sample of 2900 mounds/cairns. For these we have the exact polygon describing the area of the mounds. LiDAR and LAStools have been extensively used in the creation of this database.

the same  mound seen from the ground

the same mound seen from the ground

The height of the monuments however could only be gathered from the national monuments and sites database where the heights are stored as text. These were gathered by field surveys in the 1970s to 1990s. Then the only tools available to archaeologists for estimating the height were yardsticks or soil probes. Mapping the sites was done on aerial photos at a scale of 1:16.000. The height data gathered from the database is very variable in quality, which has to do with how they were generated. Mostly those that did the original surveys had to estimate the height of the monuments.

a first-return DSM and a ground-return DTM of the same mound

a first-return DSM and a ground-return DTM of the same mound

This summer I discovered that the lascanopy tool of LAStools could measure the min/max elevation for an area-of-interest. Using lascanopy I generated a csv report of elevations (min/max) within a polygon in a shapefile:

lascanopy -lof steinkjer.txt ^
          -keep_class 2 ^
          -lop tessem.shp ^
          -height_offset -1000 ^
          -centroids -min -max ^
          -o tessem.csv

Here I’m inputting a text file ‘steinkjer.txt’ with the list of LAS files to be queried and a shapefile ‘tessem.shp’ with the polygons of the mounds I want to know the height above the ground for. The output ‘tessem.csv’ looks like this:

index,min_x,min_y,max_x,max_y,centroid_x,centroid_y,min,max
0,616277.76,7108569.01,616290.33,7108581.37,616284.04,7108575.19,76.72,77.98
1,616292.40,7108572.37,616299.98,7108580.46,616296.19,7108576.41,77.83,78.96
2,616310.04,7108585.13,616320.96,7108596.96,616315.50,7108591.05,79.93,81.15
3,616714.65,7108371.35,616734.75,7108392.03,616724.70,7108381.69,83.47,86.73
4,616681.74,7108412.71,616699.80,7108429.61,616690.77,7108421.16,86.13,87.97
5,616672.13,7108436.30,616694.56,7108453.78,616683.34,7108445.04,86.55,89.19
6,616666.01,7108449.99,616696.89,7108475.04,616681.45,7108462.52,85.74,90.79
7,616665.14,7108471.25,616687.86,7108494.26,616676.50,7108482.76,86.81,90.35
8,616673.88,7108488.44,616691.35,7108505.91,616682.61,7108497.18,86.72,89.35
9,616695.43,7108602.90,616724.26,7108632.90,616709.85,7108617.90,81.18,85.27
10,617066.09,7108807.01,617080.97,7108819.01,617073.53,7108813.01,87.44,89.59
11,616010.62,7108764.98,616025.46,7108780.39,616018.04,7108772.68,88.3,90.78
12,621229.46,7111180.27,621246.66,7111197.69,621238.06,7111188.98,111.3,112.86
13,621196.44,7111192.72,621216.57,7111208.55,621206.50,7111200.63,110.18,112.08
14,621183.77,7111206.97,621206.39,7111226.65,621195.08,7111216.81,109.69,111.89

The resulting CSV file can be imported to QGIS with the centroid x/y as point location. In QGIS I can then generate a spatial join between the CSV file and the shapefile containing the surveyed mounds/cairns. Then using the field calculator on the table, I can compute the height as a difference of max and min elevation for the each mound/cairn. About 2600 of the 2900 monuments could get their height measured using lascanopy.

The results are shown in the three plots. These have been generated in R and ggplot2. The figure below shows the diameter plotted against the height gathered by lascanopy.

Diameter plotted against height

Diameter plotted against height

Height and diameter correspond to a large degree. One marked difference between mounds and cairns is that some of the larger mounds are lower than their expected height. This can have two explanations; one is that mounds are more likely to be affected by cultivation activities (i.e. they were plowed over by farmers) that have reduced their height. Mounds are more likely to be situated close to farms, while cairns are more likely to be sited along the coast or on hills.

histogram of diameter of monuments

histogram of diameter of monuments

The above histogram of the diameters of the monuments shows a skewing of the data towards the left. Mounds above 20 m of diameter are considered to be large, while those above 40 m are called “kongshauger” or “Kings mounds”. There are 19 such in North-Trøndelag. A normal – Gaussian – curve is fitted over the histogram.

histogram of heights of monuments

histogram of heights of monuments

Finally, the above histogram of the heights – as measured by lascanopy – for aproximately 2600 monuments. This shows that the maximum height lies at about 12.5 meters. The histogram of heights is again skewed to the left. The large mounds mostly seems to be above 20 m of diameter and above 4 m of height.

References:
Gustavsen, L., Paasche, K. 1964-, & Risbøl, O. 1963. 2013. Arkeologiske undersøkelser: vurdering av nyere avanserte arkeologiske registreringsmetoder. Oslo: Statens vegvesen.
Risbøl, O. 1963-. 2011. Flybåren laserskanning til bruk i forskning og til forvaltning av kulturminner og kulturmiljøer: dokumentasjon og overvåking av kulturminner. Oslo: NIKU.

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