locating German bunkers concealed by canopy

After accidentally finding Russian tanks in Polish forests I was curious to see if there was something else hiding under the forest canopy. Remember, I randomly picked a 500 by 500 meter LiDAR tile as example data to introduce a group of forestry students to LiDAR processing with LAStools during the ForseenPOMERANIA camp. After extracting ground points with “lasground.exe“, strange bumps appeared in the bare-earth hillshades generated with “las2dem.exe” for terrain that was supposed to be completely flat … they turned out to be Russian WW-II positions.

I met Achim when returning to teach the next two groups of students. His hobby is a mix between geo-caching and conflict-archaeology: locating old German bunkers based on approximate coordinates available in historic records and tourist maps and then mapping them precisely with GPS. Achim had a list of longitude/latitude positions as KML files where he was planning to search for known bunkers. I used “lasboundary.exe” to create polygonal outlines in KML format for all areas where we had LiDAR from the forestry project. With Google Earth it was easy to find overlaps between his target areas and our LiDAR coverage.

I extracted the ground points and created bare-earth DTMs of the relevant area with a LAStools batch processing pipeline of “lastile.exe“, “lasground.exe” and “las2dem.exe” and used “blast2dem.exe” to create a seamless hillshading with proper KML referencing (here is a tutorial for such a pipeline). What I found was pretty amazing.

At first glance it looks like a maze of little creeks that are running alongside the ridges of the hillsides but we know that water flows downhill and not “along-hill”. What we see is a network of WW-II trenches that are connecting the bunkers Achim is looking for. A closer look also reveals the likely location of those bunkers.

I placed a pin on each of them and exported their longitude and latitude coordinates for upload into Achim’s GPS device. The next day we set out to verify our LiDAR findings on the ground.

It was a rainy day. Walking through this maze of green and overgrown trenches from one moss-covered bunker ruin to the next felt oddly quiet and peaceful. Achim explained that these bunkers were originally built to defend the border with Poland – long before the Second World War broke out. Only when Russian soldiers were advancing on Germany after the collapse of the Eastern front, the young boys of the Hitler Youth were commanded to dig this network of trenches in order to fortify the bunker and stop enemy lines from gaining ground. After the war the Russians blew up all bunkers that were facing East so that their troops would not ever have to face them again.

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