Validating the UK LiDAR inventory/SfM products

On September 1st the geomatics section of the UK Environment Agency released it’s LiDAR inventory for free (including commercial use). I thought I’d take the chance to compare it with an SfM survey which was carried out on a relatively flat field in Damerham, UK. It was the subject of a georeferenced point cloud I generated previously (viewable here), and I was wondering what kind of differences we would see (or would expect to see) vs. what will presumably be the new national benchmark in an area which shouldn’t change much topographically.

First, I generated a geotiff using a new function in CloudCompare for the Damerham data. I then needed to find the tile reference where the field was located and requested that data from the environment agency’s new portal. I loaded both of these into QGIS and generated a difference DEM based on these inputs, shown below.

Difference in raster grids (LiDAR vs SfM survey)

Difference in raster grids (LiDAR vs SfM survey)

Next we can do the reverse. First we load our Damerham cloud, which was made previously and georeferenced in Agisoft’s SfM package. We then convert the ascii grid to a LAS file using one of the many very handy tools found in Lastools toolbox, las2las can do this for us. Now with the two clouds ready we can use the cloud-to-cloud distance tool to measure the difference between the two.

c2cdist_DSM

Histograms for cloud-to-cloud distance of LiDAR vs SfM clouds

Interesting! There seems to be a pretty big offset between the two. I decided to filter out all points <25cm and all points >60cm as it was such a small amount of the cloud, and generated a new extract which is presented below.

C2C_Filtered_With_hist

Cloud-to-cloud distance between LiDAR data and SfM survey using GCPs

It’s a bigger difference then I was expecting to see, and would love to test a few more SfM surveys in areas of simple topography which don’t change often to see how they fair against what will become the national LiDAR.

I had one other dataset to hand today with which to try, a terrestrial LiDAR survey of a coastal cliff in Wales, featured in this paper. Here‘s an SfM cloud I produced from using the imagery from that paper. I loaded the relevant tile into QGIS, but was required to do a reprojection as the survey was done in UTM30N, a different coordinate system to the OSGB system of the LiDAR data. After performing the reprojection I continued in much the same way, though I won’t present the QGIS screengrabs as they leave something to be desired! On loading both clouds into cloud compare I was greeted with quite the difference, as shown below.

Reproj

Offset after reprojecting

This is the nature of reprojections and coordinate systems, I just did a simple shift in Z to get it to line up more or less to where it would sit to visually check the fit, it looked pretty good!

Consti_translate

SfM cloud draped on the LiDAR

The LiDAR data (This is 1m, not even the highest!) is actually really amazing, it’s accuracy rivaling the result of this survey done just 3 years ago. I’ll include 1 more screen capture of the coastal town, bonus points for whoever can tell me what the strange streaking effect across the cloud is!

LiDAR_Coast

Town beside constitution hill

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