Saturday, 27 December 2014

Using QGIS profiler plugin to compare 2m resolution LiDAR vs SRTM

I have an idea to produce an elevation aware cycling route planner, and the first step in that is to have some elevation data in the form of a Digital Terrain Model.

The most commonly used freely available digital elevation data is that derived from the Shuttle Radar Topography Mission. At the latitude of Cornwall (about 50 degrees N) it has a resolution of about 73m x 73m per cell.

There is however a possibility of getting LiDAR data (free for non-commercial use) from the UK Environment Agency, which has various resolutions, and I obtained some 2m resolution data for an area around Truro.

For a cycling route planner, it would be nice to be able to keep track of the smaller bumps that would be smoothed out by the SRTM data. However it might not be practical to use the high resolution data because once one considers larger areas the volume of data becomes very large indeed.

There is also the Terrain 50m raster data available through the Ordnance Survey website.

I wished to see what difference it would make on a simple test circular route in Truro. I chose one which starts off climbing a hill, then goes along relatively flat terrain and then back down the hill again by a different route.

The QGIS screenshot below shows using the "Profile Tool" QGIS plugin, where the darker shade of green is the lower elevation terrain. You may need to click on the image to view it in a larger format.

The actual profile itself showing the elevation as a function of distance along the path is as follows:

SRTM = red, OS Terrain50 = black, 2m LiDAR = green
 I have had an issue with the QGIS Profile tool, in that I can't currently get it to work from a saved track, rather than 'live' by point and click each time, therefore the 2nd image above is a slightly different path.

Unfortunately the LiDAR has some wildly oscillating values, particularly around 400m from start where the road is in a cutting near the top of Chapel Hill. I have therefore done a moving average smoothing to make them more comparable.
Although the OS Terrain 50m is a higher resolution dataset, it appears to have more artifacts than the SRTM which is ~73m at this latitude

Measuring total ascent, counting the total height gain considering only positive values:
Not a big difference in total ascent between SRTM and Terrain 50. I also made an estimate by inspection of a 1:25k OS map of 55m ascent.

The total ascents were 52m, 55m, 60m, and 75m respectively for the smoothed LiDAR, SRTM, Terrain 50, and unsmoothed LiDAR respectively.

In the early part of the track, the SRTM (red line) appears to be seeing the treetops or roofs of buildings either side of Chapel Hill, but levels out at a lower peak than the full resolution LiDAR (yellow line). It appears than the LiDAR is overestimating due to proximity of artificial structures, or possibly even vehicles in a traffic jam on the A390. The smoothed LiDAR follows the SRTM fairly closely.


  1. Interesting. If the estimate by inspection of Chapel Hill's ascent was nearlybout correct, then the shuttle's data would appear to be more reliable than the OS's. On Chapel Hill anyway.

    1. (Oops. Apologies to the OS - as David points out, the LiDAR data came courtesy of the Environment Agency.)

  2. OK, you did mean the LiDAR then. Yes, I think it starts measuring the treetops and top of the cutting rather than the road surface accumulating extra bumps. There is actually a load of new LiDAR available from a survey in 2013:

    I think I should probably retest it in a more open area to avoid some of these issues.