Today, an update from the Mapping4OPS project...
LIDAR stands for 'light detection and ranging', and uses lasers from above to measure distances to reflective surfaces (and hence heights). "DTM" shows the ground of the earth and nothing else; no houses, buildings or vegetation, whereas "DSM" shows features such as buildings and trees.
I have implemented these within M4OPS, our prototype mapping system for One-Place Studies, for my own study (Holywell-cum-Needingworth) and others. The DTM layer helped to confirm for me (among other things) the location of a suspected track that I had noticed on old maps.
If you want to see this on-line go to mapping4ops.org/M4OPS, and in the dropdown named “Base Layer: Category” select Lidar (be aware it can be a little slow to show). By default this shows the DTM layer and is overlaid by Open Street Map. You can change how much of each you see (opacity) using the blue slider.
Lidar data and maps are available for other countries, and I would be happy to add any layers that we can get reasonable access to.
If you would like your study to be implemented within M4OPS and see maps for your OPS conveniently overlaid with each other, contact firstname.lastname@example.org.