LiDAR Data Explained
When LiDAR is used over a light-porous surface, such as a forest, some of the laser energy is reflected back from the canopy and some is reflected off the forest floor. Two sets of data are normally derived:
- A Digital Terrain Model (DTM) that accurately picks up the ground surface.
- Digital Surface Model (DSM) that shows the vegetation (in the case of a field or other open area these two surfaces are coincident).
Mapping packages, including up-to-date OCAD, can process the LiDAR in different ways, for example creating detailed contours or subtracting one layer from another to determine tree heights. Line features such as paths, ditches and streams show up clearly but typically have to be on-screen digitised rather than just pressing a button to create a base map.
LiDAR has been flown at various resolutions i.e. level of detail but only in the past couple of years has suitable LiDAR been available for Scotland. An initiative by SEPA, Scottish Water, the Scottish Government and Local Authorities has led to something like 17% of Scotland being “captured” by LiDAR. The percentage cover is nearly 100% in the Central Belt and river valleys and coastline prone to flooding are well covered. Following rather protracted discussions with the Scottish Government, and with major assistance from sportscotland, it has been agreed that the Scottish Orienteering Association can have access to all existing data free of charge. Usual conditions apply – not for commercial gain, not to pass on to third parties etc.
The complete data set will be held by the Professional Officer (a 2 TB disk purchased for this) and site specific data will be “extracted” using software called Global Mapper. LiDAR has already been successfully used for the new Faskally map, and the SOA paid for LiDAR to be flown for Highland 2015. The results are impressive and Stirling Surveys and DOLM (Jon Musgrave) have incorporated this data into new and updated mapping very effectively. Details of the coverage available to the SOA here.
As with any data collection caution must be exercised –I am not sure if sheep and/or deer have been picked up as boulders but experience in Glen Strathfarrar highlighted a couple of issues. In one area the birch saplings were too thin and not in leaf and what appears as a fairly substantial area of light green forest is nowhere to be seen on the LiDAR DSM. In another area the trees were too thick and though the DSM is well represented the DTM was incomplete as the laser energy failed to hit the forest floor – an indication here that this area is best avoided? There is still no substitute for good field mapping, but LiDAR is making a significant difference to the way we can map areas.
British Orienteering funded a pilot scheme in 2012 and details can be found here
There is a blog that covers LiDAR News if you wnat more.
Enjoy running on some superb new maps in 2015.