Occassional thoughts about orienteering
Sunday, June 07, 2009
A Lidar contour comparisonI found this comparison of different basemap contours on the Chatahoochee Bend Mapping Project web page. The image shows three sources for contours: green are USGS, blue are USGS DEM, and purple are Lidar. The main lesson to take from this comparison is probably that USGS DEM are nearly worthless as base material for orienteering maps. Now, I knew that already. Some years ago I ran an A-meet on maps made with USGS DEMs and the contours were just terrible.
Back to okansas.blogspot.com. posted by Michael | 9:05 PM
A DEM is just a model in a particular file format. So if a USGS DEM (or STDS) is derived from accurate elevation data - such as from sufficiently high quality, dense LiDAR data - it will also be of high quality. The key is therefore to know what data source a given USGS DEM was derived from.
Beyond using DEMs, though, the LiDAR data can also be directly analyzed to determine features of interest to orienteers beyond just contours, as explained in the 2005 O/NA article "LiDAR Basemaps Come of Age". This paper (along with other information) is on the LiDAR Basemaps for Orienteering website.
Does that mean there are good USGS DEMs? By "good" I mean contours that would be useful as an O' basemap (better than the old photogrametry-based USGS contours). Is there some way to find the good USGS DEM?
USGS Level 4 DEMs are generally high quality, and often derived by LiDAR. (USGS DEM levels are described here.)
Unfortunately the "old fashioned" Level 1 DEMs are generally what you'll readily find. Unless it's registered in the National Elevation Dataset, it can take quite a while to find Level 4 data, if it exists at all.
Thanks. I learned something new.Post a Comment
I'm fortunate to be in a part of the country where lidar is readily available, though I still have a lot to learn about how to use the various data files.