Simon Polster
Tom Allen@cartisan interesting... utility and readability over physical accuracy
@tomrallen 😁 actually I would argue the smooth contours more closely resemble reality: looking at the jagged "all over the place" contours of the original DEM in the flat areas one would assume a clefted rocky terrain while in reality there are flat smooth fields. My assumption is that in flat areas small elevation differences from buildings, ditches etc. create artefacts in the DSM which result in these chaotic contours. Or does anybody else have a better explanation?
@cartisan I suspect you might find they're very accurate cross-sections at very specific elevations. Zoom right in on flat smooth fields and they're lumpy as hell. I'm trying to think of an analogy but I can't right now 😅
@tomrallen I finally remembered what the actual issue with contours and flat terrain is (👋 Herr Friedmann from my Introductory GIS course 10 years ago). It's not artefacts in the DEM data, but rather the contour algorithm. In flat areas there are a lot of cells with similar values and the algorithm goes a little bonkers trying to connect them all. The result is overly jagged contours (and the solution is to generalize the DEM in flat areas, as I did, except that I forgot the reasoning for it 🫣)
@cartisan Nice. You should pass that on to UK Ordnance Survey 🤣
@tomrallen No need, it appears they also took that course! (the blue contours are from an unprocessed DEM, the greyed out area is my slope mask, the background is an OS Landranger map)