I'm a new user of ArcGIS, and I hope that someone can help me with my query.
I'm trying to create a DSM for viewshed analysis in 10.4.1 which has 1940s above-ground features for an 8km x 10km area in the UK. In short, I want to be able to 'subtract' areas from a LiDAR DSM which weren't there 70 years ago (mostly modern urban development), and perhaps 'add' elevation data in some areas where woodland used to exist, but has been cleared in recent years.
I have 2m LiDAR DSM and DTM ascii data tiles for the area, that I have mosaic'd to create rasters. I understand that if I subtract the DTM from the DSM I am left with a layer that just has the above-ground features, and it seems to me that it is this new layer that I should be working on to create my historic DSM. However, I'm not sure how to go about it. I can obtain the differences in elevation data easily enough by overlaying a modern map with a historic one, but I'm scratching my head over the best way to reflect these elevation changes in my model. The landscape hasn't changed that much since 1940, so using a modern DSM as the base, and mapping the differences, seems to make more sense than starting from scratch with a bare earth raster and adding all of the many above-ground features (buildings, woodland, hedges, etc) that were there in 1940.
Is this a viable method? If so, I'd be very grateful if someone could briefly explain how best to go about it.
Many thanks in advance for all suggestions.