Value Incongruity in DSM and DEM

FaithWebster — Wed, 13 Jun 2018 23:18:24 GMT

Hello, I posted about this last week, but realized i didn't provide enough information. I need to create a Canopy Height Model. I am running ArcGIS Desktop 10.5.1. I have a "DSM" that includes man-made structures. and a DEM. The structures have to be removed from the raster for it to be a usable DSM, so I digitized the structures, turned them into a raster using "Polygon to Raster" and used a Con statement in Raster Calculator. This made an output of raster with the shape of the digitized polygons, and the values of the DEM. I then mosaiced the DSM into the structures raster layer. I used the appropriate mosaic operator so that the structure raster values stayed put. So, hypothetically I would has a DSM with no man-made structures as the output. It looks right visually. I then used Raster Calculator to Subtract the DEM from the DSM (DSM - DEM) to get a CHM (relative height of trees). the output raster has a bunch of values from -23 to 0... both where the structures are and elsewhere. In theory, there should be nothing lower than 0. The DSM and the DEM are both 3 ft resolution from the same lidar flight. Does anyone have any ideas what may have gone wrong, or what I can do to fix it?

I symbolized the problem values in red. this image has 3 structures. and if it were just the structures, I would know I did the raster calculator wrong, or something like that. but because there are so many other errant values throughout, I'm perplexed.

Thank you!!

topic Value Incongruity in DSM and DEM in Elevation Data Questions

Value Incongruity in DSM and DEM