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Value Incongruity in DSM and DEM

Question asked by faithgreycloud on Jun 13, 2018

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!!