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Same Data, Different DEM's - Why?

Question asked by burthardin on Nov 12, 2017
Latest reply on Nov 12, 2017 by burthardin

I'm working with a lidar dataset (.las file). I created a DEM of the data by first extracting a portion of lidar file using Extract LAS, and selecting an extraction boundary (polygon shapefile), then using LAS Dataset to Raster. I then created a contour map from the DEM. 


I later needed to expand the AOI to include a portion of the original clipped lidar data and areas beyond. Since the lidar data that I had originally downloaded was in four large tiles, I had enough coverage to use the same - original - dataset. 


Going through the same process to create a new DEM as I had done with the original clipped lidar data, I produced a new DEM and contour map. I was able to verify that I had used the same geoprocessing by reviewing the Item Description of the original processed DEM. 


The problem is and the reason for this post, is that the overlapping portions of the DEM (and subsequent contour models) are not the same, i.e., the elevations are different. For the most part the differences are minor, but locally they are as much as 8 feet different. 


I could imagine that there could and likely would be differences in the two DEM's at the margins of where the two DEM's intersect, however many of the most significant differences are well within the boundary of the original DEM. 


I have run and rerun all of the original work described above (with the same anomalous result), and ran another test by processing the entire set of tiled .las files. This last DEM synced (matched elevations) with the second DEM, but was still off when compared to the original DEM I produced. 


My question is; Why are the two DEM's not showing the exact same elevations?