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How does clipping a DEM (raster) affect the data?

Question asked by mdiskin_usfs on Feb 15, 2019
Latest reply on Mar 8, 2019 by mdiskin_usfs

I am working on a viewshed analysis using the USGS National Elevation Data.  An area covering western NC and a small portion of eastern TN was originally extracted from Google Earth Engine.  This data was transformed to NAD83 NC State Plane.  I am using the Oberserver Points and Viewshed tools using a z factor of 3.281 (xy in feet, z in meters), which I believe is correct.  To speed analyses, I initially clipped the data further using the Clip Raster Tool (with "Maintain Clipping Extent" unchecked). 

 

For some of the points, the viewshed analyses with the clipped data are VERY different than using the original data.  Large viewshed areas areas are missing compared to analyses using the unclipped data.  That said, some of the viewsheds are exactly the same between the two DEMs.  It sees to happen to viewpoints that are mostly in one area...

 

It seems that there are patterns...areas to the east and south are missing compared to the original data...and there are some seemingly unnatural straight lines.  See two examples below (these patterns repeat for about 1/4 of my total viewpoints).  The viewshed using the original, unclipped DEM is in yellow, and the viewshed using the clipped DEM is blue.  There is yellow underneath all the blue.  What causes this?  Checking "Maintain Clipping Extent" mostly fixes the problem, although there are still some small discrepancies...

 

Mostly just curious if somebody can explain what is going on?  Thanks...

 

 

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