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DEM Accuracy   Julia Roberts Mar 16, 2005
Re: DEM Accuracy   William Huber Mar 21, 2005
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Subject DEM Accuracy 
Author Julia Roberts 
Date Mar 16, 2005 
Message I am wondering if there is a general rule of thumb regarding the vertical accuracy of DEMs, without a stated vertical accuracy. For instance, it is common to say that the vertical accuracy of a topo map is that of 1/2 the contour interval. Is there something similar to be said for DEMs? Also, knowing the DEMS are equally-spaced grids of elevation data, what contour interval relative to the DEM resolution, can best represent the accuracy of the DEM? Thanks. 
   
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Subject Re: DEM Accuracy 
Author William Huber 
Date Mar 21, 2005 
Message Julia,

USGS DEMs have published accuracy statements. Visit http://rockyweb.cr.usgs.gov/nmpstds/demstds.html for links to the National Map Accuracy Standards document and review the documentation in Part 2.

There is no general rule because, as the USGS states, the vertical accuracy depends on how the DEM was processed.

There is no general, well-founded rule for basing contour intervals on the DEM accuracy, because usually the contours serve multiple purposes. DEMs that have poor absolute vertical accuracy may very well have excellent relative accuracy among the contours and be able to support very detailed contours; indeed, they may need closely-spaced contours to reveal the information they contain. 
  --Bill Huber
Quantitative Decisions (http://www.quantdec.com )
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