Slope: Spatial Analyst vs Image Analysis

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03-09-2015 11:44 AM
AlexandreLavoie
New Contributor III

Hi

I have a DEM (1 m resolution, Z in meter) derived from lidar data.

When I run Slope from ArcToolBox (Spatial Analyst, Surface) and i compare my result with the Slope function from Image Analysis tools, i don't get the same result slope image.

The second one is approximately 80% higher.

I've found the help for "How Slope works" in Spatial Analyst but nothing in Image Analysis.

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/How_Slope_works/009z000000vz000000/ 

Does anyone know why I do not get the same results?

Alexandre

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11 Replies
DanPatterson_Retired
MVP Emeritus

Why not get desperate...convert both to integer grids for a small area so that it can be examined for how slope is calculated.
I would put in a bug report...or the help file is wrong, assuming you tried the scaled with the default values or 1 since it calculated scaled values for you based upon your coordinates.  I am surprised it would calculate values since it only appears to be a visualization thing (or is it).  Maybe it isn't  In any event it is not using the method that is purported in the help files and documented in Burroughs book and elsewhere and no other method other than maximum drop would exaggerate slopes at the high end

AlexandreLavoie
New Contributor III

Hi Dan!

Thanks for your support!

Don't worry i'm not desperate...

I prepared you 2 images. 1 DEM in 8 bits and 1 in 32 bits

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