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Classifying percent slope

September 16 2011 | 1 comment
Categories: Data Modeling

How should I understand values in a Percent Slope raster?

Spatial Analyst > Surface > Slope tool (percent) produces a percent slope raster. When attempting to classify the output display (3 classes, Manual), I have options to set Break Values (toggle between) as either a.) % of total pixels that range 0-100 or b.) slope pixel values that extend into the 10's of millions. Setting the BVs is no problem; understanding the values is. Neither appears to be in units of Percent Slope. Are values in a.) actually the percent slopes or percent of pixels in raster?

Mapping Center Answer:

How Slope Works describes in detail what the values represent.  The percent value is, effectively, calculated by rise (vertical distance between cells) divided by run (horizontal distance between cells), multiplied by 100. So a 45 degree slope (where the rise and run are the same) is 100% and as the slope gets steeper the percent values continue to rise, potentially to infinity.

Blog entry: Expressing Slope posted by Aileen Buckley on Sep 21 2011 8:48AM
I wrote a blog entry about this a while back - this might help, too. It si called "Expressing slope":

http://blogs.esri.com/Support/blogs/mappingcenter/archive/2008/06/12/expressing-slope.aspx

It was some of the research I did for the last Map Use book (6th edition) which you can get through Esri Press -- it is a great source for understanding a LOT of the concepts related to map use and consequently GIS use.

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