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Using surface - slope analysis tool box. All values for the slope calculation are in between 80 and 89. There are no values below 80. What am I doing wrong?

Paul
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GCCNY
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    Welcome to GIS.SE! Could you add some more information to your question? We don't know what program you are using or what exactly you are trying to do. – Paul Oct 12 '13 at 19:11
  • I am sorry I did not write details. I am using ArcGIS 10.1. I downloaded all tiles from GTOPO30 and mosaic them into one global DEM raster. When I am trying to perform "slope" analysis I keep getting values between 80-89 nothing different. I have read it could be a "projection" problem but what would be the appropriate one? – GCCNY Oct 15 '13 at 18:58

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The problem is not really your projection, but your units. Specifically, you need to change the setting called "Z Factor". I found that 0.000009 worked for me. However, the correct value depends upon what units the vertical and horizontal elements are stored as.

An alternative is to reproject the data into a system (cough, UTM) that uses the same units for horizontal measurement and vertical cell values. Then you can leave Z factor at its default of 1.

Ref: http://www.geo.utexas.edu/courses/371c/labs/Software_Tips/GTOPO30_import.htm

Thomas
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  • The "alternative" is the only valid solution: using a Z factor on unprojected data produces incorrect slopes. For an analysis of this see http://gis.stackexchange.com/questions/14750/global-dem-to-slope-calculation/40464#40464. – whuber Dec 04 '13 at 15:52