topic Slope calculations way too high in ArcGIS Spatial Analyst Questions

Slope calculations way too high

NathanHautala — Tue, 31 Aug 2010 19:34:29 GMT

When I run the slope analysis tool, the slopes calculated wind up being way too high... (see pictures) Using "degrees" as the units, almost everything is near 90 degrees slope. Using "percent" as the unit, almost everything is greater than 100. (I am using 1 as the z factor)

Using "percent", while the highest overall slopes are still in the right locations, the absolute values are just way too high. My best guess is that the values got multiplied by 100 one time too many. (i.e. rise/run*100*100)

Using "degrees", in addition to being too high, the highest slopes are also in the wrong locations. (i.e. the steepest areas according to the analysis are the tops of lakes)

Has anyone run into this problem before?

Thanks!
-Nathan

Re: Slope calculations way too high

DanPatterson_Retired — Tue, 31 Aug 2010 20:06:04 GMT

searching the forums (previous and present) would indicate that the most common cause of this situation is that you have a file with decimal degree coordinates and elevations in either meters or feet. Projecting the file to a suitable projection (ie UTM, state plane etc etc) which has the same x, y, z coordinates will probably solve your problem. Search the help files for "Project Tool" to start

Re: Slope calculations way too high

Anonymous User — Tue, 31 Aug 2010 20:33:09 GMT

Original User: Dragontoad

You're right.. after projecting into UTM, the slopes came out normal. Thanks for your help!