topic Re: Need Help with analysis in ArcGIS Spatial Analyst Questions

Need Help with analysis

GregSoviak — Fri, 18 May 2012 17:33:58 GMT

Ok, I am sure this has been asked but i cant seem to find the answer. I need to create a "slope analysis" where i determine the slope percentage and classify by usability by a vehicle (1-5% flat to mild, unrestricted, 6-12% mildly restricted, etc) . I am using NED 1/3 data from USGS. I have run the "slope" tool a few times and used the "percent Rise" option and i get weird numbers like 1,219. I thought this option would give me the percentages so I assumed 100% would be the greatest number. I also used the degrees and i but i see that the slope is counted in degrees so percentage is out. And maybe degrees makes more sense for this problem but I am not sure. I am new with this type of analyis and have scoured the help functions and cannot seem to get an answer that helps. Just so you (but you can probably tell) i am an occasional user and a novice. Any help would be greatly appreciated. Thanks.

Re: Need Help with analysis

DanPatterson_Retired — Fri, 18 May 2012 17:45:20 GMT

Project your data and ensure that you X, Y and Z values are in the same units (ie meters or feet), then run the slope tool. You can't use decimal degree coordinates mixed in with elevations in other units.

Re: Need Help with analysis

EricRice — Fri, 18 May 2012 20:15:23 GMT

Greetings,

I'd like to add one more thing aside from ensuring the x,y,z are all the same unit. It is still technically possible to get slopes in excess of 100%, in particular if your data is in rugged terrain. After all, a slope of 100% is equivalent to a 45 degree slope. Knowing that degree slope ranges from 0 -90 (flat to vertical) we can easily infer slope can exceed 100%. Our own graphics in the help system show a slope of 373% as an example output. It may help if you review, How Slope Works.

The reason your numbers are so high right now is because your dividing the rise (in feet/meters) by the run (in decimal degrees). Essentially your denominator is a very small number compared to the rise value. We also multiply the quotient by 100 making that large value even larger.

Best Regards,
Eric