topic Re: HELP!! I'm going downhill (very slowly apparently...) in ArcGIS Spatial Analyst Questions

HELP!! I'm going downhill (very slowly apparently...)

SarahBaughman — Tue, 14 Nov 2017 00:07:46 GMT

I need a solution as soon as possible, and any help would be appreciated. I am an archaeologist as well as a GIS Tech for a small engineering firm. I have been tasked with using the slope tool (in the spatial analyst toolbox) to find the slope in a project area to see what areas we can write off that have a slope of 15% or higher. I reviewed the process, found an elevation raster for the area, and went to work. However, whenever I go to use percent rise, it is giving me inaccurate slope percentages. It is saying the area has 5% or less of a slope, which I can say for a fact is not true. The area is along PA I-80, so it has a lot of cut banks and high sloped hills. HOWEVER, whenever I have to tool create a raster using degrees, it makes sense. Are there any suggestions as to what I may have been doing wrong with the percentage? Am I reading the numbers wrong? Not sure how many people on here have had a lot of experience with this tool and I am hoping it will save me a lot of hours at work wasted in trying to find my own fix.

Re: HELP!! I'm going downhill (very slowly apparently...)

DanPatterson_Retired — Tue, 14 Nov 2017 00:18:50 GMT

are the raster elevation units in meters? feet?

what about the x, y units? are they the same?

If you you have to apply an appropriate scaling factor.

On on a worst case... you have an unprojected dem, whose units are in decimal degrees and elevation in feet... In that case, Project to a suitable projection (not Define Projection) and redo the slope, ensure you have selected percent.

Slope How it works.....

Re: HELP!! I'm going downhill (very slowly apparently...)

SarahBaughman — Tue, 14 Nov 2017 00:33:16 GMT

I had to enter a z factor cause the elevation was in feet and my horizontal was in meters, so that I at least made sure was right. I never did think to double check the projection till I was looking thru here at other problems. The metadata said it was in NAD83 State Plane, so I just assumed that it was. It is going to be the first thing I try in the morning though. Thank you very much Dan for some suggestions! I really appreciate it.

Re: HELP!! I'm going downhill (very slowly apparently...)

DanPatterson_Retired — Tue, 14 Nov 2017 00:45:46 GMT

No problem Sarah