topic Re: Calculating correct slope values after projecting raster issue? in ArcGIS Spatial Analyst Questions

Calculating correct slope values after projecting raster issue?

RachelPinkston — Mon, 16 Jan 2012 22:27:37 GMT

Hi,

I have been struggling to get spatial analyst to return accurate percent slope values off a set of 1/3 second NED's downloaded off the USGS. The NED's were downloaded in GRID format. I brought them in to ArcMap version 10 and projected them into a UTM projection (to convert the units from DD to meters) using the project raster tool. Then I calculated slope percent (using bilinear interpolation), but for some reason the percent values it returns are really high (highest being 504%). Has anyone run into this issue? or does anyone have any suggestions for what I might be doing wrong? Thanks.

Rachel

Re: Calculating correct slope values after projecting raster issue?

DanPatterson_Retired — Tue, 17 Jan 2012 09:29:39 GMT

the z values also need to be in meters if the x and y values are. As a check, you can also try to calculate slope in degrees to ensure that the output is between 0 and 90 degrees

Re: Calculating correct slope values after projecting raster issue?

JeffreyEvans — Tue, 17 Jan 2012 14:52:26 GMT

The Zunits could be a problem but you should take note on how slope is actually calculated (http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/How_Slope_works/009z000000vz000000/). You will notice in ESRI's description that as the slope angle approaches vertical (90 degrees) the percent slope approaches infinity. Thus, it is not uncommon to get very large slope percents. If you want a bounded range use Dan Patterson's suggestion of calculating slope in degrees.

Re: Calculating correct slope values after projecting raster issue?

curtvprice — Tue, 17 Jan 2012 16:40:21 GMT

The Z units in NED grids are often stored in centimeters so that that full precision can be maintained while taking advantage of the run-length compression used in integer grids. You can either convert the grids to meters by dividing by 100 or use a z factor of .01 to compensate for the difference between meters and centimeters.

Re: Calculating correct slope values after projecting raster issue?

RachelPinkston — Thu, 19 Jan 2012 22:24:25 GMT

Thank you so much for the helpful posts, all the x,y, and z values were in the same units (after being projected). I was just overlooking the fact that it is possible to have really high slope percentages, especially in mountainous terrain. Thank you for all the suggestions and help!

Rachel