topic Re: Classifying slope raster in ArcGIS Pro Questions

Classifying slope raster

AlfredBaldenweck — Tue, 06 Dec 2022 17:35:55 GMT

Hi all,

I have a slope raster created from a DEM with the raster functions.

I set it up to give it to me in % slope.

The raster initially gives me a range in slope from 0 to 11%.

I want to make it so all slope above a given percentage is one color. I figured the best way to do that would be with the "Classify" symbology, which computes a histogram.

After the histogram is created, I now have a range of values from 0 to 4379.

What do these values represent, and how can I view this as the original percentage range again?

Re: Classifying slope raster

Robert_LeClair — Tue, 06 Dec 2022 17:52:08 GMT

Alfred - I'm working with a slope layer derived from the Slope Raster function. Following your workflow, I too see the first class with values almost up to 11%. I changed my symbology to Classify like you did and did have raster statistics calculated. Then if I wanted to change the Upper Values to be something different, I double click the value in the Upper Value column for the first color and type in say 15. Then for the second color, I type 25 and so on until I have all 5 values set precisely. Now to take it a step further, let's say that I want 3 symbols for my symbol values, I change the method back from Manual Interval and choose Natural Break (Jenks). I change the classes to 3 and then repeat the workflow mentioned above. Is this what you're attempting to do?

Re: Classifying slope raster

AlfredBaldenweck — Tue, 06 Dec 2022 17:54:15 GMT

Hi Robert, thanks for the reply.

Yes, that is my intended effect.

Changing the actual symbology is not the problem; I'm more wondering where a number like 4000 came from when there was a maximum slope of 11%.

Re: Classifying slope raster

Robert_LeClair — Tue, 06 Dec 2022 18:03:39 GMT

I wonder if it's because you're using % for the slope parameter?

From the help - "When the slope angle equals 45 degrees, the rise is equal to the run. Expressed as a percentage, the slope of this angle is 100 percent. As the slope approaches vertical (90 degrees), the percentage slope approaches infinity." So as you're getting closer to 90 degrees the value is say 4000 (not infinity, granted) but getting larger. I'll keep digging...

Re: Classifying slope raster

AlfredBaldenweck — Wed, 07 Dec 2022 14:03:43 GMT

It looks like you're right about where that number came from.

I ended up finding/making a calculator for future use to help me out.

I am confused why the histogram accounts for values up to 90 degrees when the initial input only goes to 6.3 degrees (11% ~~ 6.3 degrees).