topic Re: Display A Slope Raster in ArcGIS Spatial Analyst Questions

Display A Slope Raster

JoeBorgione — Fri, 13 Mar 2020 21:44:19 GMT

ArcGIS Pro 2.5

This is new territory for me; I haven't created surfaces since ArcInfo 6.x with TINs....

I have a number of tif rasters that I am attempting to create a slope analysis from. I think I've created what I want, but I don't understand how to display it in a map. Basically I want to show slopes less than 25% in green, 25%-35% in yellow and anything greater than 35% in red. See screen capture below.

I took a look at https://community.esri.com/message/895485-re-slope-percent-symbology?commentID=895485#comment-895485 but it applies to ArcMap, and I don't see a clear correlation to ArcGIS Pro.

The original tif raster has elevation in meters, hence the range of 1681-2050. Using the Spatial Analyst tool box, I just used the Slope geoprocessing tool; entered my input raster, output raster, setting my output measurement to Percent Rise. So far so good.

Choosing Classify as the 'Primary symbology' scheme, I can only set the number of breaks; what are the units it is displaying? How do I display my slope raster described above?

Re: Display A Slope Raster

DanPatterson_Retired — Fri, 13 Mar 2020 22:32:24 GMT

Slope in either degrees or percent... 45 deg = 100%

Show names is the key... there others as well

Re: Display A Slope Raster

ShaneMiles — Fri, 13 Mar 2020 22:48:39 GMT

Hi Joe Borgione‌,

You are correct in converting your slope raster into % rise. Once you have set the amount of breaks you want, change the method to manual interval. Then specify the upper values of each by clicking on the values and changing them for each class like the image below.

This then lets you change the colour for individual classes. You can also go into the color scheme and format it using more complex options. As seen below.

Good luck! I hope this helped answer your question.

Shane

Re: Display A Slope Raster

JoeBorgione — Mon, 16 Mar 2020 17:22:19 GMT

Thanks for the replies, but I'm still missing something:

Dan Patterson‌ I try your suggestion but I'm still faced with the values I don't understand:

Shane Miles‌: I pick 3 classes and then switch to Manual Interval but the Field pulldown is still grayed out. As you can see I change the the two values as you describe, but then there is that 1884 value once again that I have now idea what it is telling me.

I don't know if I have create the slope raster incorrectly or not; it seems like a fairly straight forward process. I'll get on a screen share with tech support and see what I'm doing wrong.

Here is a screen shot of my slope raster showing two parallel tow land limited access highways. My assumption is the slope if these items would be minimal, however as indicated by the popup, the value is 581 and some change (what ever that means).

Thanks-

Re: Display A Slope Raster

DanPatterson_Retired — Mon, 16 Mar 2020 17:34:52 GMT

1884 is the slope in percent rise. Reaaaalllly steep. But I am guessing you understand that.

Doesn't take much using a D8 configuration to get that value, you are moving close to infinity percentwise.

Have you narrowed down where you are getting those slope values? Along the ridges probably.

Why not calculate slope in degrees to have a look. 90 degrees would be a cliff an 80 degree slope gives 567 is percent.

Doing a manual classification is the best to use if you are using this for planning/zoning purposes (ie a driveway can't exceed 13% for a 10 meter run)

Re: Display A Slope Raster

JoeBorgione — Mon, 16 Mar 2020 17:52:56 GMT

I get that... sort of. When I look at those two two-lane roads, a value of 581 translates to a slope of 80 degrees. That just doesn't make a whole lotta sense to me....

Re: Display A Slope Raster

DanPatterson_Retired — Mon, 16 Mar 2020 18:04:06 GMT

It does to me... steep ditches with DEM's derived from hires sources. The ditches entering our village are near vertical.

Also 'phantom' dem values, like elevations from telephone poles. Don't assume that the dem is 'clean', especially in the era of lidar... all those cool rectangular boulders at the edge of roads

Re: Display A Slope Raster

curtvprice — Wed, 18 Mar 2020 18:19:04 GMT

Joe, to answer your question directly about percent units, the units of slope are rise over run. If the DEM is in geographic, your slope values are invalid and really really high because the x is in degrees (makes no sense) and the y is in meters or feet. The way to fix this is to project the raster to a projected coordinate system like UTM, so units in x and y are the same. May not be your issue, but I thought I'd add that to the thread.

Re: Display A Slope Raster

JoeBorgione — Wed, 18 Mar 2020 18:29:01 GMT

Been doing a bit more work with this; I now understand that percent rise can go to infinity (and beyond!). Also, it turns out that those features I thought were roads are in fact overhead power lines with towers along the way; the steepness values make sense there! As I pan around and zoom in I am seeing all sorts of anomalies as you mentioned; building footprints, trees, etc....

Re: Display A Slope Raster

JoeBorgione — Wed, 18 Mar 2020 18:30:53 GMT

Thanks Curtis; I don't have the data open at the moment, but it should be all utm and meters. The elevation values make sense in that respect.

Re: Display A Slope Raster

DanPatterson_Retired — Wed, 18 Mar 2020 18:33:22 GMT

so the source isn't 'bare earth', contact the source for a better source of data, you can't 'clean' it up without filling/removing real things like ditches

Re: Display A Slope Raster

JoeBorgione — Wed, 18 Mar 2020 18:35:27 GMT

That's another thing I need to figure out. It's great being a 63 yo rookie...