topic Slope: Spatial Analyst vs Image Analysis in ArcGIS Spatial Analyst Questions

Slope: Spatial Analyst vs Image Analysis

AlexandreLavoie — Mon, 09 Mar 2015 18:44:37 GMT

I have a DEM (1 m resolution, Z in meter) derived from lidar data.

When I run Slope from ArcToolBox (Spatial Analyst, Surface) and i compare my result with the Slope function from Image Analysis tools, i don't get the same result slope image.

The second one is approximately 80% higher.

I've found the help for "How Slope works" in Spatial Analyst but nothing in Image Analysis.

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/How_Slope_works/009z000000vz000000/

Does anyone know why I do not get the same results?

Alexandre

Re: Slope: Spatial Analyst vs Image Analysis

DanPatterson_Retired — Mon, 09 Mar 2015 20:19:37 GMT

Perhaps this page in degrees, or scaled I suspect. To test use the method in the spatial analyst and choose the opposite of what you selected last time.

Re: Slope: Spatial Analyst vs Image Analysis

AlexandreLavoie — Mon, 09 Mar 2015 20:27:30 GMT

Both my tests are in degree.

Re: Slope: Spatial Analyst vs Image Analysis

DanPatterson_Retired — Mon, 09 Mar 2015 20:33:24 GMT

A quick test is to subtract the two rasters to see if there are real differences and if there are, then where they are. If the results are both in degrees. Slope can be determined in a variety of ways (ie maximum drop, 4 direction, 8 direction etc) and I don't think that they would use different methods without specifying the differences. So if there are real differences it is either due to a different representation of slope (ie degrees slope versus % slope ) of it is just a symbology issue. If the subtraction yields a grid of 0's, then it is a symbology issue solely

Re: Slope: Spatial Analyst vs Image Analysis

AlexandreLavoie — Mon, 09 Mar 2015 20:45:37 GMT

It's not a symbology issue, i used the import symbology. It's the same classification.

I Did subtract the two raster. There's big differences. Mostly where slope are higher.

Looks like ESRI is using a different formulas.

Thanks for you support!

Alex

Re: Slope: Spatial Analyst vs Image Analysis

DanPatterson_Retired — Mon, 09 Mar 2015 20:48:46 GMT

Ok, I just wanted to make sure it isn't a degrees slope issue versus a % slope issue ... ie 45° = 100% 22.5° = 50%... slope in degrees versus % slope

Re: Slope: Spatial Analyst vs Image Analysis

AlexandreLavoie — Tue, 10 Mar 2015 16:06:39 GMT

I understand your suspicious question...

All my process are with the "DEGREE" parameter.

Results are simply different.

Alex

Re: Slope: Spatial Analyst vs Image Analysis

AlexandreLavoie — Wed, 11 Mar 2015 13:29:10 GMT

Step by step:

1. Slope with Spatial Analyst

2. Slope with Image Analysis

Result gp00:

Result Func_ge00:

Same Symbology

Difference

I really think the formulas are not the same!

Alex

Re: Slope: Spatial Analyst vs Image Analysis

NeilAyres — Wed, 11 Mar 2015 13:52:00 GMT

In the image analysis window, what do those 2 extra variables do? Pixel Size power etc

Re: Slope: Spatial Analyst vs Image Analysis

AlexandreLavoie — Wed, 11 Mar 2015 14:05:01 GMT

The 2 parameters are not considered with the Slope option

They are valid only with the "SCALED" option.

I did change the value, and then came back to DEGREE option, there's no consequence.

Alex

Re: Slope: Spatial Analyst vs Image Analysis

DanPatterson_Retired — Wed, 11 Mar 2015 14:27:56 GMT

Why not get desperate...convert both to integer grids for a small area so that it can be examined for how slope is calculated.
I would put in a bug report...or the help file is wrong, assuming you tried the scaled with the default values or 1 since it calculated scaled values for you based upon your coordinates. I am surprised it would calculate values since it only appears to be a visualization thing (or is it). Maybe it isn't In any event it is not using the method that is purported in the help files and documented in Burroughs book and elsewhere and no other method other than maximum drop would exaggerate slopes at the high end

Re: Slope: Spatial Analyst vs Image Analysis

AlexandreLavoie — Wed, 11 Mar 2015 15:33:12 GMT

Hi Dan!

Thanks for your support!

Don't worry i'm not desperate...

I prepared you 2 images. 1 DEM in 8 bits and 1 in 32 bits