Slope: Spatial Analyst vs Image Analysis

[AlexandreLavoie]

Hi

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

[DanPatterson_Retired]

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.

[AlexandreLavoie]

Both my tests are in degree.

[DanPatterson_Retired]

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

[AlexandreLavoie]

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

[DanPatterson_Retired]

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

[AlexandreLavoie]

I understand your suspicious question...

All my process are with the "DEGREE" parameter.

Results are simply different.

Alex

[AlexandreLavoie]

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

[NeilAyres]

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

[AlexandreLavoie]

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