topic Re: Slope analysis in ArcGIS Spatial Analyst Questions

Slope analysis

lorenzogratton — Sat, 03 Sep 2011 13:19:32 GMT

Hi everyone, here's my situation:
I'm analyzing Mt Cameroon and I need to get the average slope gradient(s) around the volcano edifice. I'm working in arcGIS 9.3 on a SRTM90m DEM of the area.

I don't want that the steep flanks of the volcano can interfere with my calculation of the average slope gradient of the surrounding area; so I was asked to find a way to literally "remove" the portion of the volcano edifice that we don't want to include in the analysis.
I delimited, with a polygon shapefile, the volcano edifice sensu strictu and now I need to find a way to "eliminate" this area from the DEM, in order to calculate the average slope gradient of the surrounding area. I was also asked to get the direction of the inclination of the volcano surrounding areas.
Needless to say that, at the moment I'm not able to accomplish any of these two goals...

so...I'd like to have your help, if anyone has a little time to spend for me!

Thanks a lot to everyone in advance!
lore

Re: Slope analysis

TimMinter1 — Sat, 03 Sep 2011 22:22:15 GMT

hi lore,

As with most ArcGIS things, there are usually several ways to get where you want to go. After a brief review of your description, it looks to me like you can use ArcToolbox tools to do the geoprocessing you need to do.

Use your shapefile that delineates the area for your analysis to clip the DEM. The output is just the data you need to analyze. So, if you have Spatial Analyst extension, check into the Extract by Mask tool. You may also be able to use the Data Management Tools > Raster > Raster Processing > Clip tool to do the same thing, but it looks like you'll have to convert your shapefile with the boundary to a raster (there's a tool for that under Conversion Tools).

Afterwards, to get your direction of declination ("aspect" in geo-parlance), you'll need to use the 3D Analyst extension and run the Aspect tool.

That should do it for you.

A friendly suggestion based on a few years of experience - go at these assignments with a mindset of "there's a way to do it, I just need to figure out what that way is." Personally, I drag up my "find a way" mindset and then dive head first into the help documentation. At each software upgrade, I take a few hours to review the "what's new" section, then poke around in the help again to see where stuff has been moved and generally just refresh my familiarity with it. Hit the Help.

Cheers and happy surface analysis!
tim

Re: Slope analysis

lorenzogratton — Mon, 05 Sep 2011 07:44:13 GMT

hi tim!
thanks for your quick reply! I wish I could write back that all is done and I've already finished my work but the reality isn't quite so sweet....

I tried to run the Extract by mask tool but it didn't work: <<ERROR 010267: Syntax error in parsing grid expression. Failed to execute (ExtractByMask).>>
I checked on the web and I saw sometimes the error is due to the name you choose for the output raster (but it's surely not my case, cause I follwed all the "rules") or by the environment settings > general settings > current and/or scratch workspace, but I actually don't understand what these environment settings are and how they work....

Anyway I tried the other way you wrote me and I used the Clip Management tool...
This tool worked, only I couldn't convert my boundaury shapefile to a raster 'cause (dunno if it's me that I have bad luck or it's the program) it kept giving me errors: Convert > Feature to Raster --> <<ERROR 010328: Syntax error at or near symbol SPACE>> and <<ERROR 010267: Syntax error in parsing grid expression.>>
In any case, running the tool with a feature Output Extent it seemed to work too and, as result, I got a clip of the dem I'm using inside my boundary.
Now, for what I know, I should use the Raster Calculator tool, but all my calculations didn't give me the result I wanted....I mean, logically, I tried to subtract to the entire dem the dem I clipped inside my boundaury to get the dem only OUTSIDE my boundary in order to work with the external part only, but all the results gave me a dem still inside my boundary and, plus, with no data in it.........so, I tried different calculations, different ways to do it and I tried to do it with different rasters (dem, slope) but I couldn't get anywhere....

Also, I was suggested to convert my slope to a raster, but first...isn't it a slope (format: grid) already a raster? I mean, I can also work on it with the Raster Calculator, so...it logically seems to be it...secondly, if it isn't, it's neither a feature cause I can't find it in the Feature to Raster tool....

....to sum up, I still haven't find the right way to get my dem or slope only outside my boundary....
any suggestion?
I thank you very much in advance again and I hope this doesn't make you loose a lot of time!

lore

Re: Slope analysis

lorenzogratton — Thu, 08 Sep 2011 08:04:48 GMT

problem solved!

Re: Slope analysis

EmilMonroy — Tue, 27 Sep 2011 10:02:06 GMT

Hi I also have a problem in Slope Analysis. I created a slope map from a DEM using the spatial analyst surface tool. What I need now is to produce a table showing the total land area of for the different slope category. How will I do it? What tools do I need to use?

Please help!!

Re: Slope analysis

EricRice — Tue, 27 Sep 2011 16:36:13 GMT

Greetings,

It sounds like you are interested in Zonal Histogram.

Eric