DEM statistics

[annelisebissett]

Hi,

I am fairly new to using ArcGIS (GIS software in general, actually!) and I am trying to generate info for some points (sample locations) using a DEM. I've tried to figure out how to do this using the manual but am a bit overwhelmed & unsure about the correct methods!

Using the DEM, I hope to determine the slope, upslope contributing area and topographic wetness index for each of my sample locations. At present, my sample points are a separate layer to my DEM.

Do I need to merge or overlay these points to the DEM before I can calculate slope, UCA and TWI? If so, how is this done?

I have not delineated the boundary of the catchment I am working in, and my DEM extends beyond the catchment boundary. I have the catchment boundary in a separate layer - is it possible to clip the DEM using this boundary layer?

Is Spatial Analyst the correct extension to carry out these processes?

I know this is a lot of questions, but if anyone can help me with just one of them, that would be great!

Thanks for your help,
Anna

[TimothyHales]

Hi Anna!

Yes, the Spatial Analyst extension tools are what you should be using.  You can either clip the DEM to the boundary or you can set the processing extent to the boundary in the environment settings of the tool.  You will need to first calculate the slope, upslope contributing area and topographic wetness index.  Then with these rasters use the Extract Values to Points tool to get the values of the underlying raster for each point.

I hope this helps.  Let me know if you have any questions.

-Timothy

[TimothyHales]

To add to my previous post...

You can used the Hydrology toolset within Spatial Analyst to calculate the upslope contributing area with probably the Flow Direction tool.  It all depends on what exactly you are looking for.  There are some other tools within the Hydrology toolset that may be helpful to your project.

The slope can be be calculated by using the Slope tool within the Surface toolset.

[annelisebissett]

Thank you for your help Timothy, it is good to know that I am on the right track.

To determine Upslope contributing area I have created a flow direction raster to use in the watershed tool. I tried creating a watershed raster, however I don't think I'm using the correct input for the Pour Point Data. The Arc manual I am using said pour points were the areas I wish to determine the contributing area for, so I input the shapefile of my sample points. Was this the correct step to take? I am unsure which four point field I should be using, as well.

When I followed these steps, Arc calculated tiny watersheds for each point (only a few cells) and I'm not sure this is very accurate.

You mentioned I should create the watershed raster and THEN extract values for my points. Should I be using something different for my pour points?

I hope this makes sense!
Anna

[TimothyHales]

Anna,

Your sample points and pour points are different.  The pour points are usually the high values from a flow accumulation raster.  In essence it is the lowest point along the boundary of a watershed.  Once you manually determine these points then you can use the Snap Pour Point tool to snap the points to the highest value in that particular are of the point.  Check out the following documents for more information:

How Watershed works
Understanding drainage systems

-Timothy