topic Re: Watershed Creation in ArcGIS Spatial Analyst Questions

Watershed Creation

Anonymous User — Mon, 04 Mar 2013 18:32:51 GMT

Original User: evierra

Hello,

I am working with the hydrology tools in the Spatial Analyst toolset. I have successfully created two sub-watershed maps. I am now attempting to create a new one and the watershed tool only creates small sections that do not include any of the upstream flow. I have used the Fill tool, Flow Direction, Flow Accumulation as well as the Snap Pour. i have also defined all of my environments to the flow direction raster (as suggested in a previous post). I still cannot get the tool to create accurate watersheds. Help! I have attached photos of the "good/successful" map and my current problem!

Re: Watershed Creation

MarkBoucher — Mon, 04 Mar 2013 20:02:39 GMT

Check the watershed dataset and look at the how many watersheds there are. See if you have some that are very very small, you may have put your points off the stream grid. They should be exactly on a stream grid. If they are not, they may delineate a "watershed" that is only a few cells in size. In fact, I think I can see a small "blob" near a point to the right of the green watershed in the image you posted and some others also in different places. I use Arc Hydro more than Spatial Analyst. Arc Hydro has a setting to "snap" the points to the stream grid to avoid this.

Re: Watershed Creation

Anonymous User — Wed, 06 Mar 2013 15:52:54 GMT

Original User: evierra

The Hydrology tools also have a "Snap to Pour" option where the pour points created will be snapped to the grid. I have completed this and I still get the little blobs. i have also attempted converting my stream shapefile to a raster and snapping to that instead. I will try to re-create my pour points and work with different tools. But if you can think of anything else that would be great!

Re: Watershed Creation

MarkBoucher — Wed, 06 Mar 2013 17:27:21 GMT

Make sure your snap_distance is large enough to make the pour point select a major stream in the stream grid.

snap_distance = Maximum distance, in map units, to search for a cell of higher accumulated flow.
From: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009z00000055000000.htm