topic Re: Flow direction values 1 - 255 in ArcGIS Spatial Analyst Questions

Flow direction values 1 - 255

toddsams — Fri, 01 Jun 2012 14:44:22 GMT

Flow direction returned a raster with values from 1 through 255. Most of the cells have the the standard values of 1, 2, 4, 8, 16 etc., however, there is at least one entry for every other number from 1 to 255.

Does anyone know what might be happening here?

Re: Flow direction values 1 - 255

DanPatterson_Retired — Fri, 01 Jun 2012 15:14:59 GMT

check the explanation in the help
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009z00000052000000.htm
if the flow direction is equal in 2 directions etc, then it will be given the sum of both

Re: Flow direction values 1 - 255

toddsams — Fri, 01 Jun 2012 15:18:07 GMT

Thanks Dan. Do you know this will affect the resultant flow accumulation?

Re: Flow direction values 1 - 255

DanPatterson_Retired — Sun, 03 Jun 2012 14:38:19 GMT

Todd
Check the second bullet in the help discussion on usage
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009z00000051000000.htm

Re: Flow direction values 1 - 255

boonejardot — Sun, 03 Jun 2012 21:55:06 GMT

Okay I havent read the other posts, But if your flow direction is ANYTHING OTHER THAN the default values 2 4 6 8 so forth, then you have SINKS. Sinks are a basically errors in your DEM. The way to fix this is running the Sink tool on the flow direction raster. This will correct invalid flow directions. Let me know. Thanks.

Re: Flow direction values 1 - 255

DanPatterson_Retired — Sun, 03 Jun 2012 23:03:30 GMT

Actually it is quite possible to have a flow directions that don't conform to the cardinal directions without them being sinks, but running a fill would be an obvious prelude to a flow direction and a flow accumulation.

Re: Flow direction values 1 - 255

boonejardot — Mon, 04 Jun 2012 01:14:01 GMT

Thanks Dan for that clarification. Yes Fill would be the proper tool. Sinks will identify if there are sinks, and yes there are real world sinks, but they do mess up flow direction in ArcMap. For you to know if its a true sink IE a lake, you would need real world knowledge, on the ground knowledge. Thanks again Dan.

Re: Flow direction values 1 - 255

toddsams — Tue, 05 Jun 2012 15:44:44 GMT

After fill sinks my flow direction raster had only the expected values of 1, 2, 4, 8 etc. Thanks for your suggestions!