topic Watershed Aspect - Problems with Raster Calculator in ArcGIS Spatial Analyst Questions

Watershed Aspect - Problems with Raster Calculator

BrittanyTrushel — Fri, 25 Jun 2010 16:36:59 GMT

Hi All,

I am trying to A) determine the mean aspect for a watershed and B) determine the mean aspect for the stream network in that watershed. I have tried both determining the aspect from the DEM/NHD and then clipping the raster to the watershed/stream and I have tried clipping the DEM/NHD first and then determining the aspect. When I attempt to clip a raster and then use the raster calculator, ArcMap entirely shuts down and a "Send Error Report" box pops up. This happens after I attempt to recalculate the new raster. Any ideas why this maybe happening and a solution around this? Many thanks in advance.

Brittany

Re: Watershed Aspect - Problems with Raster Calculator

JeffreyEvans — Fri, 25 Jun 2010 18:38:14 GMT

The watershed mean can be calculated using a zonal function.
1) Calculate aspect for your dem ASPECT([dem])
2) If you have a raster for your watersheds use ZONALMEAN([watersehds], [aspect])
3) If you have a polygon coverage use the Zonal Statistics menu option in Spatial Analyst. This will produce a dbf table so make sure to select the option "join output table to zone layer".

You should take note that aspect is a circular measure so the mean value is fairly useless. I would recommend using a transformation.

Re: Watershed Aspect - Problems with Raster Calculator

BrittanyTrushel — Mon, 28 Jun 2010 16:06:05 GMT

Thanks William and Jeffrey. I was aware of the conversion due to the value being in degrees. I am going to calculate the N-S aspect (1-cos(asp))/2 and E-W aspect (1-sin(asp))/2, as well as the Heat Load of the watershed and stream segment within the watershed. I was just having trouble getting to that point! Many many thanks for the tip, it worked like a charm! Still not sure why the raster clip did not work for this scenario.

Brittany