Merging Elevation Data for GeoRAS

[GregSullivan1]

Hello,
I am preparing a GeoRAS model and I have three separate elevation data of varying accuracy and extents. I have a .dtm from a field survey of the river's channel, a .dtm from an aerial for the roadway over the stream and the embankments and I have a 1/9th Arc Second DEM from USGS for the floodplain. I am having trouble merging the files since they overlap. The USGS DEM is a much larger area and the other 2 areas are within the DEM area. I want the areas where I have field survey to take priority and be used where available then I want the aerial data to be used then where neither of the 2 are available I want the USGS DEM to be used. Is there anyway to combine these three with having the more accuarate files be used in the overlapping areas?

[MattMead]

This uses the Spatial Analyst extension.

In the Spatial Analyst toolbar->options->set the working directory(or something like that, I'm not at my machine at the moment) to where you want the new DEM to be saved.

In Spatial Analyst->Raster Calculator type "newDEMname=merge(DEMsurvey,DEMfromAerial,USGSDEM)" without the quotes.  That will combine your DEM's into a single DEM of whatever you call "newDEMname" in the working directory you set.

The DEM will be built using the order going right to left(ie USGS will be base, then written over by Aerial, then written over by survey)

newDEM=Mosaic(same DEM order) will also work, but when i have overlapping data I prefer merge.

NOTE: If your cells are offset or of different sizes, the results might be a little funky

Also, if the USGS DEM has way more data than you need, you can clip it down using a polygon shapefile and Spatial Analyst->Extract by Mask if you want to make it a bit more manageable.