arcgis desktop - mosaic dem with multiple coordinate systems

I'm struggling to mosaic multiple DEMs (~40) into one large raster. The DEMs are 30m NED rasters from the NRCS data gateway, for the New England region. They are in two different coordinate systems; NAD 1983 UTM Zone 18N and 19N.

I've read several posts debating whether or not DEMs should be projected or unprojected before running slope and hillshade tools. In the end, it sounds like running slope and hillshade on an unprojected mosaic of all DEMs would be most appropriate. That being said, I can't figure out how to create the mosaic dataset with DEMs from 2 different coordinate systems.

I have only a VERY basic understanding of ArcGIS (using ArcGIS 10.2).

Answer

The DEM tiles are already projected if they are in NAD 1983 UTM Zone 17/18N. From what I understand, a Mosaic Dataset IS able to handle multiple projections: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009t0000000n000000 just search this page for "different projections" and you will find a brief paragraph explaining this.

You will, however, have to specify a co-ordinate system for the Mosaic Dataset, which would probably end up being GCS North American 1983...just go to the properties of each raster, and where it tells you the 'Projected Co-Ordinate System', scroll down a bit to see the 'Geographic Co-Ordinate System'. The Geographic Co-Ordinate System should be the same across all of the rasters, so whatever that is that's the one you will want to use for the Mosaic Dataset. More about that here: http://blogs.esri.com/esri/arcgis/2013/01/16/how-does-a-mosaic-dataset-handle-rasters-in-different-projections/.

Then, in order to do your analysis, you will probably have to project the Mosaic Dataset a few different times depending on the layers you are creating due to the distortion that is associated with different projections. Read this resource: http://mappingcenter.esri.com/index.cfm?fa=ask.answers&q=435.

So, to reiterate:

• Your DEM tiles are already projected


• A mosaic dataset can handle the different projections...it is basically projecting each tile 'On the Fly' into the Geographic Coordinate System that you specify



• In order to work with the mosaic dataset, you will have to project it so that it also has a Projected Co-Ordinate System as well...remember this will have to be done a few times for different distortions/outputs

You will have to do some reading about how layers such as slope and hillshade are derived, as well as projections and distortion. The last link I included in this answer is a good starting point to understand this concept. From there, you should be able to find resources on Google.