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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).

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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.

  • Thanks for the quick response! One quick question: I can't seem to find the projected vs geographic coordinate system. In properties, the only thing I have is "Spatial Reference: NAD 1983 UTM Zone 19N." If I scroll down, I see "Datum: D North American 1983." Projected is UTM, but I'm still unsure what the GCS should be. Thanks! – KKL234 Dec 30 '14 at 16:51
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    Unfortunately, changing a projected raster into geographic units profoundly changes the character of the data. While mosaic datasets can project on the fly, it is an expensive operation, which shouldn't be performed regularly. Choosing a different projection, like an State Plane or Albers Equal-Area covering the study area, or using the UTM zone of the principal study area (as long as the other data is not too far out of zone) would probably be a better plan. – Vince Dec 30 '14 at 17:00
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    @Vince Unfortunately my study area is relatively large; I have sample plots in both UTM 18N and 19N. There's not a specific study area I can focus on. I suppose I could run my calculations on 2 separate mosaics; one for 18N and one for 19N. But then I run into the issue of a seam between the two zones, and not being able to process everything at one time. – KKL234 Dec 30 '14 at 17:08
  • @KKL234 the GeoCRS/GCS is NAD 1983 aka North American Datum of 1983. – mkennedy Dec 30 '14 at 17:12
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    Thanks for all the comments. @mattmar and Vince: If I make the mosaic Albers Equal-Area, which order should I work in? Option A: (1) Project (using Data Management Tools, Projections and Transformations, Feature, Project) all DEMs to Albers, (2) mosaic (using Data Management, Raster, Raster Dataset, Mosaic to New Raster) newly projected DEMs. Or Option B: (1) Use Mosaic to New Raster with the original DEMs, and simply define the coordinate system as Albers? – KKL234 Dec 30 '14 at 17:26

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