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I am having an issue in ArcGIS Pro with my undergraduate ecology / herpetology research project which requires me to create a Total Wetness Index (TWI) from elevation data (which comes from a digital elevation model or DEM).

My issue centers on working with large amounts of high quality DEM files in ArcGIS Pro. I’m trying to use USGS Elevation Products 3DEP (1/3 arc-second DEM) files that cover most of the eastern United States (see polygon image below). I plan on merging all of the raster data and clipping the merged DEM layer down around points on the map (which represent salamanders) into 15 square meter pieces and then calculating the TWI around each point.

My issue is that I need about 440 files from USGS to cover most of the eastern and southeastern United States for my project. These files all together take up an extremely large amount of space (It says that to download less than half of these files would take up 80 GB of space). Have a 2020 Macbook Pro (M1, 8GB Memory, running ArcGIS Pro on Parallel). Only uploading two of these files to my computer took quite some time to work with and I don’t know if it would be worth trying to import all of them and then clip them.

I am looking for either a method to allow me to merge these files and clip them without using as much processing power or a different fine grain DEM file(s) that covers the eastern and southern United States that is easier to work with.

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    You should be able to create a mosaic dataset. This way, you can keep them all as separate rasters, but they will be treated as one large single mosaic raster. Search for the 'Create Mosaic Dataset' tool and the 'Add Rasters to Mosaic Dataset' tool. (NB: Do NOT use the 'Mosaic' or 'Mosaic to New Raster' tool, which do merge them all into a single raster.) Commented Apr 26, 2024 at 2:12
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    Would it be possible to do this in pieces and keep adding files to the same mosaic data set? I would download maybe 50 files, create a mosaic dataset and then clip that mosaic down around my points. I would then repeat this by adding more files to the dataset and clipping again. Commented Apr 26, 2024 at 17:32
  • Yes, you can add new rasters to an existing mosaic dataset at any time using the 'Add Rasters to Mosaic Dataset' tool. I should write this up as an answer... will do so now... Commented Apr 28, 2024 at 21:38

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You could use a mosaic dataset, which leaves all of the component rasters as separate raster datasets, but they will be treated as one large single mosaic raster for the purpose of viewing and/or analysis

Create your mosaic dataset using the 'Create Mosaic Dataset' tool and then add rasters to it using the 'Add Rasters to Mosaic Dataset' tool

(NB: Do NOT use the 'Mosaic' or 'Mosaic to New Raster' tools, which do merge them all into a single raster.)

You can add more rasters to the dataset later on if required.

Note that you should re-generate the mosaic dataset's overviews when adding rasters (this is the mosaic version of pyramids - ie, a series of zoomed-out versions of the raster for quicker viewing). However, do not use the default method of interpolation, which seems to be quite buggy. Use the Cubic method (takes longer, but works better).

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