Merging two rasters with different extents, taking the minimum value

Question

I have two raster data sets:

• a DTM that covers my desired extent
• river bathymetry data that has the correct river depth data, but only an extent that covers the river itself, everything else is no data

I want to create an output raster that merges these two, using the bathymetry data if it is available, and the DTM data if it is not.

On the assumption that bathymetry data will have a lower value than the DTM data for a given cell, I have tried:

1. The "Mosaic to new Raster" tool with a mosaic operation of MINIMUM
2. The "Cell Statistics" tool, also using MINIMUM

Both of these result in an output raster that almost looks correct, but the extent does not match that of the DTM and the minimum and maximum values for the output raster do not match my expectations as the lowest value in either the DTM or the bathymetry data is -20, and the output raster goes to -50.

Am I doing something majorly wrong, or is there a better way to do this?

Answer 1

I think Cell Statistics is the right approach as you tested but I have no idea why you have drastically different values than your expectations. You may try Resample your data into one of your datasets and rerun the cell statistics. Also, have a look at this.

The extent issue is related to the Environment settings. This link gives information about what they are and in there, you need to set Processing Extent> Extent item to "Union of Inputs".

Answer 2

How to merge raster datasets,

https://www.youtube.com/watch?v=JWeYsAFPfxU

To change the projection of raster

https://www.youtube.com/watch?v=fn0dCTlswjU

Answer 3

Interestingly, using GDAL Merge within QGIS did just want you wanted.

I had a larger DEM that captured the lake surface and a smaller DEM of the lake bathymetry. I don't understand why, but when I merged both, instead of combining the two as an average, the merge seemed to favor the bathymetry where present over the original DEM surface.