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I have a DEM that includes bathymetry and terrain elevation and I would like to split it at the coastline. Ideally, I would end up with one tif that's land and one tif that's bathymetry.

I have searched for this feature in GDAL commands and in QGIS but I can't find anything that extracts based on an elevation range.

  • 2
    do you have any python experience? this is a very simple task if you create two identical dimension rasters and copy the values to one if <= 0 and the other if >= 0. I can't see a tool for it straight away in GDAL, if you have access to ESRI you would be wanting extract by attributes (geoprocessing tool) – Michael Stimson Apr 12 '14 at 3:10
  • Thank you! I think I know enough python to figure this out. – jfact0ry Apr 12 '14 at 5:35
  • Awesome! It should be fairly straightforward but if you run into any difficulties there are plenty of 'GDAL in python' posts to help you through the bumpy bits. It's a bit of a shame that there's not an off-the-shelf solution for this, it seems like something that would be required occasionally. – Michael Stimson Apr 13 '14 at 4:59
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Basically, you can make two rasters on the same extent, one with height and the other with bathymetry, but rasters are always rectangle so you will have a lot of "zeros". If you only use gdal tools, you could use gdal_calc, for example:

gdal_calc.py -A dem.tif --outfile=bathymetry.tif --calc="(A<=0)*A"

gdal_calc.py -A dem.tif --outfile=elevation.tif --calc="(A>=0)*A"

A<=0 will evaluate as 0 or 1.

Remark 1: Be careful that land surfaces are sometimes below the sea level, so a simple threshold is sometimes not sufficient.

Remark 2: I also suggest you to add '-co COMPRESS=LZW -co TILED=YES' as tif creation options.

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