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I am currently working with a 15-ARCSEC DEM of Central Europe in QGIS and there are several areas that have an elevation of 0 m. This leads to the problem, that the software is unable to differentiate between land and sea and gives both the same colour. I have tried to cut the DEM by a mask layer, so that it's only on land, but unfortunately now the whole globe outside the original DEM is also calculated to be at an elevation of 0 m. this is especially frustrating, since areas with negative elevation can easily be coloured differently by assigning the respective value in the symbology.

Here you can see the inevitable problem, when working in an area like The Netherlands.

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    – Ian Turton
    Dec 18, 2021 at 11:03
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    It is a bit unclear to me what you want to do. To me, it makes sense that a DTM does not manage to distinguish between sea level and areas < [vertical resolution of DTM] above sea level If it is just a display issue, you can try to filter out everything at 0m and put your polygon land maps behind with whatever color you want for 0 meters. If the issue creates some problems for some analysis you want to do, you need to explain a bit more. Dec 18, 2021 at 11:44

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Get a polygon layer representing the sea, put this over the DEM and give it a color for the water body. Then assign a different color to the DEM for elevation = 0.

If you don't have a polygon for teh sea, use the land polygon you have and set its style to inverted polygons.

Here, I used the pre-installed worldmap in QGIS (type world in the coordinates field) for Inverted Polygon layer rendering style enter image description here

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    This is exactly how I handle water bodies in general (not just those at sea level). Dec 19, 2021 at 0:45

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