Making contours from elevation rasters

A friend asked me to make them some contour maps of Kullberg, given I clearly had all the data necessary, and given it was not something I'd tried before, it seemed like a fun thing to learn. In the end it was quite easy, but with two slight gotchyas I had to work around. The first thing you need is your elevation map, in this case a tile on the area of interest from Lantmäteriet 1m Markhöjdmodell: In theory all you need to do to generate a contour map from that is use a single GDAL command, gdal_contour: $ gdal_contour mhm-70_6_707_60_2500.tif contours.geojson -i 1 The -i parameter is a distance in map units between contours. In this map each pixel is a floating point value of height over sea level in metres, and so I'm going to generate a contour at 1m steps. This will give you this map: Success, right? Well, it's fine, but it has issues. To help discuss the first one, let's quickly look at the local land cover map, taken from Nationella Marktäckedata: In the land cover data you can…