High-resolution vector shaded relief

As pointed out elsewhere, for the vector JuraMap, in order to extend 3d-relief and hillshading beyond the Swiss border it is necessary to merge the Swisstopo swissALTI3D (0.5m resolution) Digital Elevation Model (DEM) with the 1m resolution RGE ALTI DEM for France to obtain a Terrain-RGB tileset and a hillshade geotiff file with a resolution of 0.5m.

Terrain-RGB raster tilesets were originally developed by Mapbox. They contain elevation data encoded in raster PNG tiles as colour values that can be decoded to raw heights in metres. These tilesets are used for a wide variety of applications, notably for styling terrain slope and hillshades so that terrain can be rendered in three-dimensions.

Terrain-RGB tilesets usually use the well-known PNG format that effectively involves lossless compression. Consequentially, compression or a special file format is not needed. An image is saved as PNG tiles for each zoom level where the three PNG channels allow 3 bytes of storage per pixel instead of the usual minimum of 4 or 8 bytes per pixel.

It was known as long ago as 2018 that hillshades (shaded reliefs) generated from Terrain-RGB tilesets using Mapbox-gl, the precursor to Maplibre-gl that is used to display the JuraMap vector map, displayed artifacts called "artificial visual lines" at zoom levels above about 15.5 even for a nominally high-resolution Digital Elevation Model (DEM). So generating high-resolution Terrain-RGB files is a challenge. It is made more so because the DEM for the region of France surrounding the south-western part of Switzerland currently has a resolution of 1m (a 0.5m LiDAR HD programme DEM is in preparation).

Using QGIS we have been able to aggregate the Swiss ALTI3D and the French RGE ALTI in such a way that the hillshade effect in Switzerland is preserved whilst the effect for the region in France adjacent to the Swiss-France border is considerably more precise and accurate than for the current Mapterhorn offering. It must be said however that Mapterhorn successfully removes edge artifacts which can arise when DEMs overlap. In our case an edge artifact arises (a 0.5m wide line on a hillshade) that it is not visible in a 3d-terrain display.

TO BE CONTINUED (25 Aug 2025)