On 3/17/2020 2:24 PM, Danilo da Rosa
wrote:
Do you think it would be a good idea to do some kind of interpolation to smooth the DEM file or the slope file? Do you have any recommendations on how to do that using gdal? The idea is to use the gdaldem color-relief command to generate a coloured and easy to read map. The problem is that this lines makes the map more difficult to understand, which is a priority in this case.
as has already been pointed out, the root of your problem is that your DEM appears to have been created from contours with no interpolation, resulting in a series of terraces with steps between each level, giving the appearance that the terrain consists of flat areas bounded by 90° slopes. Interpolating the existing DEM would relieve this problem, but there is no reason to believe the interpolation represents the actual surface. You would be better off starting with a new DEM that accurately represents the region of interest. I used the SRTM downloader plug-in QGIS to download a new DEM. I saved this DEM in a projected coordinate system to get away from the problems of dealing with degrees (as issue pointed out by others). I then used the slope tool (which actually just calls gdal) to create this slope image:
The slope ranges from 0 (white) to about 30° (black).
SRTM data is 1 arc-second, or about 30 meters, which is coarser than you previous data, but at least it's the actual surface. There are higher resolution DEMs for much of the US on the US National Map. Maryland (where your patch appears to lie) is covered by 1/9 arc-second data and parts are covered by 1 meter DEMs derived (I think) from LIDAR.
You state your goal is a colored, easy to read map. I've seen articles about combining hillshade, elevation coloring, and slope, though I've more often seen just hillshade + elevation (the latter part being hypsometric tinting). I think you will find it easier to achieve this goal in Qgis, which will allow you to experiment interactively with the various parameters in tinting, hill-shading, and combining the layers. For most of these operations Qgis uses gdal behind the curtain, so you can even see the gdal calls if you want to replicate the results from the command line.
_______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org https://lists.osgeo.org/mailman/listinfo/gdal-dev
