GDALDEM Hillshade – How to Fix Grid Artifacts in QGIS

czech-republicgdalgdaldemhillshadeqgis

I'm trying to create hillshades from Czech elevation data (Fundamental Base of Geographic Data of the Czech Republic (ZABAGED®) – altimetry – grid 10×10 m) – a demo files is available here: http://geoportal.cuzk.cz/UKAZKOVA_DATA/GRID10x10.zip

I combine some of their txt files using some bash scripts and then create a GeoTiff using gdal_grid. The resulting GeoTiff looks like this when imported in QGIS:

enter image description here

As a next step I'd like to create hillshades using the Raster->Analysis->DEM and the result looks like this:

enter image description here

I made sure to use the bilinear option when rewarping the raster and already tried basically all available algorithms of gdal_grid.

Not sure if this is relevant, but that's how the hill shade TIFF looks like when opend in OS X Preview:

enter image description here

What's the source of these artifacts and how to avoid them?

Best Answer

Use QGIS Vector to Raster to convert the shapefile points into raster, I will try to explain why:

Using the GDAL_Grid utility has interpolated incorrectly, that is where the stepping is coming from, you just don't see it in a black to white renderer. This is how I see the sample data interpolated using GDAL_Grid in Esri: enter image description here Note the Horizontal banding.

Using parameters:

gdal_grid -ot float32 -of GTIFF -zfield Z -l grid10x10 -outsize 517 422

enter image description here

The interpolation works better (517 by 422 was calculated from the extent divided by 10) producing the hillshade:

enter image description here

Note: the banding is better but can still be seen.

The banding is being introduced by the GDAL_Grid program! Using QGIS Raster::Conversion::Rasterize (Vector to Raster), fill in cell size so that it's not interpolated and then hillshade from Raster::Terrain Analysis::Hillshade: enter image description here

As the image is not interpolated there are points that aren't filled in and may need to be addressed with a focal mean. After you sort that out it should be fine!

-r bilinear

Did your hillshade worked if reprojected using

gdalwarp -of GTiff -s_srs EPSG:4326 -t_srs EPSG:3857 -r bilinear input.tif reproj.tif

? It didn't for me.

Resizing

After some tests :

data => hillshade : fine (good)
data => resizing => hillshade : stripped (bad)
data => resizing => reprojected => hillshade : stripped (bad)

Commands for 3. :

gdalwarp  -s_srs EPSG:4326 -t_srs EPSG:4326 -te -5.8 41 10 51.5 \
    -ts 1980 0  input.tmp.tif resized.tmp.tif # looks good but data is somehow corrupted
gdaldem hillshade resized.tmp.tif shadedrelief-resized.tmp.tif -s 111120 -z 5 -az 315 -alt 60 -compute_edges  # stripes visible

after some further processing we see a lightly stripped hillshade (larger version):

enter image description here

I'am resizing the input.tmp.tif gis data from 819*791px into 1980*1912px. My common resizing is to get the large GIS into smaller size. But if my memory is correct, both resizing up and down create strip artifacts. To avoid the strip I sorted my commands in a different way, from resizing => hillshade => reprojection (bad) into reprojection => hillshade => resizing (good).

The cost is more processing at native data quality, which is usually 10 to 100 times more massive than the resized file.

[GIS] Hillshade shows grid texture artifacts

The problem is because the DTM has a high-resolution pixel size when data (in my opinion) doesn't have the same resolution is some areas.

For example, direct hillshade raster:

Check pixel values (using Raster values to points over the hillshade ugly raster):

That's why look so ugly hillshade or slope outputs. You need to aggregate to obtain a better look output. I'm a R user for mostly all raster processing, so you can use a custom R script inside QGIS to work with R and raster package. Also, Aggregate function from SAGA toolbox only applies by sum, min or max, not mean.

In this case, aggregate by mean could be an excellent choice:

##Input=raster
##Factor=number 2
##Output=output raster
library(raster)
Output=aggregate(x=Input,fact = Factor,fun=mean)

Check documentation to know about parameters and function description. After this, use it in QGIS:

With output raster, compute hillshade:

And slope:

Best Answer

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[GIS] How to remove strange gdaldem hillshade artifacts

-r bilinear

Resizing

[GIS] Hillshade shows grid texture artifacts

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