[GIS] Hillshade shows grid texture artifacts

Question

I tried to make an hillshade and a slope map from a DTM using QGIS but the result shows some artifacts.

As you can see in the picture there is a grid texture in the hillshade and the slope map.

I downloaded the DTM files from:

http://opendata.regione.abruzzo.it/content/modello-digitale-del-terreno-risoluzione-10×10-metri.

The DTMs have the UTM-WGS84 coordinate system and a resolution of 10m. Every DTM is a GeoTIFF floating point 32 bits with a TFW associated.

I used the VRT builder to generate e virtual raster from the DTMs, then I used Warp to reproject the VRT into the same coordinate system of the original DTMs but using bilinear interpolation as "resampling method". I used the reprojected raster to make the hillshade but the grid texture was still there. Then I changed the "output raster type" to Int32 in the Warp tool, and I used the output to generate another hillshade but the texture was still clearly visible.

Do you have any idea why this is happening?

Answer 1

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: