Subsection "17.15 Clipping and merging raster layers" is outdated and incorrect

[pajot]

Description

The user manual subsection 17.15 Clipping and merging raster layers describes a process for clipping and merging raster tiles into a single layer.

It relies on the SAGA Processing algorithms. There are a few problems with this.

1. The examples use SAGA 2.1, which is by now ancient.
2. In QGIS 3.40 installed via the OSGeo4W network installer, the SAGA algorithms are not automatically included in the Processing toolbox, and only become visible if the Processing Saga NextGen Provider is installed.
3. In the screenshot, the algorithm is called Merge raster layers, but in the text description, it is called the "Mosaic raster layers algorithm". The SAGA algorithm is now called Mosaicking and is under "Raster tools".
4. If you select grids in the Grids field but leave the Input File List field empty, the operation fails with "Wrong or missing parameter: Input File List" even though this is supposed to be supported. I was unable to find an explanation for this behaviour in the documentation.

Here are four example DSM tiles:

Using the i.image.mosaic algorithm from GRASS "works", but delivers a result that is not usable (the scales are stacked and the tile boundaries are visible):

The operation required in the tutorial is a simple merge, which I accomplished with the GDAL Merge algorithm, giving this result:

I fear that correcting all this will require rewriting the whole subsection, which is a lot of work. But the distance between the doc and the reality is now so large that the doc is rapidly losing its usefulness.

Page URL: https://docs.qgis.org/3.40/en/docs/training_manual/processing/cutting_merging.html

[selmaVH1]

Hello @pajot, we are aware of the SAGA drop-off from the QGIS processing, but it can be difficult to find all the places where we need to remove SAGA, so thanks for reporting this issue.