What is going on ?

[dlejay]

Description

Page URL: https://docs.qgis.org/3.28/en/docs/training_manual/rasters/terrain_analysis.html

The tutorial says

What is going on?

but never tells the reader what is actually going on.

It took me time to realise that the “aspect” algorithm spits out “nodata” values of the type -9999 ; That the “raster calculator” spits out “nodata” values of the type -3.4e+38 ; And that “GDAL sieve” algorithm spits out “nodata” values of the type -2.1e+9.

Why are the first two correctly identified and displayed as “nodata” in QGIS but not the last one ? Why is one having “no data value not defined” in the “Transparency” tab of the layer ? What is going on here ?

BY THE WAY, the exercice is teaching theoretically wrong things. Since “nodata” arises in the “aspect” calculation when the slope is zero, these are thus pixels that satisfy “all conditions” and should be attributed a “1” in the respective layer, not a “0” as is currently done. What is really wrong here is the line of thoughts “oh, what is going on ?” → “quick and dirty cover up” instead of really understanding the source of “what is going on”.

[DelazJ]

@dlejay Thanks for walking through the manual and reporting issues you encounter. Much appreciated.

What is really wrong here is the line of thoughts “oh, what is going on ?” → “quick and dirty cover up” instead of really understanding the source of “what is going on”.

If you feel you know better or there are some mistakes/omissions in the docs, please do not hesitate to provide a direct fix. It mainly takes hitting the "Edit on Github" button in the page and follow the steps Github will show. If necessary, you could find some more details at https://docs.qgis.org/3.28/en/docs/documentation_guidelines/first_contribution.html

[dlejay]

I'd love to. Unfortunately, I am a real beginner at everything GIS (that's why I am following the official Training Manual) ; so I can catch things that don't add up, but I cannot recommend a replacement.

For example, in QGIS I don't know the reason why there is are two different ways of computing the aspect of a DEM : one is from GDAL (this I understand, is an external library) and the other one is the “QGIS one” ? currently presented in the Training Manual.

And I'd really want to understand why there is a problem with displaying the GDAL sieve step of the Training Manual, if someone had the answer.