[Qgis-developer] Re: [Qgis-user] Re: SEXTANTE - using calculator tools from SAGA and GRASS

Pedro Venâncio pedrongvenancio at yahoo.com
Thu May 3 09:03:50 PDT 2012

Hi Victor,

Thanks for your reply!

Yes, I think after the initial stabilization of the Sextante, it would be good to work in the documentation and, for example, include a help menu for the SAGA (and or other libraries) similar to the GRASS one, and the GRASS help might be adapted to the specifics of Sextante.

Regarding to the SAGA Grid Calculator, apparently it works the same way with a, b, c, ... and with g1, g2, g3, ..., from the top to the bottom of the Multiple Selection window.

However, it seems I found a strange pattern that looks like a bug.
If you make a simple calculation (a*2) when a single raster layer is selected (in the Multiple Selection window), the result appears to be resampled and the extent increases. 
However, if the same calculation is done, but selecting an even number of layers (2, 4, 6, ...), the result is shown well, with the original raster extent and without resampled values. When selecting an odd number of layers (1, 3, 5, ...), the extent increases by a half of the pixel size and pixel values appears to be resampled.

I created a short screencast that shows this behavior: http://goo.gl/Noflw 

Making the same operation directly in SAGA, there is no problem, as can be seen in this animated gif: http://goo.gl/LtRFc

Can anyone confirm? Should I fill a ticket?

Best regards,
Pedro Venâncio

----- Original Message -----
From: Victor Olaya 


SAGA calculator does not work like that

To include a raster layer in the formula you have to use  single
letters (a is the first one, "b" is the second one...), but not using
the names of the layers that you see in QGIS

I guess that we need more documentation for algorithms, or the user
should have a good knowledge of external tools like this...


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