[Qgis-user] Raster calculator questions

[Nicolas Cadieux]

Hi,

I am not sure of what you mean in the first part of the question but yes, you must create the output file to receive the output from the calculator.

As for the second part, QGIS currently has a big limitation with the raster calculator. I have been trying to get them to work on this for the past few years because I cannot use it for lidar data.  The problem is (as I understand it) that the rasters must all be loaded into memory in order to do the math.  Before, QGIS just crashed.  Not, they have that message.  I can complain all I want but I am not able to be part of the solution (beginner coding skills) so I gave them rasters to test so they can figure out the problem.

I can only imagine that your rasters, once loaded into memory, are probably close to the max memory of your computer.  If you change the CRS, you probably add a new layer into the memory.  My guess is that you are therefore running out of memory.  Therefore you get the error.

I am only speculating.  The obvious step is to ask the coders to rework the QGIS raster calculator but that is complicated.  Ultimately, the new calculator would need to   be able to chop the problems in little bits, multiprocess the math, possibly offload the math to the GPU and be able to work without loading the files into memory.  For now, I use the gdal calculator but that has more limitations (same CRS, same pixel posting and extent... ) than the QGIS calculator but it can work with huge raster.

Nicolas

> Le 3 mai 2018 à 21:43, Chris House <housec01 at gmail.com> a écrit :
> 
> When using raster calculator in QGIS 3 I  have to create a new raster first to serve as the output. Is the intent of this to overwrite a input raster (I haven't tried this) or something else? I'm confused by this. 
>  
> Secondly when I specify a CRS in raster calculator I get "insufficient memory for this operation" error.  Using the default output CRS works. This usually requires me to add a reprojection step on the output raster.
>  
> Windows 10
> Samsung SSD
> AMD 8 core processor
> 16 GB RAM
> NVIDIA Graphics Card 2GB RAM
>  
> Thanks
> --Chris
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