[postgis-users] Help with SQL query?

[Rasmus Aveskogh]

Though this is probably possible in PostGIS nowadays I would personally solve such a use case with GMT (http://gmt.soest.hawaii.edu/), for example with grdmath (http://gmt.soest.hawaii.edu/doc/5.2.1/grdmath.html) which lets you perform rather complex computations on multiple grids (for examples rasters) using reverse polish notation.

-ra

On 23 Nov 2015, at 13:20, Darrel Maddy <darrel.maddy at newcastle.ac.uk> wrote:

> Dear all,
>  
> As you know I am relatively new to postgis and SQL and therefore  I have much to learn. However, I am facing a paper deadline and need to do some quick analysis of the data I have and I am struggling to figure out how best to pursue what I need to do.
>  
> I have a significant number of rasters which have double precision values.  Without going into detail about what the rasters represent, I need to extract and sum values from one set of rasters in say table A based upon  values in another set of rasters in say table B  where the pixel value in the raster from Table B exceeds a threshold. Both tables are the same size (rasters are tiled) but I also need to figure out how I make sure the correct rasters are compared.  They have filenames like this rastervariable_10.tif, rastervariable_100.tif , presumably I need to use a logical expression to strip the numerical value (in this case this represents the year) and then order on that basis?
>  
> I can do this in QGIS one at a time but that is a little clumsy and rather time consuming.
>  
> If someone can just point me in the right direction I am sure I can figure out the rest for myself.
>  
> Apologies once more for asking what is probably a rather trivial question and yet again demonstrating my ignorance.
>  
> Many thanks
>  
> Darrel
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