[Qgis-developer] Problem with GRASS Region Extent and Region Cellsize on Processing

[Pedro Venâncio]

Hi Martin and Stefan,

I understand, if we do not use the -a flag, we can not ensure that the
resolution is the set.

What if there is another option, "align=", as Stefan suggested, which
allows to choose a raster from the canvas? This would ensure that the
region would be aligned with a selected raster?

Thank you very much!

Pedro




2016-03-12 10:44 GMT+00:00 Blumentrath, Stefan <Stefan.Blumentrath at nina.no>:

>
> ________________________________________
> Von: Qgis-developer [qgis-developer-bounces at lists.osgeo.org] im Auftrag
> von gene [martin.laloux at gmail.com]
> Gesendet: Samstag, 12. März 2016 10:46
> An: qgis-developer at lists.osgeo.org
> Betreff: Re: [Qgis-developer] Problem with GRASS Region Extent and Region
> Cellsize on Processing
>
> To keep the raster resolution.
>
> In the book "OPEN SOURCE GIS A GRASS GIS Approach Third Edition",   p.57
>
> "It may happen, that region boundaries lead to a modified, non-integer
> raster resolution. If this is not desired, the -a flag can be used to align
> the region to resolution. It adjusts all four boundaries to be even
> multiples of the resolution by slightly enlarging the current region. To
> see
> the effect, compare:
> g.region res=15 -dp
> g.region res=15 -adp"
>
> Look also:
>
> -  suggestion for g.region -a
> <
> http://osgeo-org.1560.x6.nabble.com/suggestion-for-g-region-a-td4014517.html
> >
> (Michael Barton)
>
> "Currently, if you use the -a flag with g.region it will align the region
> exactly to match the resolution of a map (e.g. g.region -a rast=myrast will
> set the region to exactly the resolution of myrast regardless of the
> extents) or the resolution specified in the res or ewres/nwres fields (e.g.
> g.region -a n=523 s=15 res=10 will adjust the extents a little to make sure
> the resolution is exactly 10)"
>
> - or in  proposal: make -a flag the default behaviour for g.region
> <https://trac.osgeo.org/grass/ticket/1745>  (Moritz Lennert)
>
> "In my experience, when working with raster, having a precise resolution is
> the most important issue, while exact extension is less important. I often
> use existing vector data or bounds coordinates to determine the extent, but
> then have to use the -a flag to make sure that the desired resolution is
> kept.
> I would like to propose that in grass7, the behaviour is switched, i.e. to
> make the -a flag (Align region to resolution) the default behaviour, and
> "align to bounds" the option"
>
>
>
> I guess it depends a bit what the most suitable solution is... There is
> also the "align" option which aligns the resolution to an input raster.
> Personally, i only use tha a-flag only in combination with the res option...
> Cheers,
> Stefan
>
>
>
>
>
> --
> View this message in context:
> http://osgeo-org.1560.x6.nabble.com/Problem-with-GRASS-Region-Extent-and-Region-Cellsize-on-Processing-tp5255693p5255960.html
> Sent from the Quantum GIS - Developer mailing list archive at Nabble.com.
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