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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">GRASS adjusts the resolution to the bounds (extent) you specified, either as n, s, e, w coordinates (like in Processing)
or using the “vector” or “raster” option. This can conflicting with information provided through the user specified resolution; meaning that either bounds or resolution has to be adjusted. By default priority is given to the bounds (and resolution is adjusted
if necessary) in GRASS, while setting the a-flag gives priority to the resolution (and bounds are adjusted).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">However, if one wants to align the cells with a specific raster map, the “align” option should be used where only the
resolution and placement of the cells (not the extent) is picked from the provided raster map. See: https://grass.osgeo.org/grass70/manuals/g.region.html<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Personally, I would say the “align” option (e.g. as “align to input”) is more suitable for modules like r.aspect (and
the resolution option could be removed) ...<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Specifying a resolution can make sense for modules like v.to.rast. However also here alignment with a specific raster
(e.g. a DEM might be even more desirable).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">There was another thing in Grass7Algorithm.py that surprised me: The use of r.out.gdal.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Using r.external.out befor running the algorithm, should be able to save you one read- and write step:
<a href="https://grass.osgeo.org/grass70/manuals/r.external.out.html">https://grass.osgeo.org/grass70/manuals/r.external.out.html</a>
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Was it not applicable in Processing?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Cheers<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Stefan
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Pedro Venâncio [mailto:pedrongvenancio@gmail.com]
<br>
<b>Sent:</b> 13. mars 2016 17:47<br>
<b>To:</b> Blumentrath, Stefan <Stefan.Blumentrath@nina.no><br>
<b>Cc:</b> gene <martin.laloux@gmail.com>; qgis-developer@lists.osgeo.org<br>
<b>Subject:</b> Re: [Qgis-developer] Problem with GRASS Region Extent and Region Cellsize on Processing<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Hi Martin and Stefan,<br>
<br>
I understand, if we do not use the -a flag, we can not ensure that the resolution is the set.<br>
<br>
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?<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Thank you very much!<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Pedro<br>
<br>
<br>
<o:p></o:p></p>
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<p class="MsoNormal">2016-03-12 10:44 GMT+00:00 Blumentrath, Stefan <<a href="mailto:Stefan.Blumentrath@nina.no" target="_blank">Stefan.Blumentrath@nina.no</a>>:<o:p></o:p></p>
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________________________________________<br>
Von: Qgis-developer [<a href="mailto:qgis-developer-bounces@lists.osgeo.org">qgis-developer-bounces@lists.osgeo.org</a>] im Auftrag von gene [<a href="mailto:martin.laloux@gmail.com">martin.laloux@gmail.com</a>]<br>
Gesendet: Samstag, 12. März 2016 10:46<br>
An: <a href="mailto:qgis-developer@lists.osgeo.org">qgis-developer@lists.osgeo.org</a><br>
Betreff: Re: [Qgis-developer] Problem with GRASS Region Extent and Region Cellsize on Processing<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
To keep the raster resolution.<br>
<br>
In the book "OPEN SOURCE GIS A GRASS GIS Approach Third Edition", p.57<br>
<br>
"It may happen, that region boundaries lead to a modified, non-integer<br>
raster resolution. If this is not desired, the -a flag can be used to align<br>
the region to resolution. It adjusts all four boundaries to be even<br>
multiples of the resolution by slightly enlarging the current region. To see<br>
the effect, compare:<br>
g.region res=15 -dp<br>
g.region res=15 -adp"<br>
<br>
Look also:<br>
<br>
- suggestion for g.region -a<br>
<<a href="http://osgeo-org.1560.x6.nabble.com/suggestion-for-g-region-a-td4014517.html" target="_blank">http://osgeo-org.1560.x6.nabble.com/suggestion-for-g-region-a-td4014517.html</a>><br>
(Michael Barton)<br>
<br>
"Currently, if you use the -a flag with g.region it will align the region<br>
exactly to match the resolution of a map (e.g. g.region -a rast=myrast will<br>
set the region to exactly the resolution of myrast regardless of the<br>
extents) or the resolution specified in the res or ewres/nwres fields (e.g.<br>
g.region -a n=523 s=15 res=10 will adjust the extents a little to make sure<br>
the resolution is exactly 10)"<br>
<br>
- or in proposal: make -a flag the default behaviour for g.region<br>
<<a href="https://trac.osgeo.org/grass/ticket/1745" target="_blank">https://trac.osgeo.org/grass/ticket/1745</a>> (Moritz Lennert)<br>
<br>
"In my experience, when working with raster, having a precise resolution is<br>
the most important issue, while exact extension is less important. I often<br>
use existing vector data or bounds coordinates to determine the extent, but<br>
then have to use the -a flag to make sure that the desired resolution is<br>
kept.<br>
I would like to propose that in grass7, the behaviour is switched, i.e. to<br>
make the -a flag (Align region to resolution) the default behaviour, and<br>
"align to bounds" the option"<br>
<br>
<br>
<o:p></o:p></p>
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<p class="MsoNormal">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...<br>
Cheers,<br>
Stefan<o:p></o:p></p>
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<p class="MsoNormal"><br>
<br>
<br>
<br>
<br>
--<br>
View this message in context: <a href="http://osgeo-org.1560.x6.nabble.com/Problem-with-GRASS-Region-Extent-and-Region-Cellsize-on-Processing-tp5255693p5255960.html" target="_blank">
http://osgeo-org.1560.x6.nabble.com/Problem-with-GRASS-Region-Extent-and-Region-Cellsize-on-Processing-tp5255693p5255960.html</a><br>
Sent from the Quantum GIS - Developer mailing list archive at Nabble.com.<br>
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