[Qgis-user] Raster calculator question

[kimaidou]

Thanks Alex for your help here. I keep this in mind for next time.

Michael


2014-04-29 5:41 GMT+02:00 Alex Mandel <tech_dev at wildintellect.com>:

> GDALInfo "Band NODATA value. " if known. In the case where it's not set
> in the header of the file then you have visually inspect the raster
> using the info tool to figure out what nodata is in use. Generally all
> rasters in the same set from a given source use the same nodata (for
> batching purposes).
>
> Then you can use GDAL_Translate to assign -a_nodata value:
>
> Thanks,
> Alex
>
> On 04/28/2014 10:27 AM, kimaidou wrote:
> > Hi Andreas
> >
> > About no data, I tried many things with no luck : NULL, none, 0, etc. If
> > someone knows, please help (and update documentation if possible).
> >
> > I would like to change the nodata value, but could not find any way. It
> > seems Grass can do it via the r.null alg, but it is not yet in
> Processing.
> > Some tools in QGIS, related to GDAL, seem to be able to replace a know
> > nodata value into another one. But it is often a chicken and egg problem,
> > since we do not always know the nodata value (or I am missing some other
> > tools like command line gdal to get it ?)
> >
> > Cheers
> > Michael
> >
> >
> > 2014-04-28 16:26 GMT+02:00 Andreas Neumann <a.neumann at carto.net>:
> >
> >> Hi,
> >>
> >> Again on the raster calculator: I noticed that nodata raster cells
> >> cannot be detected. They seem to be ignored. Is there any trick to query
> >> nodata cells? They are explicitly set to nodata, not to 0.
> >>
> >> Andreas
> >>
> >> Am 24.04.2014 12:57, schrieb Carlos Cerdán:
> >>> Hi Andreas:
> >>>
> >>> Yes, it always evaluates to true because of "OR" condition , so try:
> >>>
> >>> (lake_mask > 0) * lake_mask + (lake_mask<=0)*dtm
> >>>
> >>> Good luck
> >>>
> >>> Carlos
> >>>
> >>>
> >>>
> >>>
> >>> 2014-04-24 1:53 GMT-05:00 Andreas Neumann <a.neumann at carto.net>:
> >>>
> >>>> Hi,
> >>>>
> >>>> I have a hard time understanding the raster calculator syntax.
> >>>>
> >>>> I would like to merge two rasters: DTM and lake mask. Let's call the
> one
> >>>> dtm and the other lake_mask.
> >>>>
> >>>> When the lake_mask has a value > 0 then I would like to take the value
> >>>> of the lake_mask, otherwise the value of the dtm.
> >>>>
> >>>> What would an appropriate syntax look like?
> >>>>
> >>>> I tried something along
> >>>>
> >>>> ((lake_mask > 0) * lake_mask) OR dtm
> >>>>
> >>>> But all I get in the resulting grid are values of 1. I guess it is
> >>>> because it always evaluates to true?
> >>>>
> >>>> Thanks for your help.
> >>>>
> >>>> Andreas
>
>
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