[gdal-dev] re-gridding to a coarser grid

[Tim Keitt]

Sometimes I think GRASS can do anything...

In my case I was using a 1km equal area grid over North and South America
and needed to implement an interpolation scheme not pre-packaged anywhere I
had looked, so I worked with large tables of points. Not highly recommended
unless you have other things to do while you wait.

THK

On Thu, Dec 15, 2011 at 12:39 PM, <Doug_Newcomb at fws.gov> wrote:

>
> For what it's worth,
>
> What do you consider large and what kind of method for downscaling you
> wish to use?
>
>   I do this in GRASS using the r.neighbors command, http://grass.fbk.eu/grass64/manuals/html64_user/r.neighbors.html.
>   I've done an averaging method to convert a 20 ft elevation grid  (
> statewide coverage)  of approximately 7 billion cells to an averaged 60 ft
> grid ( 777 million cells)  by performing a 3x3 neighbor hood analysis  on
> the 20 ft grid, setting the GRASS workspace resolution to 60 ft and
> multiplying the 20ft grid created by the  3x3 neighborhood analysis by 1 to
> create a new grid (<http://grass.fbk.eu/grass64/manuals/html64_user/r.neighbors.html>http://grass.fbk.eu/grass64/manuals/html64_user/r.mapcalculator.html)
>   .  GRASS will take the value of the center averaged 20 ft cell in the
> 60ft block as the value of the cell in the new 60 ft raster.
> <http://grass.fbk.eu/grass64/manuals/html64_user/r.mapcalculator.html>
>
> options for r.neigbors calculations are :
>  average,median,mode,minimum,maximum,range,stddev,sum,variance,diversity,interspersion
>
>
>
> Doug
>
>
> Doug Newcomb
> USFWS
> Raleigh, NC
> 919-856-4520 ext. 14 doug_newcomb at fws.gov
>
> ---------------------------------------------------------------------------------------------------------
> The opinions I express are my own and are not representative of the
> official policy of the U.S.Fish and Wildlife Service or Dept. of the
> Interior.   Life is too short for undocumented, proprietary data formats.
>
>
>  *Tim Keitt <tkeitt at gmail.com>*
> Sent by: gdal-dev-bounces at lists.osgeo.org
>
> 12/15/2011 01:17 PM
>   To
> "Andreas H." <lists at hilboll.de>
> cc
> gdal-dev at lists.osgeo.org
> Subject
> Re: [gdal-dev] re-gridding to a coarser grid
>
>
>
>
> If you need complete control, and your data are not too massive in
> number, I have had good results pushing pixel coordinates as points
> into postgis and then using spatial queries to aggregate in various
> ways (eg averages a hexagonal grid). It seems round-about, but it
> works. I've actually done it on some pretty massive grids.
>
> THK
>
> On Wed, Dec 14, 2011 at 7:26 AM, Andreas H. <lists at hilboll.de> wrote:
> > Travis,
> >
> > thanks for your answer!
> >
> > Regarding the resampling methods: Do they all just interpolate the data?
> I
> > mean, when downsampling, usually I would use mean() or something similar
> > to fill the new (coarser) grid cells. Doas gdalwarp actually do this and
> > I'm not able to understand the documentation, or is it different?
> >
> > Thanks again!
> > Andreas.
> >
> >
> >> Andreas,
> >>
> >> gdalwarp can be used to resample images using the -tr flag or -ts flag.
> >>
> >> For example resample 1m image to 10m using cubic resampling and
> >> "target resolution'
> >>
> >> gdalwarp -r cubic -tr 10 10 input_1m.tif output_10m.tif
> >>
> >> You may have an issue determining the output resolution as I believe
> >> the resolution will be in decimal degrees rather than arc-seconds.
> >>
> >> Regards
> >>
> >> On 13 December 2011 14:52, Andreas H. <lists at hilboll.de> wrote:
> >>> Hi,
> >>>
> >>> let's say I have a GeoTIFF file with a global grid in a 30 arc-second
> >>> resolution. Which would be the appropriate GDAL command to spatially
> >>> down-sample this file to say 0.125°?
> >>>
> >>> Thanks for your insight,
> >>> Andreas.
> >>> _______________________________________________
> >>> gdal-dev mailing list
> >>> gdal-dev at lists.osgeo.org
> >>> http://lists.osgeo.org/mailman/listinfo/gdal-dev
> >>
> >
> >
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>
>
>
> --
> Timothy H. Keitt
>  <http://lists.osgeo.org/mailman/listinfo/gdal-dev>
> http://www.keittlab.org/
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>


-- 
Timothy H. Keitt
http://www.keittlab.org/
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