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, <[email protected]> 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 [email protected] > > --------------------------------------------------------------------------------------------------------- > 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 <[email protected]>* > Sent by: [email protected] > > 12/15/2011 01:17 PM > To > "Andreas H." <[email protected]> > cc > [email protected] > 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. <[email protected]> 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. <[email protected]> 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 > >>> [email protected] > >>> http://lists.osgeo.org/mailman/listinfo/gdal-dev > >> > > > > > > _______________________________________________ > > gdal-dev mailing list > > [email protected] > > <http://lists.osgeo.org/mailman/listinfo/gdal-dev> > http://lists.osgeo.org/mailman/listinfo/gdal-dev > > > > -- > Timothy H. Keitt > <http://lists.osgeo.org/mailman/listinfo/gdal-dev> > http://www.keittlab.org/ > _______________________________________________ > gdal-dev mailing list > [email protected] > <http://www.keittlab.org/> > http://lists.osgeo.org/mailman/listinfo/gdal-dev > > -- Timothy H. Keitt http://www.keittlab.org/
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