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.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. 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 -- Timothy H. Keitt http://www.keittlab.org/ _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
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