On Tue, Sep 6, 2016 at 10:22 PM, Markus Metz <[email protected]> wrote: > On Tue, Sep 6, 2016 at 8:12 PM, Helmut Kudrnovsky <[email protected]> wrote: >>> According to gdalinfo, the pixel sizes are >>> Pixel Size = (0.008333333300000,-0.008333333300000) >>> should be >>> Pixel Size = (0.008333333333333,-0.008333333333333) >>> i.e. fp precision is too low which explains why west is not exactly on >>> -180 but -180.0001389, just as south is not exactly -90 but >>> -90.0001389. >>> >>> There are 43207 columns, but with 30 seconds there should be 43200 >>> columns meaning that there are 7 excess columns to the east. >>> Considering that the 7 westernmost columns and the 7 easternmost >>> columns overlap, their values should be identical. >>> >>> There are 21599 rows, but with 30 seconds covering -90, 90 there >>> should be 21600 rows. The northernmost row is missing and north should >>> be 89:59:30. The data would need to be prepared with gdal_translate, >>> cutting off the 7 excess columns to the east and fixing the extents, >>> before they can be imported into GRASS. >> >> I had a conversation with the data provider. >> >> according to them, the climate rasters are based upon GMTED2010 [1] >> >> in the description of the GMTED2010 data [2] >> >> 30 arc-seconds GMTED2010 data: >> Resolution (decimal degrees) 0.0083333333 > gdalinfo says 0.008333333333333 which is exactly (within double > precision floating point accuracy) 30 arc seconds, while 0.0083333333 > is not (documentation is inaccurate, data are accurate). > >> West extent: minimum X-coordinate (longitude) -180.0001388888 >> South extent: minimum (for mean): -90.0001388888 (for others: >> -56.0001388888) >> East extent: maximum X-coordinate (longitude) 179.9998611111 >> North extent: maximum Y-coordinate (latitude) 83.9998611111 >> Rows (for mean): 20,880 (for others: 16,800) >> Columns: 43,200 >> >> it seems the original GMTED2010 data has an intentional half-pixel-shift. > > Because GMTED2010 is based on SRTM which also has a half-pixel shift. > GMTED2010 has no excess columns and covers 360 degree in longitude > while SRTM with all tiles patched together has one excess column > because SRTM tiles overlap by one row/column. > > The CHELSA extents are different from the GMTED2010 extents. If the > CHELSA climate raster grid geometry is based on GMTED2010, the CHELSA > grid geometry has been modified at some stage. The answer of the data > provider does not explain the grid geometry of the CHELSA raster data.
The CHELSA data have the same half-pixel shift like GMTED2010 with half-pixel shift meaning a half-pixel shift of SRTM at 1 arc second resolution. That means that in addition to the 7 excess columns to the east, the southernmost row of the CHELSA data also needs to be cut off with gdal_translate because it extents beyond 90S which GRASS does not allow. Markus M > >> >> any idea? > > The CHELSA grid geometry has been messed up at some stage and no > longer matches GMTED2010 at 30 arc seconds resolution. The excess 7 > columns at the east and the missing northernmost row are a mystery. A > nice test would be to check if the westernmost 7 columns are identical > to the 7 easternmost columns since they cover the same area. > Otherwise, you can try gdal_translate to prepare the data for GRASS > import. The first thing I would do is evaluate spatial accuracy > (spatial pattern matching with worldclim). > > Markus M > >> [1] >> http://erouault.blogspot.co.at/2011/12/seamless-access-to-remote-global-multi.html >> [2]https://pubs.usgs.gov/of/2011/1073/pdf/of2011-1073.pdf >> >> >> >> ----- >> best regards >> Helmut >> -- >> View this message in context: >> http://osgeo-org.1560.x6.nabble.com/CHELSA-climate-data-set-tp5284115p5284378.html >> Sent from the Grass - Users mailing list archive at Nabble.com. >> _______________________________________________ >> grass-user mailing list >> [email protected] >> http://lists.osgeo.org/mailman/listinfo/grass-user _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
