[GRASS-user] CHELSA climate data set

[Markus Metz]

On Tue, Sep 6, 2016 at 8:12 PM, Helmut Kudrnovsky <hellik at web.de> 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.

>
> 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.
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