[GRASS-user] CHELSA climate data set

[Markus Metz]

On Tue, Sep 6, 2016 at 10:22 PM, Markus Metz
<markus.metz.giswork at gmail.com> wrote:
> 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.

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.
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