[GRASS-user] CHELSA climate data set

[Markus Neteler]

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.

Helmut and me have contacted one of the CHELSA authors. He eventually
acknowledged that their coordinates are suboptimal and said that the
coordinate issues would originate from the GMTED2010 data set. What
they wrote on http://chelsa-climate.org/known-issues/ for their V1.0
is not sufficient. The CHELSA author yesterday said that he'll fix the
coordinates for the next dataset release. Let's see...

For now I have added a new CHELSA section in the Wiki:
https://grasswiki.osgeo.org/wiki/Global_datasets#CHELSA_climate_maps

... if that's right, no idea - some statistical analysis in complex
terrain like the Alps or similar would probably tell if the coordinate
fix is right.

best,
markusN


>> 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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-- 
Markus Neteler
http://www.mundialis.de - free data with free software
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