[R-sig-Geo] netcdf data from Antarctica in rotated latlong - how to reproject to normal geographical coordinate system

[Roger Bivand]

On Thu, 5 Apr 2018, Matthias Boer wrote:

> Hi all
>
> I am having trouble extracting some climate time series for a series of field sites in Antarctica from a netcdf file with a rotated pole projection. The file (gridded monthly temperatures for 1979-2016) includes these details about the projection:
> float rotated_pole[]
>            grid_mapping_name: rotated_latitude_longitude
>            grid_north_pole_latitude: -180
>            grid_north_pole_longitude: -150

>            proj4_params: -m 57.295779506 +proj=ob_tran +o_proj=latlon 
> +o_lat_p=-180.0 +lon_0=30.0

>            proj_parameters: -m 57.295779506 +proj=ob_tran +o_proj=latlon 
> +o_lat_p=-180.0 +lon_0=30.0

>            projection_name: rotated_latitude_longitude
>            long_name: projection details
>            EPSG_code:
>
> I can read the netcdf file into R:
> tst.s <- stack("filename.nc", varname="var1")

You'll need to look for the arguments supporting +proj=ob_tran 
(use_ob_tran=) in rgdal::spTransform(). +proj=ob_tran is a complete mess, 
and until the next release of GDAL, the GDAL netcdf driver will not 
understand these either. Your best bet may be not to warp the input grids, 
but to transform the field sites point coordinates to ob_tran to extract 
the data you need (checking that the transformation seems sensible).

Roger

>
> but this gives a couple of warnings that raster can't understand the projection:
> Warning messages:
> 1: In .stackCDF(x, varname = varname, bands = bands) :
>  tskin has 4 dimensions, I am using the last one
> 2: In .getCRSfromGridMap4(atts) : cannot process these parts of the CRS:
> grid_north_pole_latitude=-180; grid_north_pole_longitude=-150; proj4_params=-m 57.295779506 +proj=ob_tran +o_proj=latlon +o_lat_p=-180.0 +lon_0=30.0; proj_parameters=-m 57.295779506 +proj=ob_tran +o_proj=latlon +o_lat_p=-180.0 +lon_0=30.0; projection_name=rotated_latitude_longitude; long_name=projection details; EPSG_code=
> 3: In .getCRSfromGridMap4(atts) : cannot create a valid CRS
> grid_north_pole_latitude=-180; grid_north_pole_longitude=-150; proj4_params=-m 57.295779506 +proj=ob_tran +o_proj=latlon +o_lat_p=-180.0 +lon_0=30.0; proj_parameters=-m 57.295779506 +proj=ob_tran +o_proj=latlon +o_lat_p=-180.0 +lon_0=30.0; projection_name=rotated_latitude_longitude; long_name=projection details; EPSG_code=
>
>
> Could anyone point me to some code to assign the correct CRS to the 
> imported stack 'tst.s' and then to project it to the 'normal' 
> geographical coordinate system, so I can extract time series for the 
> field site coordinates which are in 'normal' latlong.
>
> Thanks a lot for your time.
>
> Cheers,
> Matthias
>
>
>
> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> Dr. Matthias M Boer
> Hawkesbury Institute for the Environment<http://www.westernsydney.edu.au/hie> | Western Sydney University | Hawkesbury Campus, Richmond, NSW 2753, AUSTRALIA
> P: +61-(0)2-4570-1373 (direct) | P: +61-(0)2-4570-1941 (admin) | E: m.boer at westernsydney.edu.au
>
> Postal address: Locked Bag 1797 | Penrith, NSW 2751, AUSTRALIA
> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
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-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; e-mail: Roger.Bivand at nhh.no
http://orcid.org/0000-0003-2392-6140
https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en