Importing/exporting rotated-pole GRIB files into… – Dev001

If you have GRIB2 files with rotated north pole, you can’t import it on the fly into ArcGIS because of two issues: The DWD COSMO-EU weather model works in a rotated-pole coordinate system where the geographic north pole is at λ=170 °W and φ=40 °N. (0°E,0°N)rot in this rotated coordinate system corresponds to (10°E,50°N) in the common unrotated coordinate system (where 0 °E means Greenwich and 90 °N means north pole). These values are used in this example. First, the GRIB data have to be imported (while ignoring the unsupported projection). The correct grid definition is ArcGIS won’t be able to operate on the .grb file directly, so copy it to a geo-database: Data Management Tools / Raster / Raster dataset / Copy raster. After copying, ArcGIS should be able to show the GRIB data, having the left bottom pixel at (0,0). Use Data Management Tools / Raster / Raster processing / Shift to correct the origin of your raster (in the example, the raster has to be shifted -18° and -20°, see xfirst and yfirst in the grid definition). It doesn’t matter whether you do this now or after applying the custom transformation. Define a custom GCS (e.g. D_COSMO_EU_RotatedPole) and assign it as projection: Data Management Tools / Projections and Transformations / Define Projection (you can define the coordinate system details just there). Make sure it’s a GCS without further projection and with correct parameters (earth radius etc.). Now the tricky part: Create a custom transformation (Data Management Tools / Projections / Create Custom Geographic Transformation). The Coordinate Frame and Position Vector transformations allow rotations, so we’ll use one of them. In my case, the (0,0) origin of the grid shall move to (10°E,50°N), so there are two rotations required: The custom transformation will do that de-rotation, so call it “derotate-cosmo” and use these parameters: If everything is done right and the imported GRIB file has been assigned the D_COSMO_EU_RotatedPole projection, you can now choose either D_COSMO_EU_RotatedPole or WGS84 as your view projection in Layers / Properties / Coordinate System. Then click on “Transformations” and make sure that the custom transformation is applied. For instance, if you want to view your map in WGS84, set the transformation from D_COSMO_EU_RotatedPole to WGS84 to derotate-cosmo. Viewing the data should now work, but of course calculations are very fragile in this coordinate system. I recommend to transform the GRIB data into a working projection as soon as possible. However, in my case, I had to create masks in the original GRIB projection for further use with cdo, which is now possible when you always project all vector features into the D_COSMO_EU_RotatedPole system. If the transformation works, but doesn’t match with other data sets exactly, you may define another custom GCS, e.g. D_COSMO_EU, and chain two custom transformations together (D_COSMO_EU_RotatedPole → D_COSMO_EU → WGS84). If you have created masks or other data in the rotated-pole projection with ArcGIS, you may want to export them for further use with cdo. To do so: Source.


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