Clarifications about the projection of the grid of the MESCAN-SURFEX outputs

1- Regarding geodetic datum and ellipsoid: the used ellipsoid is a sphere with radius 6371229 m.

Using ArcGIS to open a MESCAN-SURFEX grib file, the datum information are:

  • Datum: D_unknown
  • Spheroid: Sphere
  • Semimajor Axis: 6371229.0
  • Semiminor Axis: 6371229.0
  • Inverse Flattening: 0.0

2 - Regarding false easting and false northing: these are just used to assure that the coordinates are not negative in the Lambert Conic Conformal system. In the ArcGIS definition these are set to zero, but the data seems to be correctly projected anyway. Although with some negative coordinate values. Suggest values are:

  • false easting=2937018
  • false northing=2937031

3 - Examples of MESCAN-SURFEX projection parameters using ESRI ArcMap 10.4.

The software identifies the coordinate system as “Lambert_Conformal_Conic_2SP” with parameters corresponding to those in the user guide:

  • Lambert_Conformal_Conic_2SP
  • Authority: Custom
  • Projection: Lambert_Conformal_Conic
  • false_easting: 0,0
  • false_northing: 0,0
  • central_meridian: 8,0
  • standard_parallel_1: 50,0
  • standard_parallel_2: 50,0
  • latitude_of_origin: 50,0
  • Linear Unit: Meter (1,0)

Using GCS_Coordinate_System_imported_from_GRIB_file, the parameters are:

  • Angular Unit: Degree (0,0174532925199433)
  • Prime Meridian: Greenwich (0,0)
  • Datum: D_unknown
  • Spheroid: Sphere
  • Semimajor Axis: 6371229,0
  • Semiminor Axis: 6371229,0
  • Inverse Flattening: 0,0

The software works well also adding a layer with land outlines in a different coordinate system:

and transforming to wgs84:

4 - Example of MESCAN-SURFEX projection parameters using Qgis 2.8.6.

The software defines the projection with this proj string which also corresponds to the parameters in the user guide:

  • +proj=lcc +lat_1=50 +lat_2=50 +lat_0=50 +lon_0=8 +x_0=0 +y_0=0 +a=6371229 +b=6371229 +units=m +no_defs
  • with these false easting and false northing values: +proj=lcc +lat_1=50 +lat_2=50 +lat_0=50 +lon_0=8 +x_0=2937018 +y_0=2937031 +a=6371229 +b=6371229