Hi all,
I am opening a seperate thread at this deals merely with the problems
encountered in translating projection parameters to/from OGC WKT and
CF-1.5.
I would appreciate any guidance in resolving these issues.
In summary:
1) LCC (Lambert conformal): should include a scale_factor parameter in
the 1 standard parallel case
2) CEA (Lambert Cylindrical Equal Area): how to convert the
'scale_factor_at_projection_origin' alternative to standard parallel
to OGC WKT?
3) PS (Polar stereographic):This projection's parameters in CF-1 are
significantly different from OGC/PROJ.4 , CF-1 standard parameters
'Either standard_parallel or scale_factor_at_projection_origin' is
incompatible with OGC WKT, which requires both
More detailed information (reproduced here) can be found at
http://trac.osgeo.org/gdal/wiki/NetCDF_ProjectionTestingStatus
1) LCC 1SP (Lambert conformal)
The OGC WKT 'Lambert_Conformal_Conic_2SP' maps clearly to LCC in CF-1
with 2 std parallels.
The CF-1 spec claims that a 1SP version can be used, but it doesn't
include a 'scale_factor' as does the OGC WKT
'Lambert_Conformal_Conic_1SP'.
For import: Given discussion and example on #3324, and also support
from NARCCAP project: it seems the CF-1 LCC 1SP projection is a
'degenerate' LCC-2SP with both SP's at the same location. Thus seems
we can read with both 'std_parallel' (1 value) and
'latitude_of_projection_origin' mapping to latitude_of_origin (WKT)
and setting scale_factor=1? This is the code's current behaviour
(though perhaps could do more error-checking).
For export: given the LCC-1SP can specifically include a scale_factor
to control the projection (see
http://www.remotesensing.org/geotiff/proj_list/lambert_conic_conformal_1sp.html),
it seems this can't be directly exported to NetCDF without throwing
away the scale factor.
Perhaps the reasonable thing for now is to export the scale_factor to
NetCDF, so it can at least be read back in. Otherwise, we'd have to
try to translate the LCC-1SP to a LCC-2SP on the fly.
2) CEA (Lambert Cylindrical Equal Area):
The CF-1 conventions claim this can be encoded with a
'scale_factor_at_projection_origin' alternative to standard parallel -
how would this conversion be done from/to OGC WKT (which requires
central_meridian and standard_parallel_1)?
3) PS (Polar stereographic)
This projection's parameters in CF-1 are significantly different from
http://www.remotesensing.org/geotiff/proj_list/polar_stereographic.html.
In particular the CF-1 equivalent for 'latitude of natural origin'
seems to be 'standard_parallel'.
CF-1 defines either standard_parallel or
scale_factor_at_projection_origin, which is incompatible with OGC WKT
(and OGR) that requires both. Note: we have verified the current
approach we take to mapping is the same used by at least one other
NetCDF CF-1 user in the UCAR NARCCAP project.
Current implementation (as of Dec. 13) uses the following rules:
- WKT scale_factor maps to CF scale_factor_at_projection_origin
- WKT central_meridian maps to CF straight_vertical_longitude_from_pole
- WKT latitude_of_origin maps to WKT standard_parallel
- WKT to CF: if WKT latitude_of_origin is positive then CF
latitude_of_projection_origin is +90, else it is -90 (North vs. South
Pole)
- CF to WKT: unsupported if standard_parallel is not defined, as
OGRSpatialReference::SetPS() requires both CF
scale_factor_at_projection_origin
Thanks
Etienne
Received on Tue Dec 13 2011 - 14:21:01 GMT
