Hi Etienne:
Generally, CF has followed proj4 conventions as much as we could, and
John Snyder's reference book is often underneath:
http://pubs.usgs.gov/pp/1395/report.pdf
OGC specs tended to come from experts using the EPSG, and generally we
are less familiar with that.
some specific comments below, based on my own understandings.
On 12/13/2011 2:21 PM, Etienne Tourigny wrote:
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).
yes, we handle both 1 and 2 standard parallels in the same projection.
the 1 parallel can be thought of as a degenerate case of the 2 parellels.
AFAIU, the scale factor is determined by the standard parallels, so is
reduntant to specify, see Snyder, equation 15-4.
its possible that the more general case is to use another constant to
multiply the one given in 15-4, but i have never seen that.
Im not sure what #3324 refers to.
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)?
It does appear at first glance that there should be a seperate scale
factor. I am less familiar since I havent coded this one.
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'.
<http://www.remotesensing.org/geotiff/proj_list/polar_stereographic.html>i
agree, i dont know where these CF names came from.
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
Im guessing these rules may have to be specific to a projection, rather
than assuming you can do a global mapping of the attribute names.
I would like to incorporate your findings into the CDM library when I
can. Do you have any official documentation describing this mapping?
Thanks
Etienne
_______________________________________________
The question for CF is whether we should start to follow the WKT spec
closer, and add aliases where needed. I would prefer this since most of
us arent experts in this sort of thing. Which is pretty much the whole
argument for adopting WKT as an alternative to what we have done.
john
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