Hi Phil, all,
This latest round of discussion around using CRS WKT reminded me of a
few questions:
1) How easy is it to form a WKT that has good interoperability between
different client libraries?
I've had multiple WKT for the same CRS with differences in AUTHORITY,
AXIS, and a few other things and had trouble having certain software
read them in without some tweaking. For instance, the
http://spatialreference.org/ pages for a given CRS list both OGC WKT and
ESRI WKT.
2) Are there good specification documents for WKT?
I believe the relevant OGC document is OGC 06-103r4 which can be found
here:
http://portal.opengeospatial.org/files/?artifact_id=25355
"OpenGIS Implementation Standard for Geographic Information -
Simple Feature Access - Part I: Common Architecture"
See section 9 "WKT Representation of Spatial Reference Systems".
It has an non-normative, non-exhaustive list of units, ellipsoids,
projection names, projection parameters, and the like in Appendix B.
But in the wild, there seem to be lots of variations in projection names
and parameter names. There's even variation between them in the OGC spec
mentioned above and the EPSG data base.
3) Given that, is CRS as WKT well enough defined so that providing a
place for it in CF get us the interoperability we want?
Or, Phil, would the proposal you have in mind go into more detail (or
add to) the OGC spec?
4) One last question. The OGC CRS WKT specification is less than two
pages of EBNF (without a lot of explanation). How far is the current CF
grid mapping spec from complete coverage of the CRS WKT? Prime meridian
and some units?
Which would bring us back to the issue of maintaining controlled lists
in CF or defining how to use the EPSG database (for names rather than
code numbers, e.g., "Lambert Conic Conformal (2SP)" not 9802).
Ethan
--
Ethan Davis UCAR Unidata Program
edavis at unidata.ucar.edu http://www.unidata.ucar.edu
Received on Sat Oct 01 2011 - 00:49:17 BST
