Hi
I agree adding something like 'datum = "WGS84" is very easy for
modellers to adopt, but in geodetics terms this is very ambiguous.
While it is simple, it is simply not enough.
If you want simple, a solid approach is EPSG codes.
Take a look at the openlayers examples at:
http://trac.osgeo.org/openlayers/wiki/SphericalMercator and
http://docs.openlayers.org/library/spherical_mercator.html
Openlayers, GoogleEarth, BingMaps, WMS specifications and many others
use the EPSG codes as an unambiguous, brief yet explicit definition of
the underlying geodetic parameters.
A very good reference for the EPSG / WKT codes can be found at
http://spatialreference.org/
The most simple example is WGS84 LatLong (No projection)
Using EPSG your metadata would need to be:
"EPSG:4326"
Using WKT your metadata would need to be:
GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.01745329251994328,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]]
Existing CF metadata conventions for CRS definition will cover this
without difficulty.
If we take spherical mercator as an example (this is still a simple
one!)
Using EPSG codes your metadata would be:
"EPSG:900913"
Using WKT you would need the following metadata:
900913=PROJCS["WGS84 / Simple Mercator",
GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS_1984", 6378137.0, 298.257223563]],
PRIMEM["Greenwich", 0.0],
UNIT["degree", 0.017453292519943295],
AXIS["Longitude", EAST],
AXIS["Latitude", NORTH]],
PROJECTION["Mercator_1SP_Google"],
PARAMETER["latitude_of_origin", 0.0],
PARAMETER["central_meridian", 0.0],
PARAMETER["scale_factor", 1.0],
PARAMETER["false_easting", 0.0],
PARAMETER["false_northing", 0.0],
UNIT["m", 1.0], AXIS["x", EAST],
AXIS["y", NORTH],
AUTHORITY["EPSG","900913"]]
This is now stretching the existing CRS specification as defined in CF
convention.
Taking a more complex example, the state mapping grid for California.
Using EPSG codes your metadata would be:
"EPSG:7008, 9822"
in WKT would be something like:
PROJCS["NAD27 / California Albers",
GEOGCS["NAD27",
DATUM["North American Datum 1927",
SPHEROID["Clarke 1866", 6378206.4, 294.97869821389821,
AUTHORITY["EPSG","7008"]],
TOWGS84[-2, 152, 149, 0, 0, 0, 0],
AUTHORITY["EPSG","6267"]],
PRIMEM["Greenwich", 0, AUTHORITY["EPSG","8901"]],
UNIT["degree", 0.017453292519943295, AUTHORITY["EPSG","9122"]],
AXIS["Geodetic latitude", NORTH, AUTHORITY["EPSG","106"]],
AXIS["Geodetic longitude", EAST, AUTHORITY["EPSG","107"]],
TOITRS["NAD27 to WGS 84 (21)","1249"],
AUTHORITY["EPSG","4267"]],
PROJECTION["Albers Equal Area", AUTHORITY["EPSG","9822"]],
PARAMETER["central_meridian", -120],
PARAMETER["latitude_of_origin", 0],
PARAMETER["standard_parallel_1", 34],
PARAMETER["standard_parallel_2", 40.5],
PARAMETER["false_easting", 0],
PARAMETER["false_northing", -4000000],
UNIT["metre", 1, AUTHORITY["EPSG","9001"]],
AXIS["Easting", EAST, AUTHORITY["EPSG","41"]],
AXIS["Northing", NORTH, AUTHORITY["EPSG","42"]],
AUTHORITY["EPSG","3309"]]
We use WKT for our specification, and it works perfectly fine, but they
can get big. The largest one reported to me was 3Kbytes long. I tried
to dig it out but no luck.
In summary, my recommendation would be to support EPSG codes for
scientists who require a simple mechanism to define a CRS, but also
permit WKT to be specified in cases where a comprehensive CRS definition
is required. EPSG code "EPSG:4326" would fulfill the vast majority of
cases where basic latitude/longitude from GPS is in use.
regards
pk
-----Original Message-----
From: cf-metadata-bounces at cgd.ucar.edu
[mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Seth McGinnis
Sent: Wednesday, 5 October 2011 6:50 AM
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] Question on WKT representation of CRS
(Bentley, Philip)
On Tue, 4 Oct 2011 15:28:15 +0100
Jonathan Gregory <j.m.gregory at reading.ac.uk> wrote:
>
>The CF convention as it stands can say a lot less, but it does look
more
>self-explanatory to me! The meaning of the WKT is not clear to me. I'm
quite
>uneasy about importing a convention into CF which produces opaque
metadata
>like this, even though it is no doubt machine-readable.
I'm uneasy about opaque metadata, too, especially when it comes
to model output. (I'm agnostic about its use for observational
data, or as an optional add-on.)
Pragmatically, I think modelers could be asked to add some more
parameters to their projection metadata, things like 'datum =
"WGS84"' or 'ellipsoid = "spherical"', and that would be
successful. I think if they were asked to add something long and
mysterious like WKT, there would be a lot of model output with
metadata that's either non-conformant or flat-out wrong.
Another consideration, mentioned in a previous thread about
datums, is that the reality of atmospheric models is they
generally run on a spherical earth but use forcings taken from
WGS84 locations without any transformation. So the datum is
somewhat ill-defined in the first place. Would having WKT
available for these cases imply a misleading level of
specificity?
Cheers,
--Seth
_______________________________________________
CF-metadata mailing list
CF-metadata at cgd.ucar.edu
http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
Received on Tue Oct 04 2011 - 20:07:48 BST
