Hello John and CF-metadata folks,
--- John Caron <caron at unidata.ucar.edu>:
> Normally I dont see information in the netCDF file
> on the assumed shape
> of the Earth, eg ellipsoidal flattening or even
> spherical radius. Is
> that because it doesnt matter that much? Or is there
> an assumed standard
> that everyone just knows about?
I guess you would like to supply 'assumed shape of the
Earth' information when converting netCDF datasets into
GIS data format. If I am right, the most important thing
should be how the coordinate information is used by GIS.
GIS converts coordinate system when it overlays various
kinds of data with different coordinate systems.
For example, if we say that some meteorological data is
based on coordinates on sphere, GIS will convert it into
ellipsoid-based coordinates (in which GIS operation is
going to be performed). Then the meteorological data is
shifted equatorward.
The amount of shift should be equivalent to the difference
between geodetic and geocentric latitudes. According to
Snyder's book, it is 11'40" (21km) at 45 degree latitude
(in Clarke 1866 ellipsoid but not so much different in
others for two or three-digit precision).
In my knowledge, the reversed conversion (ellipsoid to
sphere) has not been performed in data assimilation of
numerical weather simulation. Meteorological community
naively references location using latitude/longitude
coordinates, ignoring ellipsoid by which the coordinates
are defined (in short, people get observation and lat/lon,
then put them into model at the same lat/lon). Thus the
model data basically reflects observations on coordinates
on ellipsoid, regardless the coordinate system the model
use.
I believe such data should not be shifted so much as 20km.
> Is there a different answer for global vs regional
> models? At what
> scale does this start to matter?
Recent operational global models (even excluding the Earth
Simulator) often have resolution near 20km. Thus the
problem is not only for regional models.
--
TOYODA Eizi <toyoda at gfd-dennou.org>
Environmental Remote Sensing Center
University of Wisconsin-Madison
Received on Tue Apr 05 2005 - 16:57:49 BST
