I have to agree with Jon on this one. To borrow his phrase, I wouldn't
want to be "inventing metadata".
Regarding the problem of spherical models using uncorrected WGS84
positions - two wrongs don't make a right. It's the use of the models
that need fixing. Altering the interpretation after the fact just
compounds the problem.
And since using WGS84 positions on a spherical model is tantamount to
altering the model physics, who knows ... maybe correcting the positions
would give more accurate results from the model.
Richard Hattersley AVD Expert Software Developer
Met Office FitzRoy Road Exeter Devon EX1 3PB United Kingdom
Tel: +44 (0)1392 885702 Fax: +44 (0)1392 885681
Email: richard.hattersley at metoffice.gov.uk Website:
www.metoffice.gov.uk
-----Original Message-----
From: cf-metadata-bounces at cgd.ucar.edu
[mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Jon Blower
Sent: 02 February 2011 16:17
To: John Caron; cf-metadata at cgd.ucar.edu; Java NetCDF
Subject: Re: [CF-metadata] ESRI implementation of ellipsoidal-earth
> Also, they are wondering what to use for the default ellipsoid when
these are not specified. It seems to me the obvious > candidates are 1)
spherical earth with a standard radius, or 2) WGS84 ellipsoid.
My inclination would be to treat the lack of datum as an issue to be
escalated to the user to choose what to do. Seems more complicated but
I'd be very wary of "inventing metadata" for the sake of convenience.
Maybe ESRI could provide the user with a global "datum leniency"
setting, that a user could explicitly set to avoid being bothered by
prompts, if they know what they are doing.
It's less of an issue for viewing datasets in isolation, or for
coarse-resolution data. But for high-resolution (regional- and
local-scale) data the datum shift can be significant. I see this
increasingly with people doing data assimilation in small-scale models -
the geolocation error can be greater than the size of a grid cell.
(I will declare my own hypocrisy and note that ncWMS does assume WGS84
at the moment (it won't soon)... but that's how I came across the
problem in the first place!)
HTH,
Jon
-----Original Message-----
From: cf-metadata-bounces at cgd.ucar.edu
[mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of John Caron
Sent: 01 February 2011 19:40
To: cf-metadata at cgd.ucar.edu; Java NetCDF
Subject: [CF-metadata] ESRI implementation of ellipsoidal-earth
Apparently ESRI is willing to add support for CF 1.5 grid_mapping
attributes for ellipsoidal-earth/geodetic-datum definitions in the Grid
Mappings and Projections specification.
They are looking for sample data, especially using ellipsoidal
parameters (semi_major_axis/semi_minor_axis/inverse_flattening). Does
anyone have any?
Also, they are wondering what to use for the default ellipsoid when
these are not specified. It seems to me the obvious candidates are 1)
spherical earth with a standard radius, or 2) WGS84 ellipsoid.
Opinions?
John
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Received on Thu Feb 03 2011 - 07:18:05 GMT
