Hi Phil,
Yes, I think strictly this is correct. But it does mean that, for data
that do not specify a datum, the only legitimate way to plot such data
as a map is as a latitude-longitude plot. Projection to any other
coordinate system would require knowledge of the datum in order to do
the mathematical transformation accurately (I think... possibly there
are some exceptions).
The suggestion for applications to print "health warnings" for
underspecified data is a good one.
Maybe future versions of CF should make the datum specification
mandatory, where it is known?
Jon
-----Original Message-----
From: Bentley, Philip [mailto:philip.bentley at metoffice.gov.uk]
Sent: 19 April 2010 16:52
To: Jonathan Blower; Mike Grant
Cc: cf-metadata at cgd.ucar.edu
Subject: RE: [CF-metadata] Default datum for latitude and longitude?
Hi Jon,
Indeed, the UM global climate model, for example, assumes a spherical
earth.
In the absence of specific datum information I think I would probably
caution against assuming any particular datum is applicable. I can
understand the temptation to use WGS 1984 as a default because it is
widely known and used. However, this implies a degree of accuracy in the
coordinate data which in many instances may well not be justified -
indeed its use might even compound any relative location errors.
As you rightly suggest, if the end-use/application demands a particular
spatial accuracy then any datasets which demonstrably do not not meet
those requirements should probably not be included in the analysis - or
else should be with appropriate caveats, provisos and disclaimers!
Regards,
Phil
> -----Original Message-----
> From: cf-metadata-bounces at cgd.ucar.edu
> [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Jonathan Blower
> Sent: 19 April 2010 16:23
> To: Mike Grant
> Cc: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Default datum for latitude and longitude?
>
> Hi Mike,
>
> Yes, that's what I've been doing so far, but I seem to
> remember someone telling me that models tend to be formulated
> on a spherical earth. But maybe if the precise geolocation
> of the model grid points is important (e.g. for small-scale
> models) the datum will always be specified, or a different
> projection used.
>
> Cheers, Jon
>
> -----Original Message-----
> From: Mike Grant [mailto:mggr at pml.ac.uk]
> Sent: 19 April 2010 16:07
> To: Jonathan Blower
> Cc: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Default datum for latitude and longitude?
>
> On 19/04/10 15:43, Jonathan Blower wrote:
> > specification, which datum should be assumed? Spherical Earth?
> WGS84?
>
> If you're picking one at random, I'd go for WGS84 - that's a
> pretty safe bet for a lot of remote sensed and GPS related data.
>
> Cheers,
>
> Mike.
>
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Received on Mon Apr 19 2010 - 10:04:49 BST
