All,
I don't understand what is meant "opaque" or "long and mysterious" with regard to Well Known Text crs definition. Just because the parameters stored in a wkt string are complex?
The fact of the matter is that coordinate reference system information is an integral component of any geospatial data. Wkt is the common method of defining custom crs definition. Epsg is used in common applications like web mapping to allow a client to easily request one of a list of available crs.
The discussion of datums and crs with regard to low resolution data is clear at this point. If data providers supply data without crs definition, software to access it should throw warnings to inform a user that the data is incomplete. The point is that the data is incomplete, that is ok, but compliance with the standard should entail completeness of data with no need for assumption or error in a normal workflow.
Cheers,
Dave
----- Reply message -----
From: "Bryan Lawrence" <bryan.lawrence at ncas.ac.uk>
Date: Wed, Oct 5, 2011 5:52 am
Subject: [CF-metadata] Question on WKT representation of CRS (Bentley, Philip)
To: <cf-metadata at cgd.ucar.edu>
Hi Seth
I haven't read all the threads ... but I strongly agree with your last paragraph!
I have had many conversations with folks who think that adding datums will make data more usable to the impacts community, where datum errors can move things by o(10)s of km ... and my protestation that no one should interpret as physical any differences on those scales from a (climate) model (even one run at o(km) resolution if such exists) ... was simply ignored. The reality is exactly what you say, that level of specificity is simply inappropriate.
I appreciate some of the arguments raised in the thread on storing lat/lon coordinates, about the need for the use of one in a GIS workflow - but frankly I think that's an issue about workflow metadata not source data metadata. As Balaji and others said, there might not even be *one* datum appropriate for GCM work ...
Of course observational data may well be different, and I'm not sure about NWP ... especially mesoscale models. So by all means, facilitate the provision of this information, but don't make it compulsory ... and I think it would be with WKT?
Cheers
Bryan
> 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
>
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--
Bryan Lawrence
University of Reading: Professor of Weather and Climate Computing.
National Centre for Atmospheric Science: Director of Models and Data.
STFC: Director of the Centre for Environmental Data Archival.
Ph: +44 118 3786507 or 1235 445012; Web:home.badc.rl.ac.uk/lawrence
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