Fwd: [CF-metadata] projections

From: Jonathan Gregory <jonathan.gregory>
Date: Fri, 15 Nov 2002 18:35:54 +0000

Dear John

Many thanks for your email about projections. I agree that we haven't done
enough on this yet.

> 1) i would prefer to use existing "authorities" such as FGDC to name the
> projections.

It is useful to base our standard on existing standards such as FGDC - thanks
for suggesting it. Doing so saves thinking and confusion. However, I would
prefer not to give "authority" to something outside our standard. I'm happier
to adopt/adapt parts of other standards into CF as the need arises for them,
because as we do so we can satisfy ourselves (a) that the definition is clear
and (b) that we need it. We shouldn't make CF unnecessarily complicated by
importing many projections that are not actually used in our field - I would
prefer us to limit ourselves to what CF users require, expanding as requested.
Allowing multiple external authorities would have the further drawback that
there could be more than one way to represent a particular projection, which
would make life harder for the users of data.

> 2) since its highly likely that many variables will share a projection, we
> should have an option to factor them out, eg:

I'm afraid I'm not in favour of this proposal. In general we have preferred
to duplicate metadata on a per-variable basis, because this makes it easier to
distribute and copy variables between files, and also simplifies software
which processes the data (and hence may change some variables but want to leave
others unaffected). Hence we have reduced the number of global attributes and
recommended attributes on variables where relevant. It wastes a bit of space,
but this is generally minor.

> 3) its important that the x and the y coordinates be identified, and that they
> be understood to be coordinates on the specified projection plane. You could say
> something like:
>
> "Variables representing the x and y coordinates on the projection plane must
> always explicitly include the units attribute; there is no default value. The
> units attribute will be a string formatted as per the udunits.dat file. The
> recommended unit of latitude is km. The variable representing the x and y
> coordinates must have an attribute axisType="x" and "y" respectively."

The purpose of the mapping information is to tell the software how to
compute the actual lat and lon from the x and y - yes? I suppose that the x
and y should have units which are specified for the individual projections.
For the rotated pole mapping that we have defined, x and y are in degrees
(of longitude and latitude in the rotated system).

The axis attribute may or may not be appropriate. It depends whether x and y
are in any sense "like" longitude and latitude.

> 5) FGDC would be a good start
>
> i would be happy to compile these for your appendix if you'd like.

Thank you. It would be very helpful to have proposals for those projections
that are needed.

Best wishes

Jonathan
Received on Fri Nov 15 2002 - 11:35:54 GMT