Brian Eaton wrote:
> Hi John,
>
> CF compliance requires that the horizontal location of each grid point be
> specified by latitude and longitude coordinates. In the case of a grid
> resulting from a polar stereographic projection this could be accomplished
> by the use of 2D lat/lon arrays as described in sec 5.2. Note that the
> specification of a projection and the projection coordinates is not
> sufficient for compliance.
ok. why not require the projection also?
>
> The current working draft (linked to the CF home page as CF-1.0-beta4)
> contains a new section on grid mappings and projections. The only mapping
> currently provided is a rotated pole grid. We intend to handle projections
> using the same mechanism, but haven't yet determined a source of key/value
> pairs to describe projections. It would be best to have an external
> reference for this rather than making up our own and listing them in an
> appendix as we've done for the rotated pole example (which is not a
> projection). Do you think the FGDC specification of projections would be
> suitable for this (and could you point me towards a reference for them)?
> Also, are you aware of any ISO standards related to this?
What we are leaning towards in thredds and ncml is to allow multiple
"authorities" for projection naming. eg a tag like authority="FGDC". The ISO
stuff is not freely available, but i understand they follow the work of the
European Petroleum Survey Group (EPSG) Geodesy Parameters, which is much bigger
set of specialized projections.
I think FGDC is suitable, but it would be good to survey CF users to make sure
all desired projections are covered by it. The "authority" tag is intended to
allow other standards like ISO to be used as they emerge without breaking the
standard. CF-2 could publish a list of recommended authorities and their
parameter encodings.
I will forward to this list some related work we are doing in the NcML (netcdf
markup language) group on coordinate systems, as soon as i have a readable doc
about it.
FGDC:
http://www.fgdc.gov/metadata/contstan.html
this page summarizes and may be useful as a reference:
http://www.fgdc.gov/metadata/production/prspatref.html
EPSG:
http://www.ihsenergy.com/epsg/
I will also forward you some XML encodings for FGDC that our collegue Stefano
Nativi created (as soon as i can find them).
[It turns out the problem can be more complicated than just naming a projection,
one really needs a "reference system" that might include elipsoidal earth
parameters, or more complex stuff that are needed for applications that need
extremely accurate positioning (eg oil drilling). So CF has to decide how far
down this path they want to go. ]
Received on Thu Oct 10 2002 - 13:04:17 BST
