[CF-metadata] vertical coordinates

From: Seth McGinnis <mcginnis>
Date: Wed, 06 Aug 2008 15:28:59 -0600

Hi Jon,

I don't know about the other cases, but I believe that if
you want to specify that
you're using a spherical earth or the WGS84 ellipsoid, you
do it by including
parameters like earth_radius (for spherical) or
semi_major_axis and
inverse_flattening (for WGS84) as attributes in the
grid_mapping that
describes your map projection. See Appendix F and examples
5.8 and 5.9.

That would implicitly define the vertical coordinate
system, wouldn't it?

(I'm just an end-user, though, not an expert -- I may well
be wrong.)

--Seth

----
Seth McGinnis
Associate Scientist
ISSE / NCAR
----
>Hi all,
>
>Looking through the CF conventions document (1.3) there
seems to be no
>way to distinguish some of the possible vertical
coordinate systems.
>CF distinguishes dimensioned vertical coordinates (e.g.
height or
>depth in metres) from dimensionless coordinates (e.g.
sigma) but does
>not seem to distinguish between the different possible
heights/depths.
> Here are a few possibilities that I think can't be
distinguished in CF:
>
>- Depth below the geoid (also, which geoid?)
>- Depth below an ellipsoidal approximation of the earth
(e.g. WGS84)
>- Depth below instantaneous sea surface (e.g. where depth
is inferred from pressure)
>- Depth below a spherical approximation of the earth (used
in many models, I believe)
>
>(the same goes for heights of course)  Am I right or have
I
>misunderstood something?  To provide a case study, a
colleague of mine
>is combining ocean model results with other GIS data
sources in order
>to drive a high-resolution (50m in the horizontal) model
of an
>estuary.  The ocean model data files (which I think are
CF-compliant)
>don't give a vertical datum, and this leads to an
uncertainty in
>*horizontal* positioning of around 100m, which is two
whole grid cells!
>
>This problem is most marked, of course, in very
high-resolution
>studies - it's not likely to be too important for
larger-scale work,
>although it does seem important to be able to describe
vertical
>positioning as accurately as we can describe horizontal
>and temporal positioning.
>
>Any thoughts?
>Jon
>
>-- 
>--------------------------------------------------------------
>Dr Jon Blower Tel: +44 118 378 5213 (direct line)
>Technical Director Tel: +44 118 378 8741 (ESSC)
>Reading e-Science Centre Fax: +44 118 378 6413
>ESSC Email: j.d.blower at reading.ac.uk
>University of Reading
>3 Earley Gate
>Reading RG6 6AL, UK
>--------------------------------------------------------------

Received on Wed Aug 06 2008 - 15:28:59 BST