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
--
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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
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Received on Wed Aug 06 2008 - 15:05:29 BST
