Jon, Vertical datums were originially going to be handled by CF Trac ticket 18: http://cf-pcmdi.llnl.gov/trac/ticket/18 but then were excluded in the interest of moving forward on the horizontal datum issues. I think we would need a new ticket for the vertical datum issues (and of course, someone to champion the process).
-Rich On Wed, Aug 6, 2008 at 5:28 PM, Seth McGinnis <[EMAIL PROTECTED]> wrote: > 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: [EMAIL PROTECTED] >>University of Reading >>3 Earley Gate >>Reading RG6 6AL, UK >>-------------------------------------------------------------- > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > -- Dr. Richard P. Signell (508) 457-2229 USGS, 384 Woods Hole Rd. Woods Hole, MA 02543-1598 _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
