[CF-metadata] What do models assume for the shape of the Earth?

[toyoda at gfd-dennou.org]

> >The amount of shift should be equivalent to the
> difference
> >between geodetic and geocentric latitudes. 
> According to
> >Snyder's book, it is 11'40" (21km) at 45 degree
> latitude
> >(in Clarke 1866 ellipsoid but not so much different
> in
> >others for two or three-digit precision).
> > 
> >
> which Snyder book is this? which page ?

John P. Snyder, 1987: Map Projections - A Working Manual.
US Geological Survey professional paper; 1395, pp.383. 

I was talking about Table 3 on page 18.

> >In my knowledge, the reversed conversion (ellipsoid
> to
> >sphere) has not been performed in data assimilation
> of
> >numerical weather simulation.  Meteorological
> community
> >naively references location using
> latitude/longitude
> >coordinates, ignoring ellipsoid by which the
> coordinates
> >are defined (in short, people get observation and
> lat/lon,
> >then put them into model at the same lat/lon). 
> Thus the
> >model data basically reflects observations on
> coordinates
> >on ellipsoid, regardless the coordinate system the
> model
> >use.
> > 
> >
> so observations are always using geodetic latitudes
> ?

Yes, in my experience.

There are two major source of horizontal location
information of meteorological observation: station table
(mainly compiled by WMO) and satellite observations
(usually locations are coded within data).  The latter
could be more important for this issue.  I don't think all
they use the same ellipsoid or datum, but I haven't heard
any of them are using geocentric latitude.

-- 
TOYODA Eizi <toyoda at gfd-dennou.org>
Environmental Remote Sensing Center
University of Wisconsin-Madison