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

[Rich Signell]

John Caron wrote:
 >
 > Another question is which ellipsoid is being used. According to Snyder,
 > when US switched from NAD27 to NAD83, nearly every point in North
 > America got a new lat/lon, and "the difference can reach 300 m". (p 13).
 > So perhaps we are talking about 300 m rather than 20 km ? Rich can you
 > comment more on this?
 >

John,

It can be up to 20 km if you are half way between the pole and the equator and 
converting lon,lat values from a bogus ellipsoid, like a sphere, to a real 
ellipsoid, like WGS84.

It's much less, like your 300 m value if you are converting between true 
ellipsoids & datums, such as from NAD27 to NAD83.

If you want to play with these datum conversions, you can use the "cs2cs" 
function in the freely available proj4 package, available at:
http://proj.maptools.org/

For example:

cs2cs +proj=latlong +ellps=sphere  +to +proj=latlong +ellps=WGS84
-117 45
117dW   45d11'32.334"N 3579.255

cs2cs +proj=latlong +datum=NAD27 +to +proj=latlong +datum=NAD83
-117 45
117d0'3.587"W   44d59'59.568"N 0.000

-Rich


-- 
Richard P. Signell           rsignell at usgs.gov
U.S. Geological Survey       Phone: (508) 457-2229
384 Woods Hole Road          Fax:   (508) 457-2310
Woods Hole, MA  02543-1598