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
Received on Thu Apr 07 2005 - 15:19:16 BST
