> Latitude and longitude are useless without a datum,
> since they are *calculated* and not read. For example,
> your coordinates in a GPS system would use the WGS84
> (world geodetic system survey of 1984) but if you are
> using another datum (like NAD for North-America or
> ED50 for Europe) those coordinates will be very different.

In these parts (SF Bay Area, CA, USA) the difference between NAD27 and
WGS 84 is a around 13 meters north and 94 meters east.

NAD27 is the datum the USGS still clings to since they *really* don't
want to revise all their maps.  The internal calculations in GPS's is
done in WGS84 and converted to older datums as needed.  Some of the
online mapping services still use NAD27, others use WGS84 (or its
close relative NAD83).

        http://www.colorado.edu/geography/gcraft/notes/datum/datum.html

-wolfgang
-- 
Wolfgang S. Rupprecht 		     http://www.wsrcc.com/wolfgang/
(NOTE: The email address above is valid.  Edit it at your own peril.)