"Peter J Seymour" <moz@[EMAIL PROTECTED]
> wrote in message
news:fk04pr$4n9$1$8300dec7@[EMAIL PROTECTED]
> What I find a bit disconcerting with GPS measurements is the question of
> datums. Different measurements may be relative to different dataums. For
> instance on Google Maps, the Greenwich meridian and 0 degrees longitude
> do not coincide. Has any one found cases of confusion and inaccuaracy in
> practice, such as not correctly locating a grave?
Welcome to the wonderful world of geodesy. Different datums do not exist
just for fun. They came about because the original geodetic surveys that
established the shape of the earth were not connected to each other. Thus,
each country (continent) would use which ever spheroid (datum) best fit
their local situation. Only when the shape of earth could be measured
using
satellites were we able to develop spheroidal models that provided a best
fit for the entire earth, rather than just for a particular region. ITMT,
all those legal property surveys were done and maps published using the
existing local datums. So, countries couldn't just toss out the old datum
when a better one came along. This means that the latitude and longitude
of
an object measured in the local datum (e.g. Tokyo, Gauss) can be as much
as
SEVERAL HUNDRED METERS removed from the same position described in a
geocentric datum such as WGS84. For this reason surveyor will ALWAYS
include information about the datum used on any survey he performs. ALL
accurate maps (topographic, navigation, etc) have information about the
datums and projections used. There is no way around this until the entire
world adopts a single datum and converts all the old maps, plats, etc to
that datum.
That's the bad news. The good news is that the North American Datum
(NAD27)
that has been in use for US maps since 1927 is pretty close to the WGS84
(World Geodetic System 1984) datum used by GPS at least over most of North
America.
The best policy for recording the position of anything located by GPS is
to
always also record the datum used. You also need to understand that
nothing
is ever positioned exactly, never, ever. There is always an error,
although
in some cases the error can be made very small. When using GPS you should
also record the error. (e.g. +-25m).
--
Donald R. Newcomb
DRNewcomb (at) attglobal (dot) net
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