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(meteorobs) Which 'datum' to use for meteor report longitude and latitude


As most modern meteor observers know, one of the things IMO asks the observer
to report on the IMO Meteor Report Form for each and every observing session,
is the observer's location during the session: the Form asks for not only the
place name (which is very dependent on country and regional standards, and
may not be unique), but also for latitude, longitutde and elevation, ideally
to within a FEW TENS OF METERS.

(Folks may argue that such accuracy is not necessary for visual observing, but
since I've never heard a definite statement to that effect from IMO, I always
try to report my position within a second of arc and ten meters elevation.)

Now here's my question: In trying to determine the EXACT location of a site for
an observing report this week, I asked some of my fellow local amateurs whether
they had ever measured it. No one had done a GPS measurement at this particular
spot, so I ended up using a mapping service tied to the U.S.G.S. TIGER database
to get my answer. Yet when I wondered if this was accurate, a friend shared
the following very interesting tidbit of information with me:

>A datum is a particular coordinate system for latitude and longitude. It's
>analogous to an epoch for RA and Dec. The datum used on older USGS topo maps
>is NAD27 (North American Datum of 1927).  Newer USGS topo maps use NAD83
>(North American Datum of 1983).  Then there's the WSG84 datum used by GPS
>and IOTA. According to [one source] differences in coordinates amongst these
>data can be as much as 300 meters, or 0.2 arc minutes [for latitude].

So in truth, my answer was RIGHT with the NAD83 datum, but WOULDN'T agree with
a reading from a GPS system at that same site. So my first question is simply
whether location information DOES need to have sub-minute accuracy for visual
reports? (Or for that matter, for telescopic or video data reports?) Second, if
the meteor organizations DO expect this level of accuracy, which datum do they
expect their North American observers to use? And is there a similar problem
of "competing" coordinate systems in Europe, Asia and elsewhere?

Perhaps a little off-topic, but I've always wondered about this... :)

Clear skies,
Lew


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