If you recall the start of this thread (which has change titles a few times now) it was the Lat and Long information provided for the US state quadripoint which kicked it all off - that was the initial connection to multipointing.

I believe it also has relevance to the EGLYSD class B claim discussed in another thread - the location evidently reached was the confluence according to WGS84 datum which is most likely not the same location as the actual tripoint which was probably defined using a different datum. Some of these datums can cause points to be off by up to 200m in some parts of the world so datum awareness is vital if you are going to use Lat and Long as the means of identifying multipoint locations.

From: BoundaryPoint@yahoogroups.com [mailto:BoundaryPoint@yahoogroups.com] On Behalf Of aletheiak
Sent: Tuesday, January 03, 2006 9:20 PM
To: BoundaryPoint@yahoogroups.com
Subject: [BoundaryPoint] Re: ok but why or what are we looking for

ok thanx i get it but i am also still trying to figure out if there is anything useful for trypointing in these conversations that is somehow still eluding me for reasons mostly just explained the choice of datum has rarely if ever made a difference in our multipointing tries i dont say it absolutely couldnt make a difference but it evidently hasnt yet & such variations or deviations in data as you guys are reporting here come as no surprise i trust yet their exact cause or causes are at the same time unknown & unknowable because the variables are too great as well as too numerous to permit any conclusions about any single bit of data let alone any generalizations about the whole of it except that trypointing by gps might become very slightly more or less tentative approximative fudgy & sketchy than it already is depending on which datum you choose so on both counts these observations seem to me to be not useable information but rather the absence of it for our purposes here or at best a partial description of what is for us only a hypothetical data gap anyway the data that are most useful to us here in trypointing are those which narrow & focus the search & the perception rather than diffuse them as soon as someone actually applies any of this info to a real try & it makes a difference in the outcome i would of course immediately change my appraisal of it --- In BoundaryPoint@yahoogroups.com, "Hugh Wallis" <hugh@o...> wrote: > > Each entry made records the actual WGS84 coordinates displayed on a GPSr > device that is positioned at a physical marker which is marking the Prime > Meridian. The point is to demonstrate, pursuant to the preceding > conversation here, that the choice of datum when reporting Lat and Long is > relevant. Frequently the WGS84 reading will show other than 0° 0.000" E/W > because different datums (data ?) than WGS84 have been used when placing the > physical markers. This is not a scientific survey of course, nor can it be > used in any way to deduce errors elsewhere. It is simply for illustration > and one of the larger collections of such "in the field" reports of such > deviations that I am aware of. It adds additional information to what Roger > found from Google Earth. > > > _____ > > From: BoundaryPoint@yahoogroups.com [mailto:BoundaryPoint@yahoogroups.com] > On Behalf Of aletheiak > Sent: Tuesday, January 03, 2006 4:46 PM > To: BoundaryPoint@yahoogroups.com > Subject: [BoundaryPoint] ok but why or what are we looking for > > > --- In BoundaryPoint@yahoogroups.com, "Hugh Wallis" <hugh@o...> wrote: > > > > Take a look here where the difference between WGS84 and various other > > "observations" is documented at various locations along the prime meridian > > > > http://tinyurl.com/76jv4 > > > > > > _____ > > > > From: BoundaryPoint@yahoogroups.com [mailto:BoundaryPoint@yahoogroups.com] > > On Behalf Of Roger McCutcheon > > Sent: Tuesday, January 03, 2006 6:20 AM > > To: BoundaryPoint@yahoogroups.com > > Subject: Re: [BoundaryPoint] Four pointer > > > > > > I went to maximum magnification on Google Earth and found that it did > indeed > > show zero longitude as being about three quarters of the way to the far > side > > of Black Heath Avenue, to the east of the Zero Meridian in the Royal > > Observatory, so then I went to the Prime Meridian site and found that "the > > zero meridian on the WSG84 datum, which is about 100 metres to the east of > > the line marked at Greenwich, is an average of the various continental > > movements", so we need not worry: someone is paying attention! Roger > > McCutcheon.