I'd like to comment on this. Even at the present state of the art, you cannot expect

a hand held GPS to get you to an exact set of coordinates. It will get you close enough

so that you can locate a monument or marker, if there is one. Lacking some sort of

marker, all you can say is that you were very close to the point but you couldn't put your

finger on the exact spot.

As an example, the Idaho-Montana-Wyoming tri-point which is on the continental

divide in Yellowstone Park is in a remote area, but is quite easily located by hand

held GPS. With just map and compass you would have to be very lucky to find it.

I went there in 1997, which was prior to the removal of selective availability of the

GPS signals, but we got close enough to the marker to be able to see the witness

signs on surrounding trees. Now it should be relatively easy for anyone with a recent model GPS to find such a location using WGS84/NAD83 datum. Without the presence

of a marker though, it would be impossible to locate the exact point except with modern

surveying equipment. When this location was resurveyed in 1994 the large group of

surveyors got lost on their way in because the batteries went dead in their hand held

GPS. After searching for about two hours they found the marker and were able to

document that location with surveying grade GPS.

I guess my question is, why all the verbose nit-picking?

Jack Parsell

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.

No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.1.371 / Virus Database: 267.14.12/220 - Release Date: 1/3/2006