exactly my point jack
thanx
& i think either datum would do it there at idmtwy
today too
--- Jack Parsell <
jparsell@...> wrote:
> 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.
>
>
>
>
>
>
>
------------------------------------------------------------------------------
> YAHOO! GROUPS LINKS
>
> a.. Visit your group "BoundaryPoint" on the
> web.
>
> b.. To unsubscribe from this group, send an
> email to:
> BoundaryPoint-unsubscribe@yahoogroups.com
>
> c.. Your use of Yahoo! Groups is subject to the
> Yahoo! Terms of Service.
>
>
>
------------------------------------------------------------------------------
>
>
>
>
>
------------------------------------------------------------------------------
>
>
> 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
>
__________________________________________
Yahoo! DSL Something to write home about.
Just $16.99/mo. or less.
dsl.yahoo.com