violent what
hahaha
all you need to dispel the misconception that datum doesnt matter is to actually &
preferably nonviolently find a tripoint where the datum really would make a difference
between a hit or a miss
we dont yet actually know of one
it is yours to demonstrate
i will try to help you if i can
since i would love for you to be right
but its your thesis
not mine
--- In BoundaryPoint@yahoogroups.com, "Hugh Wallis" <hugh@o...> wrote:
>
> I think we are in somewhat violent agreement. But, as I mentioned, some
> datums (e.g. Tokyo datum) can be up to 200m off from WGS84 which could put
> you too far from finding a physical marker - possibly the wrong side of a
> river or cliff face. NAD83 and WGS84 are pretty darn close to each other so
> it is unlikely to ever become an issue in the USA. (but the world is larger
> than the USA).
>
> >>why all the verbose nit-picking?<< Someone chose to question whether
> datums have relevance.
>
> >>it would be impossible to locate the exact point except with modern
> surveying equipment<< - surveyors using such modern equipment would still
> need to be aware of datums when making their calculations as would the
> people who placed any marker there in the first place. You have to have SOME
> frame of reference (which is essentially what a datum is). It is just that
> with the universal availability of GPS technology people have started to
> think that datums are only relevant to the use of GPSrs which is a
> misconception that I would think the readers of this list might benefit from
> having dispelled.
>
>
> _____
>
> From: BoundaryPoint@yahoogroups.com [mailto:BoundaryPoint@yahoogroups.com]
> On Behalf Of aletheia kallos
> Sent: Tuesday, January 03, 2006 11:17 PM
> To: BoundaryPoint@yahoogroups.com
> Subject: Re: [BoundaryPoint] Re: ok but why or what are we looking for
>
>
> exactly my point jack
> thanx
> & i think either datum would do it there at idmtwy
> today too
>
> --- Jack Parsell <jparsell@t...> 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.
> >
> >
> >
> >
> >
> >
> >
>
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