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.
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.
>
>
>
>
>
>
>
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