--- In
BoundaryPoint@yahoogroups.com, "Dave Patton [DCP]"
<dpatton@c...> wrote:
> At 09:27 AM 2004/07/27, aletheiak wrote:
> >--- In BoundaryPoint@yahoogroups.com, "Dave Patton [DCP]"
> ><dpatton@c...> wrote:
> > > > > I've posted the narrative and photos detailing my
> > > > > recent visit to the British Columbia/Idaho/Washington
> > > > > (BCIDWA) tripoint and 49N 117W degree confluence on
> > > > > the Degree Confluence Project website:
> > > > >
> > >
> >
>http://www.confluence.org/confluence.php?lat=49&lon=-117&vi
sit=4
>
> >& this double check only substantiates further the hypothetical
2
> >or 3 meter offset
>
> What method/software are you using for your calculations?
i dont know how it is done but i just use this online converter
http://www.ngs.noaa.gov/cgi-bin/nadcon.prl
which is presumably to nad27 conus rather than nad27 canada
but please see below
> > > I have sent an email to the IBC asking what version of
NAD27
> > > they have used, but haven't yet received an answer.
> > > I assumed NAD27 CONUS.
> >
> >hm
> >please explain if & how this might affect the accuracy range
>
> Because unless you know the datum used to express
> coordinates, you don't really know the "exact" location.
>
> There is, for example, a difference in the 3-parameter
> transformation parameters for NAD27 Canada(Alberta/BC)
> and NAD27 CONUS. Whether this has a measurable effect
> on the possible distance between the line connecting
> boundary markers 194 and 195, and BCIDWA, I don't know.
right
i understand
& thats just what i was wondering
according to this page
http://www.sarinfo.bc.ca/Library/Navigation/datumcon.nav
a shift from
nad27 canada
to
nad27 conus
in this particular geographical area
would cause an apparent southing of 5 meters
& this apparently means
oddly enough
that our hypothetical offset of 2 or 3 meters too far north
could become a hypothetical 2 or 3 meters too far south
so in a sense a difference of datum would make not much
quantitative difference in the offset here
but only a difference in its algebraic sign
yikes
what a freaky coincidence
still i am sitting on the edge of my chair waiting to learn how the
ibc will answer your question
in order to judge which if indeed either of these 2 essentially
mirror images of the supposedly true bcidwa is the really true
one
& then of course
once we know this for certain
we could use the last significant digits of whatever we have
which will probably be hundredths of seconds
or roughly a 1 foot square target
regardless of which datum they are all correctly expressed in
to actually compute the truest available estimated position in
relation to the disk center
& you may well have visited this point class c as in see
or touched it class b as in be
since it now appears to be only about 8 feet from the disk
one way or the other
& you may also have framed it in one of your pix
so i think we really are getting somewhere
both toward finding the trupoint & classifying your visit
>
>
> --
> Dave Patton
> Canadian Coordinator, Degree Confluence Project
> http://www.confluence.org/
> My website: http://members.shaw.ca/davepatton/