>>because the far greater component of any datum shift
displacement
both potentially & typically i believe
is in the longitude rather
than the latitude
<<
At http://www.colorado.edu/geography/gcraft/notes/datum/edlist.html (a.k.a.
http://tinyurl.com/8sexy - a fortunate
coincidence - easy to remember at any rate - hope it makes it through eveyone's
spam filter) you can see the datum shift from Local to WGS84 for many
datums for long and lat in the dx and dy columns (in metres). While the
above assertion is true in some cases, if we look at the datums that have
typically been used for Egypt (European 1950 giving dx = -130 and dy = -117, Old
Egyptian 1907 giving dx = -130 and dy = 110), the lat and long displacements are
not really "far greater" although they are greater. If we look at those
typically used for Sudan (Adindan gving either dx = -166 and dy = -15 or dx =
-161 and dy = -14) then they are, however. Libya does not appear in the table.
In some parts of the word the shift difference is the other
way around - e.g. Indonesia and much of South East Asia, some users of the
European 1950 datum such as Spain and Portugal, and various remote islands,
almost all users of NAD27 i.e., Canada, USA, Central America, much of Eastern
Europe using S-42.
It seems as though a general statement cannot be made
therefore and that every datum comparison needs to be done
independently.
On Brownlie - if anyone has access to a copy, some relevant
references are found at http://arabworld.nitle.org/texts.php?module_id=3&reading_id=119&sequence=6 (a.k.a.
http://tinyurl.com/b89pu )
summarised therin thus: "Libya's eastern borders result from agreements between Egypt and Italy in
1925 and 1926, which superceded (sic) an earlier arrangement between the Ottoman empire and
Egypt in 1841, and from agreements in 1934 between Britain and Egypt (as the
condiminium powers in Sudan) on the one hand and Italy on the other. The latter
agreement transferred the Sarra triangle to Libya - the territory allocated to
Sudan under the 1899 convention and lying to the south of the 22N parallel
(Brownlie 1979; 102-109, 133-140).". This secondary (or is it
tertiary) source is quite likely accurate, but, of course, may not be complete
(which is what you are hoping I guess).
thanxx
i appreciate the question no less than the
critical
review behind it
but first
with regard to the egsd
signs
all indications thus far are that they date from the
militarily
tense early to mid 1990s period
so their accuracy was very likely limited
by selective
availability in any case
& regarding egly
yes
indeed i could & probably should ask this just to
rule it out with
greater certainty
tho i have already given my reasons for doubting
there
is a bilaterally official datum even there
in my previous
post
to which i should now add that the elusiveness or
formlessness of
all those so hopefully lovely egly
border arcs
which arent at all
discernible as arcs on any maps i
have yet seen
is just one more
indication of the crudeness & hence
datumlessness of the 1927 egly
demarcation survey
& yikes reason even to doubt that the
commissioners
would have been proud enough of their handiwork in
the
wilderness to even plat it out
or worry about it in any other way
much beyond
degmindec or degmindodec
let alone degminsec
etc
but in the meantime i also realized that tho my
proposed 487
mile extrapolation from the nearest egly
rock would nail the longitude of
eglysd with as much
precision as that rock itself had already been
nailed
with
it still wouldnt at all necessarily nail the latitude
of
eglysd any better than we already have it
22nd parallel
hahahahaha
ahhh
nevertheless i believe this will
prove to be a smaller
shortcoming than one might initially think
because
the far greater component of any datum shift
displacement
both
potentially & typically i believe
is in the longitude rather than the
latitude
since latitude is first a sidereal fact while
longitude is only
a terrestrial & far more subjective
one
meaning all possible
versions of the 22nd or any other
parallel will bunch up much closer
together generally
than will all possible versions of the 25th or
any
other meridian
for example the example i think we may have
begun with
azconmut shifting between nad27 & nad83
has 59 meter
longitudinal displacement & only 2 meter
latitudinal
which btw per
pythagoras means more than 99point94
percent of the distance of any shift
there is
contributed by the longitudal portion of the
shift
alone
so our range of possible latitude values here
at
eglysd should be a correspondingly small fraction of
the distance
from the 25th meridian of the
southernmost egly rock
in whatever datum
it is bagged & or expressed
which means
using say pilotage gps
on the southernmost egly rock
we would get the correct submeter wide ribbon
of
longitude for eglysd in any datum
& but only the correct say
meter or at most several
meter broad ribbon of latitude for it
&
thus using commercial gps any datum shift in the
latitude would most likely
be imperceptible
& tho i thought of this saving grace shortly
before
falling asleep last night
the confirmation of it came to me in a
dream
from which i awoke laughing out loud
in the dream i had just
reached eglysd by car & found
a border guard there with his young son
who was
operating a walkie talkie
of course the guard asked me to
open my trunk
which i did
& inside it
much to my surprise
was
his other son
an identical twin for all i could tell
talking back to
them on another walkie talkie
hahahahaha
& smiling &
shaking my head in amazement ever since
can you dig it
but
anyway
where do i think we are now
i think
we are beginning to
think mohamed wont answer perhaps
because he doesnt know
& we are
looking for brownlie just to check as much as
we can of our guesswork to
date
as well as to get a better fix on any potential
source
docs
but actually
it just occurs to me too
since this
southernmost egly rock
which we are trying to hang our hat on
is so
beguilingly close to the 29x25 project
intersection point
why dont we
just sponsor a fellowship for some
deserving confluencer to bag it for
us
--- "Lowell G. McManus" <lgm@...> wrote:
>
While you're corresponding with them, couldn't you
> just ask if there
is an
> official datum for the delimited but undemarcated
>
portions of the EGLY boundary
> and what datum they used when erecting
the
> (presumably) unilateral signs on
> EGSD? Wouldn't
that beat trying to extrapolate from
> afar?
>
> Lowell G.
McManus
> Leesville, Louisiana, USA
>
>
> -----
Original Message -----
> From: "aletheiak"
<aletheiak@...>
> To:
<BoundaryPoint@yahoogroups.com>
> Sent: Tuesday, January 10, 2006
9:36 PM
> Subject: [BoundaryPoint] Re: but why suppose an
> eglysd
datum has ever been
> stated or even can be synthesized
>
>
> > aha & far better
> > we just ask the egypt
border force if it happens
> to have gps era survey data
> >
on this
> > southernmost egly marker at any level of
>
exactitude
> > & extrapolate eglysd from it
> >
aha
> > of course
>
>
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