Re: MI Mysterious coordinate system

[Cliff Mugnier - University of New Orleans]

Alex Eshed wrote:
> 
> Season's Greetings, List.
> 
> Here in Israel the use of WGS 84, Zone 36 Northern Hemisphere,
> is pretty common. Digitized maps (from paper maps) show a high
> level of accuracy compared to ground surveying data in this
> projection. However, GPS (differential) readings are off by a few
> dozen meters.
> 
> But... when using a projection called UTM ED 50 the GPS readings
> are far more accurate.
> 
> Can anyone explain to me what's happening? And what is UTM ED
> 50 anyway?
> 
> TIA to all responders.
> 
> Best regards,
> Alex Eshed
> Digi-Tek Ltd.
> 12, Homa St., Rishon LeZion, 75655, Israel
> Tel: +972-3-961-5840
> Fax: +972-3-961-5877
> mailto:[EMAIL PROTECTED]
> http://www.dgtek.com
>

Alex,

Here's the explanation for your question in as short a treatise as I can make
it:

A "Datum" is a system of Latitude and Longitude that is traditionally
established by individual countries. In Israel, the traditional system is the
Palestine Datum of 1928.

After WWII, the U.S. Army Map Service (AMS) decided that it was going to "unify"
all of the individual Datums that had connections with each other from classical
surveying observations.  That new "Datum" was going to be started in 1950 in
Europe. In particular, France was the first country to be "converted" to the
European Datum 1950 (EU50).  Each country in Western Europe was connected to
EU50 through their connections to France, then Scandanavia was connected in the
Northern European Block, and Institue Geographique National (IGN) in France was
contracted to do the connection computations in Northwest Africa (through AMS
computations in Spain and Portugal).  After the Danube region of Europe was
adjusted and connected with Western Europe, the chains of triangulation into
Greece and Turkey were computed on EU50, and then the Palestine Datum was
recomputed onto EU50 in the process of bringing EU50 to Egypt and connection
with EU50 in Algeria as computed by the French IGN.

These computations were done with the use of the Universal Transverse Mercator
(UTM) Grid which is composed of 60 zones, each 6 degrees wide in longitude.  UTM
is a Grid, and is independant of ellipsoids and Datums.  UTM is based on the
Transverse Mercator projection, and the mathematical formulae are specifically
called the Gauss-Kruger model.

For further information on this stuff, I publish a monthly column on the topic
of "Grids and Datums" (each month is the history of all systems, new and old,
existing in a particular country).  I publish in "Photogrammetric Engineering
and Remote Sensing," the official journal of the American Society for
Photogrammetry and Remote Sensing.

You can download some of my past "Grids and Datums" columns from:

http://www.asprs.org/resources.html

The files are in Adobe Acrobat PDF format, and there is a link at that page
where you can download a free copy of Acrobat to read and print each copyrighted
article.

There's ten different countries on the list at the moment, two discuss some of
the history I just alluded to above.  See specifically my column from June, 1998
on the Kingdom of Morrocco, and my column from October, 1998 on the Kingdom of
Belgium.  (There's lots of other places, but that's what's available on EU50
stuff.  I did one a few months ago on the Kingdom of Norway, but the Society
does not have that one available as a "freebie."

It ain't a mystery; it's just remarkably obscure stuff.

One of the world's foremost authorities on Map Projections is Prof. Ron K. Adler
who teaches at several campuses (campii?), including in Tel Aviv.

Happy Holidays to you too!

-- 
Clifford J. Mugnier ([EMAIL PROTECTED])
The Topographic Engineering Laboratory
Department of Civil and Environmental Engineering
UNIVERSITY OF NEW ORLEANS
New Orleans, Louisiana  70148

Voice and Facsimile: (504) 280-7095
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