[Proj] Similar datum transforms for MGI differ too much.

[Mikael Rittri]

Hello again, 
I think I have had a flash of insight, but I don't have any hard evidence.
So I hope someone can confirm my theory, which is as follows: 

<theory>

The MGI datum, used in Austria and former Yugoslavia, was originally 
based on the Ferro meridian, not Greenwich.  This is the datum that 
EPSG calls "MGI (Ferro)", code 6805. 

When the mapmakers in Austria and Yugoslavia decided to adapt MGI 
to the Greenwich meridian (to get "MGI", EPSG code 6312), they used 
different values for the longitude of Ferro.  In Austria, they assumed 
that Ferro was at 
    
    17° 40' 00" West of Greenwich,

which is the value given by EPSG. But in Yugoslavia, they assumed that Ferro was at 

    17° 39' 46" West of Greenwich,

which is the value given Table 4.1, http://www.map-reading.com/ch4-1.php . 
This would mean that, although the MGI (Ferro) graticule is continuous, the
MGI (Greenwich) graticule is discontinuous at the Austrian-Sloven border.

In other words, MGI (Greenwich) is not a proper datum.  We should talk 
of Austrian MGI (Greenwich) and Yugoslavian MGI (Greenwich) as separate datums.

The results of my tests showed a difference in the MGI transforms for Austria 
and Slovenia, and I said that the difference was 299.7 meters, almost exactly 
east-west.  But expressed in longitude degrees, the difference is 14.092 arc seconds, 
which corresponds almost exactly with the 14" difference between the two Ferro longitudes.
(The remaining 0.092 arc seconds corresponds to 2 meters at this latitude, which
 can be explained by the limited accuracy of the datum transforms.)  

</theory>

So, can anyone confirm this theory? 

--
Mikael Rittri
Carmenta AB
SWEDEN
www.carmenta.com

-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Mikael Rittri
Sent: den 18 februari 2009 11:48
To: proj at lists.maptools.org
Subject: [Proj] Similar datum transforms for MGI differ too much.

Hello,
here is small mystery.  
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