[Proj] Cadastral Grid - Slovenia

[Mikael Rittri]

Hello,
Irwin Scollar wrote:
> Given the recent discussion concerning the change of longitude for 
> Ferro in the Austrian grid systems before and after World War One,
> can anyone tell me if this change was applied to what is now Slovenia
> or if the older Albrecht value is still used, or if a completely 
> different grid is currently in force?
> 
> Irwin Scollar 

(The recent discussion can be found in the Proj.4 February archive, 
 see http://lists.maptools.org/pipermail/proj/2009-February/004415.html )

This issue has become EPSG Change Request 2009.015, which can 
be read at http://www.epsg-registry.org (choose "query by filter", 
and let the Type be "Change Request"). 

Roger Lott of EPSG has been investigating this, but all details 
are not clear yet, and the OGP geodesy subcommittee has not made
any formal decision.  

However, Mr. Lott has been kind enough to keep me informed, and 
has allowed me to post a tentative summary here. So remember:

1) These are not official EPSG opinions, 
2) There is a risk that I have misunderstood some details. 

Anyway:

My February post (see link above) was basically correct.
That is, the MGI (Ferro) datum covered both Austria and former 
Yugoslavia, but when the countries switched to the Greenwich 
meridian, Austria assumed that Ferro was 17° 40' 00" west of 
Greenwich, while Yugoslavia assumed it was 17° 39' 46.02" west 
(the Albrecht value). 
    So, when using MGI with Greenwich, one must distinguish 
between MGI (Greenwich, Austria) and MGI (Greenwich, Yugoslavia),
since longitudes differ by 13.98", about 300 meters. The MGI datum 
was finalised in 1901, but the switch to Greenwich occurred later 
(in the 1920s?). 

A new adjustment was made in Yugoslavia in 1948.  This datum is 
known as D48, at least in Slovenia, and it was (probably) designed 
to be a minor improvement, in the sense that some distortion was 
removed, but that coordinates are not changed more than a couple 
of meters (compared to MGI 1901).  This D48 system was definitely 
adopted in Slovenia, but is hasn't been confirmed that it was adopted 
in all of Yugoslavia, although it is probable. 

Slovenia has later made a densification of the European ETRS89 datum;
this is known as D96 in Slovenia, or "Slovenia Geodetic Datum 1996" 
in the EPSG database (EPSG:6765). 

Slovenia is currently working on a new and improved densification of ETRS89,
see http://www.ec-gis.org/Workshops/inspire_2008/presentations/11_4_mozetic.pdf 

And here is a site with a lot of Slovene 7-parameter transformations between D48 and D96:
http://prostor.gov.si/vstop/sistem_zbirk_prostorskih_podatkov/drzavni_koordinatni_sistem/horizontalni_drzavni_koordinatni_sistem_d96tm/d96tm/transformacijski_parametri/ 
All these transformations are also in the current EPSG database, I think. 
But EPSG describes their Areas of Use in words; on this site you can see 
maps as well. 

I think the datum transformations currently published by EPSG from "MGI" 
to WGS84 will work fine, if you respect their Area of Use.  Just remember 
that if the Area of Use is in Austria, then "MGI" is really the Austrian MGI.  
And if the Area of Use is in former Yugoslavia, then "MGI" is really 
the Yugoslavian MGI (or possibly the 1948 version of the Yugoslavian MGI). 

Best regards, 
--
Mikael Rittri
Carmenta AB
SWEDEN
www.carmenta.com