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

[support.mn at elisanet.fi]

Hello,

I get about the same differences using xCalc.

lon=degr(14,59,42.65)=14.99518056
lat=degr(46,38,58.722)=46.64964500
lon2=degr(14,59,56.742)=14.99909500
lat2=degr(46,38,58.686)=46.64963500

d=dist(lat2,lon2,lat,lon)=298.59269980 meters

c=cog(lat2,lon2,lat,lon)=270.21464752 degrees (almost exactly west)
c1=cog(lat,lon,lat2,lon2)=90.21180106 degrees (almost exactly east) 

Regards: Janne

---------------------------------------

Mikael Rittri [Mikael.Rittri at carmenta.com] kirjoitti: 
> 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.maptoolsorg] 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.  
> [-- see rest of post at http://lists.maptools.org/pipermail/proj/2009-February/004397.html --]
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