[Proj] Some details on German Army Grid (Re: MGRS invented by Germany?)

[Gabor TIMAR]

Dear Colleagues,

the cited German paper was interesting indeed, thanks to Charles for the 
reference. I requested the full text from the author via the 
ResearchGate (first author is registered there). Perhaps I'm allowed to 
share some details of the DHG system (and sorry, nothing about the 
similarities to MGRS here):

According to my best knowledge, the German DHG and the Russian 
Gauss-Krueger belts (the latter one MIGHT be a result of the pre-war 
cooperation between these empires) were the first transverse Mercator 
zone systems with 6 degrees zones. In the Russian system (used later 
also by the Warsaw Pact military), the zones were indicated by a 
'leading digit' in the Easting coords (e.g. in the zone '34', between 
18-24d longitude, the False Easting number was 4,500,000 instead of the 
standard 500,000 of the modern UTM - this digit '4' indicated the second 
digit of the zone number. And yes, the Russian maps of the Himalayas has 
False Easting of 14,500,000 ) In some German war maps, this leading 
digit was not used, while in the restored Hungarian war basepoint list, 
it was used. The earlier introduced German civilian grid used the 
3-degree wide zones, the same system that is used nowadays in Germany.

During the WWII, the Germans made a tremendous work, integrating many 
European networks and maps to their DHG system. A key person behind this 
operation was an Austrian expert, dr. Karl Ledersteger (Austria was 
annexed by Germany in 1938), and after the war, he published some 
interesting details about this in Vienna. If someone is interested 
(Cliff maybe?), I can send one in PDF, of course it is in German language.

The hardest part of this integration was to make a more or less unified 
geodetic network, in modern terms, a horizontal datum for the DHG. Many 
national basepoint systems were transformed, in the somewhat cooperating 
countries, like Bohemia (albeit they were occupied in 1939), Hungary, 
Latvia, it was carried out by local experts, while in Poland and Russia 
(where also Bessel-ellipsoid based datums were used prior to the WWII), 
they were more or less accepted with the same coordinates. The starting 
geodetic basis was the German (in fact, the old Prussian) Rauenberg 
datum, which was called after the unification RDN (ReichsDreiecksNetz) 
1940. According to my 2004 paper (sorry, it is in Hungarian, with a 
small English summary at the end), it is practically identical to the 
present-day German DHDN77 system within an error margin of a few meters. 
However, these errors must be higher in Poland and Russia, where the 
Borowa Góra 1925 and Pulkovo 1932 Bessel datums were used, respectively. 
It was even extended to Switzerland, which was extensively photographed 
by the German planes flying to and from Italy during the war.

The RDN1940 was the very first European continental geodetic datum, and 
the second worldwide, after the American NAD1927. I also made an 
accuracy check: the unified datums transformation parameters between the 
RDN1940 and the WGS84 provided a transformation accuracy along the 
vector from Budapest to Berlin in the same range (better than 5 meters) 
than inside Germany and inside Hungary, so the consistency of the datum 
was quite good. According to other works of dr. Ledersteger (who was, 
obviously, a key person in this computation, too ), these data were used 
as input of the ED50 works. Albeit Europe was cut into two by the iron 
curtain in these years, a Hungarian paper from 1953 reported about the 
(theoretical) extension of the ED50 to Hungary, based similarly on DHG 
(by Lajos Homoródi, professor of the technical university of Budapest).

My paper is available at:
http://www.fomi.hu/honlap/magyar/szaklap/2004/06/6.pdf
and since its host institute might be a subject of re-structuring, this 
permalink is in danger, I uploaded it to ResearchGate, too.

with kinds regards,
Gábor Timár
Eötvös University, Budapest, Hungary