[Proj] DHDN / Gauss Krueger to WGS84

[Mikael Rittri]

Hello Roger,

Actually, I do believe that E 21.982946, N 48.433278 is the correct
result, or at least more correct than E 11.446588, N 49.169494.

Here is how I convinced myself:

Since x = 4459750 m and the False Easting is 3500000,
the point is 4459750 - 3500000 = 959750 m east of the
central meridian which is 9° E. 

At the equator, there are about 111120 meters
per degree, so if we were at the equator we would 
be 959750 / 111120 = 8.64° east of the central
meridian.  But at the approximate latitude 49° N, the
meridians are closer together by a factor cos(49°),
so we are really about 8.64°/cos(49°) = 13.17° degrees
east of the central meridian, which is 22.17° east 
of Greenwich. 

This is of course just a rough calculation, ignoring
the changing local scale of the projection etc, but
I think it shows that the longitude should be nearer
22°E than 11°E. 

I don't know what's wrong with the original test example.

(For a while, I was thinking it had to do with the axis order,
 since in EPSG:31467, x should be northing and y easting.
 But 4459750 m cannot be a northing here, since it would
 be too far south, somewhere in south Italy.
 So I suppose your test example does use the mathematical
 convention after all, with x as easting and y as northing.)

Best regards,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com

-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Roger Oberholtzer
Sent: Friday, March 30, 2012 12:23 PM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] DHDN / Gauss Krueger to WGS84

Bad form to reply to my own post, but...

> > I would add that epsg:31467 expands as:
> > 
> > +proj=tmerc +lat_0=0 +lon_0=9 +k=1.000000 +x_0=3500000 +y_0=0 +ellps=bessel 
> > +datum=potsdam +units=m +no_defs

> > it seems to work. A reference point of:
> > 
> > x = 4459750 m  y = 5448182 m
> > 
> > is converted to
> > 
> > E11.446588 N49.169494
> 
> 
> If I use pj_transform(), I get:
>  
>   21.982946, 48.433278

I see that cs2cs gives the same incorrect result I get, and not the
expected result:

cs2cs +init=epsg:31467 +to +init=epsg:4326
4459750 5448182
21d58'58.607"E  48d25'59.803"N -15.126


Yours sincerely,

Roger Oberholtzer

OPQ Systems / Ramböll RST

Office: Int +46 10-615 60 20
Mobile: Int +46 70-815 1696
roger.oberholtzer at ramboll.se
________________________________________

Ramböll Sverige AB
Krukmakargatan 21
P.O. Box 17009
SE-104 62 Stockholm, Sweden
www.rambollrst.se


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