[Proj] Using EPSG codes with cs2cs (the one who asked: Amro_rt_rps)

[Mikael Rittri]

Mohammad,

I'm starting to think that the long/lat of your black-box tool is not WGS84 long/lat, but DHDN long/lat.

The DHDN long/lat has EPSG code 4314, and DHDN / 3-degree Gauss-Kruger zone 4
has EPSG code 31468, and I get:

> cs2cs +init=epsg:4314 +to +init=epsg:31468

13.755944 51.0545555

4623099.53      5659041.38 0.00

Here, the default datum shift that Proj.4 uses for DHDN is not important, since both
the source and target uses the DHDN datum. The result is only 1.46 m from your
tool result. Close, but no cigar. At least you can use a smaller Finagle's constant ;-)
("That quantity which, when multiplied by, divided by, added to, or subtracted from the
answer which you got, yields the answer you should have gotten.")

I don't know if DHDN long/lat is often used for German GIS data. The analogous thing
in Sweden would be RT90 long/lat, a format that you never see for GIS data, at least not
from the Swedish Land Survey. But the practice in Germany may be different, of course.

> ... until as you said the 2012 new modifications come to Proj.4.

I don't think they will help you. I think they are identical to the current definitions, except
in the axis order and axis abbreviations. This is important in certain GIS standards from
OGC for transmitting coordinates on the web, but not for you. (Since your problem is not
that you have confused northing with easting.)

> One last question to all, how do I revers the conversion from X/Y Cartesian to Long/Lat Geographical?

When you use cs2cs on the command line, the "left hand side" is everything
between cs2cs and +to, while the "right hand side" is everything after +to.
Just exchange the left and right hand sides to reverse the direction.

Best regards,

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

________________________________
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Mohammad Amro
Sent: Wednesday, May 09, 2012 2:06 PM
To: proj at lists.maptools.org
Subject: Re: [Proj] Using EPSG codes with cs2cs (the one who asked: Amro_rt_rps)

Hi all,

[ - text deleted - ]

... may be there is no better way to do it than offsetting the different and be happy with that. May be the tool I am using (closed box) is following different methods for conversion?!

e.g., Long:  13.755944, Lat: 51.0545555

Tools coordinates:
4623100    5659040

7-parameters (X is shifted: 118 m, and Y is shifted 140 m)::
4623218.84    5659181.12

BETA2007 Datum shifts (X is shifted: 120 m, and Y is shifted 140 m):
4623220.44    5659181.52

Anyway, I think, as I am doing this conversion on a small scale area, then a linear offset (-118m, -140m) would result in <±2 meters which is the best I can go with until as you said the 2012 new modifications come to Proj.4.

One last question to all, how do I revers the conversion from X/Y Cartesian to Long/Lat Geographical?

A huge thanks you,
Mohammad Amro

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