[Proj] [Newbie] Conversion from wgs84 to tmerc

[Clifford J Mugnier]

Roland,

It's rather difficult to answer such an all-encompassing question regarding
"how to transform everything to WGS84."  The U.S. Military has NOT done it,
and you're not likely to do it either, unless you are addressing a very
loose accuracy criterion for thematic mapping purposes.  For those areas of
the world that the U.S. Military & NGA needs to "go in," they do not
normally rely on existing maps but use National Technical Means to map the
area of interest with new brand-new imagery that is already 'on' WGS84.

That's why the TR 8350.2 document (still available for download from NGA)
is now considered obsolete and is not a document used for operational
readiness surveying and mapping.  You can use that document with its tables
of transformation parameters for individual countries and regions, but the
accuracy of the possible transformations depend largely on the quality of
the classical geodetic networks surveyed before WWII with theodolites and
invar chains and wires.  (Not anywhere as good as GPS does nowadays.)  The
tables of transformation parameters in TR 8350.2 are derived from empirical
observations of existing triangulation stations in various parts of the
world.  The result is a piecemeal solution with discontinuites across
classical triangulation chains, it is based on a two-dimensional Helmert
transformation on the complex plane from all the local native datums and
grids of Europe before WWII to the European Datum of 1950.  The accuracy is
good enough for combat operations in hitting targets within the blast
radius of indirect artillery fire using 1:50,000 scale topographic military
maps.  It is NOT better than that.  It is NOT good enough for cruise
missile targeting or GPS-guided munitions.  That's why TR 8350.2 is
obsolete.

The EPSG/OPG/APSG tables are similar in geodetic provenance and reliablity.

For thematic mapping objectives, I suppose what you want to do is possible,
but will definitely be a grand pain in the behind to implement.  (The U.S.
and NATO militaries gave up trying it.)  For large-scale geodetic
applications or cadastral applications, it is impossible without your own
fleet of low-orbit satellites with digital cameras and a network of
download Earth stations.

Sorry, but it's easier said than done.

Clifford J. Mugnier, C.P., C.M.S.
National Director (2006-2008),
Photogrammetric Applications Division
American Society for Photogrammetry and Remote Sensing
and
Chief of Geodesy,
CENTER FOR GEOINFORMATICS
Department of Civil Engineering
LOUISIANA STATE UNIVERSITY
Baton Rouge, LA 70803
Voice and Facsimile: (225) 578-8536 [Academic]
Voice and Facsimile: (225) 578-4474 [Research]
================================
http://www.ASPRS.org/resources/GRIDS
http://appl003.lsu.edu/eng/ceeweb.nsf/$Content/Mugnier
================================



Hi folks,

I'd like to convert wgs84 coordinates from all over Europe to tmerc
(Gauss-Krueger).

According to David Vaz' posting "cs2cs how to use" (posted on Feb. 15,
2006, [1]) it is not that easy to convert?!

> You cannot go directly from WGS84 Datum coordinates to a projection
(tmerc)
> on another Datum without FIRST performing a Datum Shift.


Searching the web, I found the following information [2]:

> I got the same problem as Harald Wehr when tranforming from wgs84 to
> Gauss-Krueger (zone 4).
> I use cs2cs as follow
> cs2cs +proj=latlong +datum=WGS84 +to +proj=tmerc +ellps=bessel
> +lat_0=0 +lon_0=12 +x_0=4500000 +towgs84=583,68,399

Presumably, this applies (to a certain part of) Germany!?

My questions are:
how can I determine lon_0 (central meridian) and x_0 (left shift) for
many different locations in Europe? All I have are the geographic
coordinates and the geographic coordinate system. E.g.:

ID|GeographicCoordinateSystem|Longitude|Latitude|CountryID
1|"WGS84"|9.183132|53.904484|DE
2|"WGS84"|6.764922|38.33335|ES


[1] http://lists.maptools.org/pipermail/proj/2006-February/002100.html
[2] http://lists.maptools.org/pipermail/proj/2002-June/000378.html
[3] http://www.grass-gis.de/lit_html/grasshandbuch_v12/node172.html
(german)

Any hint appreciated.

TIA,
Roland R.