tmerc-coordinate system

[Martin Ameskamp]

> >I suppose you really mean transverse Mercator (e.g. Gauss-Krueger) as
> >opposed to Universal TM. We simply use UTM and pretend it's GK, but then
> >we don't have to do conversions from GK to UTM or v.v. ...

> >Martin Ameskamp, Inst. f. Informatik I (Computing Dept.)
> 
> Whoa!  Stop!  Transverse Mercator, Universal Transverse Mercator (UTM)
> and GK (USSR definition) are three different animals.  Transverse
> Mercator is the general case where the user is free to determine
> ellipsoid-sphere, central merdian, central scale factor and false
> easting-northings.  UTM is restricted to specific central meridians,
> latitude range, ellipsoids only (type dependent upon local tradition),
> scale factor of 0.9996, and fixed false eastings and northings.  GK
> follows UTM except for scale factor of 1.0 and different zone
> nomenclature and false easting may have a zone-based offset.
> 
> Thus one CANNOT "pretend" that UTM and GK are equivalent.
> 
> I am sorry, but I get apoplectic when people get loose with their
> definitions and usage of these systems.
	
	I am sorry if I caused apoplectic fits and the like (may you recover
	speedily...). Thanks for the clarifications.

	However, I did not say anything about pretending that GK and UTM
	are equivalent (see quote above). Maps in Germany use both systems
	(e.g. GK for 1:5000, UTM for 1:100000) which can be a bit of
	a nuisance if you have to use both scales.

	Now GRASS doesn't offer GK as an option when you create a location,
	so what we do is create a UTM region and enter GK coordinates as
	UTM coordinates. 

	It's fairly obvious that things like conversions to latitude/
	longitude coordinates would give spurious results and the scale
	factor you mention seems to indicate that area/distance measurements
	can be slightly inaccurate (negligible in our (mainly ecological)
	applications).

	Are there any other reasons why this 'method' should not be used?
	I'll admit that it's not a very clean way of doing things, but
	in applications that don't require great accuracy it seems a
	sensible thing to do.

	Martin
--
Martin Ameskamp, Inst. f. Informatik I (Computing Dept.)
Kiel University, Olshausenstr. 40, 24118 Kiel, Germany
Fax: ++49 431 8804054, Voice: ++49 431 8804474, 
email: ma at informatik.uni-kiel.d400.de