tmerc-coordinate system

[Gerald I. Evenden]

>From: Martin Ameskamp <ma at informatik.uni-kiel.d400.de>
>To: grassu-list at max.cecer.army.mil
>Subject: Re: tmerc-coordinate system
>
	... material deleted and covered in private mail
>
>	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).

Here is where I get confused and do not understand what you are saying.
I feel you are asking for a transformation from one carto system to
another.  That is absolutely no problem and done all the time---including
intermediate transformation through geographic coordinates.  And it
can be done accurately.  I do not understand what you mean by
"spurious results," etc.

Secondly, I presume that you are using GK as the equivalent of generic
Transverse Mercator and not the Easter Block GK system.  If so, then:

proj -I +proj=tmerc +ellps= +k_0= +lat_0= +x_0= +y_0= ... -o | \
	proj +proj=utm +ellps= +zone= -i

translates a TM (aka Gauss-Kruger) system x-y coordinates to UTM x-y.

>	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.)

Gerald (Jerry) I. Evenden   Internet: gie at charon.er.usgs.gov
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