[Proj] large difference while projecting utm to utm back and forth

[Noel Zinn (cc)]

I agree for what may be the "basic" TM in Proj.4, however Proj.4 does have 
more than one of the Scandinavian implementations of TM that are robust far 
from the TM.  Search for and use one of those and you'll get better 
round-trip closure.  -Noel

Noel Zinn, Principal, Hydrometronics LLC
+1-832-539-1472 (office), +1-281-221-0051 (cell)
noel.zinn at hydrometronics.com (email)
http://www.hydrometronics.com (website)

-----Original Message----- 
From: Mikhail Tchernychev
Sent: Wednesday, February 16, 2011 11:13 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] large difference while projecting utm to utm back and 
forth

  Hello Christoph,

I might be wrong but I'm afraid it is not simply possible. In my
experience with Transverse Mercator
formulas (not with cs2cs)  they are ill-posed when you get far from the
central meridian.
In your case you have 5 zones difference, which is 30 degrees.  In other
words transformation
is just not defined when you get so far.

It would work if you have points at the border of the zones, say 30 and
31, but
35 is way far.

Best Regards,
Mikhail

On 2/16/2011 5:19 AM, Christoph Dohmen wrote:
> Dear list,
>
> I know, I should not do it but I have to :-(
> Given is a point 600000 east and 8260000 north in utm zone 35.
> I have to project this location to utm zone 30.
> Doing some calculations I have to project this value back from
> zone 30 to zone 35.
> But I get differences of about 3km east and more than 200m north.
>
> Those are the steps I made using cs2cs:
> cs2cs +proj=utm +zone=35 +datum=WGS84 +to +proj=utm +zone=30 +datum=WGS84
> 600000 8260000
> 1451785.04      8532758.68 0.00
>
> and the way back:
> cs2cs +proj=utm +zone=30 +datum=WGS84 +to +proj=utm +zone=35 +datum=WGS84
> 1451785.04      8532758.68
> 597050.49       8260246.21 0.00
>
> Can anyone describe a way to achive the startvalues?
>
> Best regards
>
> Christoph
>
> P.S.
> I use proj 4.7.1 and the latest from svn.

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