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

[Mikhail Tchernychev]

  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.