[Proj] datum does not change?

[Robert Jaeschke]

Hello, 

Why do I always get the same result? Changing the datum doesn't seem to
have an effect on the output? I expect something that is much nearer to 
"12d10'27.403"E  51d41'38.33"N" than the result below. 

Thank you for your help!

Robert Jaeschke

-------------------------------------------------------------------------

> invproj +proj=tmerc +lat_0=0.000000000 +lon_0=12.000000000 +k=1.000000 
    +x_0=4500000.000 +y_0=0.000 +ellps=bessel << EOF
> 4512157 5728865
> EOF
12d10'33.041"E  51d41'43.018"N

> invproj +proj=tmerc +lat_0=0.000000000 +lon_0=12.000000000 +k=1.000000 
    +x_0=4500000.000 +y_0=0.000 +ellps=bessel +datum=potsdam << EOF
> 4512157 5728865
> EOF

12d10'33.041"E  51d41'43.018"N

> invproj +proj=tmerc +lat_0=0.000000000 +lon_0=12.000000000 +k=1.000000 
    +x_0=4500000.000 +y_0=0.000 +ellps=bessel 
    +towgs84=591.28,81.35,396.39,1.477,-0.0736,-1.458,9.82 << EOF
> 4512157 5728865
> EOF
12d10'33.041"E  51d41'43.018"N

> invproj +proj=tmerc +lat_0=0.000000000 +lon_0=12.000000000 +k=1.000000 
    +x_0=4500000.000 +y_0=0.000 +ellps=bessel +towgs84=606,23,413 << EOF
> 4512157 5728865
> EOF
12d10'33.041"E  51d41'43.018"N

-------------------------------------------------------------------------
proj -V
Rel. 4.4.9, 29 Oct 2004

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