[Proj] Error transformation EPSG:2175 <=>EPSG:2177

[Michal Baranski]

>> cs2cs +init=EPSG:2175 +to +init=EPSG:2177
> ...
>> Error over 100
> 
> 
> /usr/share/proj/espg's EPSG:2177 does not define a datum so no datum
> transform is applied.
>  
> specify one manually with +datum= or 3or7 term transform parms with
> +towgs84=,,,
>  
> http://trac.osgeo.org/proj/wiki/FAQ#WhydoIgetdifferentresultswith4.5.0and4.6.0
> Hamish

EPSG:2175 is defined Poland zone 5, EPSG:2177 is Poland CS2000 zone 6, 
it is not true,
when I add datum or +towgs84 then it will be true.

I am not an expert, I was surprised error.
Please help in determining the value datum or towgs84.
Best regards
Michal Baranski