[Proj] using an NTv2 grid to apply a slight shift to coordinates in a metric CS

[Andre Joost]

Am 03.01.2013 20:44, schrieb Maciej Sieczka:
> W dniu 03.01.2013 17:25, Andre Joost pisze:
>> the nadgrid shift is usually done with lat/lon coordinates. You
>> transform your cartesian coordiantes to lat/lon,
>
> EPSG:2174 is on Krassovsky/Pulkovo_1942_58. Do I convert my cartesian
> EPSG:2714 coordinates to latlon on WGS84 or Pulkovo_1942_58?

the lat/lon should be Pulkovo. Its like the constant +towgs84 value

+towgs84=33.4,-146.6,-76.3,-0.359,-0.053,0.844,-0.84

but two-dimensional. The grid-shift is usually between the Krassovsky 
ellipsoid and WGS84.


>
> What I don't know is the following:
>
>>> Let me put it this way - should my grid provide shifts between:
>>>
>>> a) the original EPSG:2174 and the EPSG:2174+shift
>>>
>>> or rather
>>>
>>> b) between WGS84 and the EPSG:2174+shift?
>>>

the second one.

Greetings,
André Joost