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

[Maciej Sieczka]

Hello,

I have a script which takes an x,y pair of metric coordinates in 
EPSG:2174 (http://www.spatialreference.org/ref/epsg/2174/proj4) and 
returns them slightly shifted, in the same coordinate system.

The difference between the input and output coordinates varies from 0 
to about 60 cm, non-systematicaly.

I would like to be able to "reproject" my GIS data from EPSG:2174 to 
EPSG:2714 + the shift described above, and the other way round; using 
GDAL/OGR, GRASS, QGIS reprojection tools.

Can anyone tell me if using a shift grid for that would make sense? I 
have a general idea how to create one from my scripts output, but there 
is something missing in my thinking. 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?

And what would be the proj4 string for the new system in either case?

Another thing is if it would be possible to utilise a similar grid to 
reproject between EPSG:2714+shift and other coordinate systems covering 
the same territory (e.g. EPSG:2180), again using GDAL etc. tools.

Ideally, I would like to achieve both goals with just single shift 
grid.

I'm afraid I might be asking my questions wrong way, but hopefully you 
get what I'm aiming at. I'd be grateful if you can correct my errors and 
give me some guidance here.

Regards,
Maciek

-- 
Maciej Sieczka
http://www.sieczka.org