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

[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?

> apply the shift, and transform into another grid.
>
> The syntax of the proj string is: +proj=tmerc +lat_0=0 +lon_0=9
> +x_0=3500000 +y_0=0 +k=1.000000 +ellps=bessel +units=m
> +nadgrids=path/to/BETA2007.gsb +wktext
>
> The grid file has to be in binary format, which can be transformed
> from an ASCII file.

I more or less know how to create a NTv2 grid (from threads
http://lists.maptools.org/pipermail/proj/2011-April/005628.html,
http://lists.maptools.org/pipermail/proj/2012-February/006093.html, and
the links mentioned in them both).

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

Maciek

> Am 03.01.2013 15:03, schrieb 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.

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