[Proj] Help with use of swisstopo NTv2 grid shift ?

Sandro Santilli strk at keybit.net
Wed Jan 25 05:24:06 EST 2012

On Wed, Jan 25, 2012 at 09:54:49AM +0000, Mikael Rittri wrote:

> I don't see that you do anything wrong with the 'trip' calculation. 

Good to know I'm on the right track !

> You say you get a mismatch of about 7e-07, which I assume is degrees,
> so that's about 7 or 8 cm.  

Right. That's what I had.

> I have tried to reproduce your results, but my mismatches were ten times
> smaller, or at most 8 mm. 
> Anyway, here is how I made my computations:
> First, convert the five test points from CH1903 / LV03 to CH1903 LongLat:
> >proj +init=epsg:21781 -f "%.9f" -I
> 602030.680 191775.030
> 7.466225970     46.878408135

Sorry about my ignorance: where does the commandline above say you're
converting to CH1903 (epsg:4149) rather than WGS84 ?

It could be I found my drift ? See this:

cs2cs +init=epsg:21781 +to +proj=longlat +ellps=bessel +towgs84=674.4,15.1,405.3,0,0,0,0 +no_defs -f "%.9f"
602030.680 191775.030
7.466225442     46.878408624 0.012045878

The destination projection above is epsg:4149

So do you confirm your multi-step would be as follows ?

 (1) convert CH1903/LV03 to WGS84 using epsg:21781
 (2) apply NTv2 on the result of step 1 (get CH1903+)
 (3) convert CH1903+ back to CH1903+/LV95 using epsg:2056


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