[Proj] +towgs84 approximation error

[Kristian Evers]

Hi Jochem, 

A few comments:

> The problem is that the Proj.4 formulas are wrong. Wrong in the
> sense that they give different results from transformation with other
> software. I used several software packages and the differences between
> them
> were 0.1 to a few mm. The only one that was 4 cm different was Proj.4.

You need to demonstrate this with some actual data and an example of 
what you are doing. Please recreate the problem with a call to cs2cs. Until
we see some results it is only going to be guesswork from our side.

> Thanks. Should I compile this to an exe or an other type of file? Where
> should I put that file in order to use it in QGIS?

It is a bit more convoluted to use a development version of PROJ.4 with QGIS. 
It is certainly possible if you know what you are doing, but I assume that you don't 
Since QGIS is a rather complicated setup. Basically you need to first build a recent
development version of PROJ.4, and then compile QGIS with PROJ.4 as one 
of the depending libraries. Look towards the QGIS documentation for a proper
write up of the process.

> This is for another country. I
> would like to make it possible for them to use Proj.4 instead of writing a
> dedicated transformation program.

This is a very sensible approach, one that I hope many other countries will adapt
in the coming years. We are doing the same thing in Denmark. Which is also why
we are expanding the geodetic capabilities of PROJ.4 at the moment. The progress 
can be followed on GitHub where development is on-going, and some background 
can be found in the archives of this list. 

Best regards,
Kristian 

> -----Oprindelig meddelelse-----
> Fra: proj-bounces at lists.maptools.org [mailto:proj-
> bounces at lists.maptools.org] På vegne af Jochem
> Sendt: 23. marts 2017 03:38
> Til: proj at lists.maptools.org
> Emne: Re: [Proj] +towgs84 approximation error
> 
> Hi Stefanos,
> 
> You are still completely misunderstanding my problem. The responsible
> agency
> already did these measurements. And the measurements are ok. They also
> computed the 7 parameters. These are ok too. I checked the least-squares
> adjustment. 
> 
> However I don't want to use one of the other software packages as these
> can
> only transform points and not vector maps like QGIS.
> 
> Ragards, Jochem
> 
> 
> 
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