[PROJ] [EXTERNAL] Re: Proj 6 API questions

[Kristian Evers]

Jochem,

We are definitely in nit-picking territory at this stage. I agree with you that it is not going to do much harm in many cases but I can think of a few where it will. Mind you, for the specific use case in QGIS a generic solution is preferable (and likely required). I believe I have proposed such a solution.

/Kristian

-----Oprindelig meddelelse-----
Fra: Lesparre, Jochem <Jochem.Lesparre at kadaster.nl> 
Sendt: 28. marts 2019 17:00
Til: Kristian Evers <kreve at sdfe.dk>; Nyall Dawson <nyall.dawson at gmail.com>; Even Rouault <even.rouault at spatialys.com>
Cc: PROJ <proj at lists.osgeo.org>
Emne: RE: [PROJ] [EXTERNAL] Re: Proj 6 API questions

Kristian,

For the Dutch national CRS (named RD), the datum transformation has an absolute precision of <1mm (since we define it by the transformation ITRS-ETRS89-RD). Here, transforming to ITRF would be more accurate since it would remove the scale error of the national CRS of 4mm/km and the NTv2-corrections of up to 25 cm. 

For some of the Dutch Caribbean islands, the scale factor is really large. Although the datum transformation is only known at metre level, I convinced that despite this inaccuracy, transforming to ITRS would give more reliable distances. I've done some consultancy in other countries too, and, I don't think the inaccuray of the datum transformation in general would harm more than the inaccuracy of legacy CRSs.

Kind regards, Jochem
 

-----Original Message-----
From: Kristian Evers <kreve at sdfe.dk> 
Sent: Wednesday, March 27, 2019 12:09 PM
To: Lesparre, Jochem <Jochem.Lesparre at kadaster.nl>; Nyall Dawson <nyall.dawson at gmail.com>; Even Rouault <even.rouault at spatialys.com>
Cc: PROJ <proj at lists.osgeo.org>
Subject: SV: [PROJ] [EXTERNAL] Re: Proj 6 API questions

This approach assumes that a sufficiently accurate transformation between the legacy CRS and the modern CRS exists. That cannot be guaranteed and it most cases I would argue that such a transformation comes with a fair amount of uncertainty. Not doing a datum shift and simply working within the original datum avoids the introduction of errors and ensures consistency with the coordinates as they were originally measured.

I am most familiar with Danish systems and for those I would not be comfortable adding a datum shift into the mix. The transformations are simply not accurate enough across the entirety of the country (near control points everything is good, between them... who knows?).

/Kristian

-----Oprindelig meddelelse-----
Fra: Lesparre, Jochem <Jochem.Lesparre at kadaster.nl>
Sendt: 27. marts 2019 11:54
Til: Kristian Evers <kreve at sdfe.dk>; Nyall Dawson <nyall.dawson at gmail.com>; Even Rouault <even.rouault at spatialys.com>
Cc: PROJ <proj at lists.osgeo.org>
Emne: RE: [PROJ] [EXTERNAL] Re: Proj 6 API questions

Kristian is right that just using a modern ellipsoid for distances/areas in an old but still used national CRS would give errors.
However, I think computing distances/areas using the old ellipsoid of that CRS is also not correct for most users. Therefore, I suggest:

1. Back-project the projected coordinates to geodetic coordinates using the Ellipsoid parameters of the CRS datum.
2. Perform datum transformation to the most recent realisation of the currently accepted scientific CRS of that planet (currently ITRF2014 for Earth) with its ellipsoid (currently GRS80 for Earth).
3. Perform geodesic calculations using this ellipsoid parameters of this CRS datum.

Kind regards, Jochem



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