[Proj] Call for review of derived NTv2 for Swiss projection CH1903

[Mikael Rittri]

Sandro wrote:

> I got a max of 9mm drift.
>
> This mail is to ask others to try the NTv2 file with real data and let me > know if it's working as expected.

Looks excellent to me.

I have used Carmenta Engine to construct two implementations of CH1903.
One uses a concatenated datum shift, where the original NTv2 file
transforms to CH1903+, and the Helmert datum shift continues to WGS84.
The other uses a direct datum shift to WGS84 based on your new NTv2 file.

Then I selected three arbitrary points expressed in CH1903 LongLat and
transformed them to WGS84 LongLat with both implementations. The largest 
difference between the two implementations was 0.008 millimeters. 

So it seems that your drift of 9 millimeters is caused only by the
difference between the official FINELTRA algorithm and the NTv2 file.

Regards,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com

-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Sandro Santilli
Sent: Wednesday, February 01, 2012 11:21 PM
To: PROJ.4 and general Projections Discussions
Subject: [Proj] Call for review of derived NTv2 for Swiss projection CH1903

I've finally handled to derive an gridshift for CH1903->WGS84
so that it can be used w/out engaging chains of projections.

The procedure I followed to generate the new file has been:

 1) Project each of the NTv2 grid points to WGS84 using the
    chained approach (CH1903 -> CH1903+ -> WGS84)
 2) Compute the shifts between output and source
 3) Store the shifts in the new NTv2

More details on the how I'll probably give later with a blog post
as it was a complex and interesting trip around GDAL and PostGIS.

The verification I performed has been to project from CH1903/LV03,
using these parameters:

<CH1903_LV03> +proj=somerc +lat_0=46.95240555555556 +lon_0=7.439583333333333 +k_0=1 +x_0=600000 +y_0=200000 +ellps=bessel +nadgrids=CHENYX06a_wgs84.gsb +units=m +no_defs  <>

To CH1903+/LV95 (using EPSG:2056) and computing the distance of each
of the 5 example points found in Chapter 7 of
http://www.swisstopo.admin.ch/internet/swisstopo/en/home/topics/survey/sys/refsys/switzerland.parsysrelated1.37696.downloadList.97912.DownloadFile.tmp/swissprojectionen.pdf

I got a max of 9mm drift.

This mail is to ask others to try the NTv2 file with real data and
let me know if it's working as expected.

The new NTv2 file is here:
http://strk.keybit.net/tmp/CHENYX06a_wgs84.gsb.gz (~ 1Mb)

Thanks in advance.

--strk;

  ,------o-. 
  |   __/  |    Thank you for PostGIS-2.0 Topology !
  |  / 2.0 |    http://www.pledgebank.com/postgistopology
  `-o------'

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