[Proj] +towgs84 approximation error

Stefanos Beligiannis smpel at yahoo.com
Wed Mar 22 21:16:15 EST 2017

Hi Jochem,
this is more clear : 1. Usually these parameters are defined by the government by LSM and their (the parameters) computations are based on some CPs of  the National HARN network with coordinates calculated on both systems.  This means they enclose the national grid internal errors plus the least square methods calculation errors. In turn it is a big luck for you that you have only up to 4.5cm error.2. To my opinion, If you want better accuracy you must re-measure with big accuracy the coordinates of those (or at least, as many as possible) common points you have on the old Hayford datum, on WGS84 with GPS static Measurements and then define new "local" 7 parameters to transform the coordinates you have, and ignore the government for this area.3. I had the same with you problem on Riyadh Airport (Ain Al Abd datum) and I solved it like above.I find the problem very interesting and I want to kindly give me feedback how you solved it.many thanksStefanos
      From: Jochem <jochem.lesparre at kadaster.nl>
 To: proj at lists.maptools.org 
 Sent: Thursday, March 23, 2017 3:35 AM
 Subject: Re: [Proj] +towgs84 approximation error
Hi Stefanos,

I have coordinates in an old national grid and a new national grid. Both
grids have TM projections on the international Hayfort ellipsoid. Both grids
have 7 parameter transformations to WGS84 defined by the government but not
published in EPSG. I want to do:

cs2cs +init=mygrids:old +to +init=mygrids:new -f %.4f <old_coords.txt

What I need is exactly what I explained in my first email: a 7 parameter
transformation without approximation error of 4 cm (this error is a bias,
not a standard deviation) in the formulas.

Regards, Jochem

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