[Proj] Calculating +towgs84 values from source / dest .xyz file

[Jose Gonçalves]

Hi
That program provides the rotation matrix and not the angles, but you can
easily convert them from radians to arcseconds, to use in string +towgs84.
Check the formulas in page 134 of document http://www.epsg.org/Portals/0/
373-07-2.pdf. <http://www.epsg.org/Portals/0/373-07-2.pdf> PROJ.4 uses the
Position Vector" standard.
You can get the angles from matrix elements:
Rz = -r12 * 3600 *180 / PI
Ry = r13 * 3600 * 180 / PI
Rx = -r23 * 3600 * 180 / PI

The scale factor must be converted to scale difference in ppm:
DS = (s-1) * 1000000

You don't need to change translation vector components.

Regards

José Gonçalves


2017-09-25 13:16 GMT+01:00 Mark Johnson <mj10777 at googlemail.com>:

> Is there a c program source that can take a list of x/y/z points for a
> source and destination and calculate a valid 7 parameter +towgs84 value
> that could be used with cs2cs?
>
> I found something similar at http://helmparms3d.sourceforge.net/, called
> helmparms3d - but it seems calculate values that cannot be used for
> +towgs84.
>
> Mark Johnson, Berlin Germany
>
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