[Proj] Meaning of 7-parameter transformation rotations

Clifford J Mugnier cjmce at lsu.edu
Wed Nov 30 13:58:19 EST 2005


Note that the SENSE of the rotations have two common usages.  The American
Standard versus the European Standard differ!  To transform in the same
direction, the signs are OPPOSITE from one "standard" to another.
Generally speaking, a 7-parameter  transformation, whether it is a
Bursa-Wolfe or a Molodensky, MUST have a "test point" provided so that the
user knows which sense the rotations need for the desired transformation

The Americans, Australians, and Luxemborgs use one SENSE, the Europeans use
the other.  In regard to the rest of the world, it depends on which
brand-name of GPS Geodetic Processing software was used to derive the
parameters from some collection of data points.  See my monthly column for
such important minutiae for country to country considerations.

Be careful of 'dem signs!  :-)

Clifford J. Mugnier
Chief of Geodesy
Department of Civil Engineering
CEBA 3223A
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536 [Academic]
Voice and Facsimile:  (225) 578-4474 [Research]

On 11/30/05, Martin Vermeer <martin.vermeer at hut.fi> wrote:
> Yes, I know. Just wanted to point out that the above quoted text is
> uninformative. Yes, the exact matrix contains sines and cosines, but of
> the angles Rx, Ry, Rz that are in seconds of arc (and thus, in any
> computation, must be converted to radians first).
> The text following "not in physical units" just expresses the author's
> lack of information, and contains a wrong guess. I propose to replace it
> by correct information.


Thanks, I have updated the paragraph to read:

"The seven parameter case uses <i>delta_x</i>, <i>delta_y</i>,
<i>Rx - rotation X</i>, <i>Ry - rotation Y</i>, <i>Rz - rotation Z</i>,
<i>M_BF - Scaling</i>.  The three translation parameters are in meters as
in the three parameter case.  The rotational parameters are in seconds of
arc.  The scaling is apparently the scale change in parts per million."

Does this look OK?  The previous text was intended to highlight my
lack of information.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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