[Proj] WGS84 to ED50

[Hamish]

Roger Oberholtzer wrote:
> > > The Applanix provides this information on their calculations:
> > ...
> > >  Coordinate transformation from WGS84 to mapping frame datum
> > >     dX = 125.098545; dY = 76.000054; dZ = 156.198703; f =
> > > 0.999991695369;
> > >     R1 = 0.000000000000; R2 = 0.000000000000; R3 = -0.000005473550;

Hamish wrote:
> > The above R3,f values makes me hesitate a little, but it may be that
> > Applanix and Trimble are both preforming a less precise 3-term transform,
> > while PROJ is doing a more precise 7-term transform. That would certainly
> > explain a 1-4m error.

WRT Cliff's comments, I should have written "that would explain the 1-4m
difference", not "error", and gone to more trouble to point out that by writing
"more precise" I did not mean to imply "more accurate".

Regardless of the correctness of it, there is some value in being able to
exactly replicate someone else's approach.


Roger:
> My understanding is that Applanix is doing a 7 parameter transform.

But if in that 7-term transform all three rotational terms are all very near 0
and the scaling term is very near to 1, you are just left with dx,dy,dz, which
is in effect a 3-term transform. If your desire is to get the same exact answer
as Applanix outputs, then feed PROJ the exact coefficients they specified, not
something different (perhaps better?) from another source.

> What I am unsure of is how to get proj to do the same. If it is the default,
> then I am curious how the 7 parameters are specified. Are these the
> values in the +towgs84 option?

Yes, 7 term is given with +towgs84=,,,,,, and 3 term is given with +towgs84=,,.


but if there's a NTv2 grid kicking around..... all the better to use that.


further (remarkably clear) reading on 3,7,NTv2 transforms:
 (see both html+PDFs)
http://www.linz.govt.nz/core/surveysystem/geodeticinfo/geodeticdatums/nzgd49tonzgd2000/index.html


always glad to provide amusement,
Hamish



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