[Proj] datum conversion, estimating parameters

Robert Jaeschke robert-j at gmx.de
Wed Mar 23 15:32:13 EST 2005


On Tue, Mar 22, 2005 at 08:22:04PM -0600, Noel wrote:

> To accomplish what you intend you need the ECEF (geocentric Cartesian) 
> coordinates for at least one point in both datums.  

Thank you. If i would have read the docs more carefully ... :-(

> Differencing those ECEF 
> coordinates will give the the three translations (the first 3 of the 7 
> parameters, the rest of the parameters being zero), which is all you can 
> hope for with one point.  Solving for the other parameters requires many 
> more points over (and this is very important) a LARGE area in order to 
> achieve any statistical significance.  The problem is ill-conditioned over 
> a small area.

That's clear and makes the problem not easier: I have only data of
Germany ...

> This problem is NOT solved in projected space.

My first thought was: Why not convert all values to geocent and then
repeat my steps? Of course, I tried it. And of course it does not work.
For conversion to geocent, level above sea is important and I don't have
these values. And there are probably more reasons for that it's impossible,
what I want. :-(

Regards, Robert

"I don't even have an e-mail address. I have reached an age where my main
purpose is not to receive messages." 
--- Umberto Eco, quoted in the New Yorker 

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