[Proj] cs2cs's +towgs84 usage

Noel Zinn ndzinn at comcast.net
Sat Apr 24 07:06:57 EST 2010

Not true.  A round trip will always close.  You’ll have learned nothing
about the correct rotation sense.  You can test this yourself.  Do one round
trip with the rotations unchanged and do another round trip with the
rotations negated.  They’ll both close on the original point.  Correctness
is determined by the intermediate coordinates (right or wrong), not by the
round-trip closure.  –Noel Zinn



From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of Ilumas
Sent: Saturday, April 24, 2010 3:02 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] cs2cs's +towgs84 usage


Well, I agree that for assessing the accuracy of the re-projection results
one needs a reference, but that should not be necessary for understanding
the behavior of the cs2cs or the towgs84 string. Therefore I still argue
that the test I suggested below is a valid way to test whether the parameter
values need to be negated or not. Anyone?



On Fri, Apr 23, 2010 at 3:39 PM, Jean-Claude REPETTO <jrepetto at free.fr>

Le 23/04/2010 14:24, Ilumas a écrit :

> Anyway, I believe that the behaviour of a certain towgs84 string is easy

> to test with cs2cs itself: just do the transformation to one direction

> (e.g. from WGS84) and convert the resulting coordinates back to the
> original datum using exactly the same parameter string. If the resulting
> coordinates equal to the original ones, no negation is needed. Right?
> ilumas

Hi,nding the behavious of the cs2cs

I think you are wrong. To check the parameters, you need to know the
coordinates of at least one point in both datums, and compare them with
the results of cs2cs.


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