[Proj] cs2cs's +towgs84 usage

[Ilumas]

Sure,

but that's not the point. The original question was if the values need to be
negated when going FROM wgs84 TO something else. If that was correct, one
should do the round trip as follows:

1.  cs2cs +proj=utm +zone=a +datum=wgs84  +to +proj=utm +zone=a
+datum=datum2 +towgs84 dx,dy,dz
input: x1,y2
output: x2,y2
2.  +proj=utm +zone=a +datum=datum2 *+towgs84 (-1)dx,(-1)dy,(-1)dz* +to
+proj=utm +zone=a +datum=wgs84
input: x2,y2
output:x1,y1

Or am I completely lost here?

Thanks for your patience...

I


On Sat, Apr 24, 2010 at 3:06 PM, Noel Zinn <ndzinn at comcast.net> wrote:

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