[Proj] cs2cs's +towgs84 usage

[Ilumas]

Anyway, I believe that the behaviour of a certain towgs84 string is easy to
test with cs2cs itself: just do the transormation 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



On Fri, Apr 23, 2010 at 12:41 PM, Ilumas <muginaa at gmail.com> wrote:

> Thanks Mikael,
>
> what if the Method is:
>
>  Method [Geocentric translations (geog2D domain)]
>
> and the Sign reversible field value is YES for all three parameters?
>
>
> best regards,
>
> Ilumas
>
>
>
>
>
>
> On Fri, Apr 23, 2010 at 11:37 AM, Mikael Rittri <
> Mikael.Rittri at carmenta.com> wrote:
>
>>
>> About the rotation sign conventions, I would
>> add that calling the two conventions "European"
>> and "American" is not helpful, since there is not
>> much agreement about which convention is which!
>> (Both conventions are used in Europe, and there
>>  is no obvious geographic pattern.)
>>
>> The terms "left-handed" and "right-handed" are
>> just as bad.
>>
>> It is better to say that cs2cs uses the sign
>> convention known as the
>>
>>     Position Vector Transformation.
>>
>> If you look up the +towgs84 parameters on www.epsg-registry.org
>> (filter by Type: Coordinate Transformation), you will see the text
>>
>>  "Warning: CARE! Ensure application uses this method! See Guidance Note
>> 7-1"
>>
>> above the parameter values.  This means that you
>> should scroll down _below_ the parameter values,
>> where you find the Method.  If it says:
>>
>>  + Method [Position Vector transformation]
>>
>> you can use the parameters directly in cs2cs.
>> But if it says
>>
>>  + Method [Coordinate Frame Rotation]
>>
>> you should reverse the signs of the three
>> rotation parameters.
>>
>> Best regards,
>> --
>> Mikael Rittri
>> Carmenta AB
>> SWEDEN
>> www.carmenta.com
>>
>> -----Original Message-----
>> From: proj-bounces at lists.maptools.org
>> [mailto:proj-bounces at lists.maptools.org] On Behalf Of Frank Warmerdam
>> Sent: den 21 april 2010 18:14
>> To: PROJ.4 and general Projections Discussions
>> Subject: Re: [Proj] cs2cs's +towgs84 usage
>>
>> Ilumas wrote:
>> > Dear list,
>> >
>> > could you kindly confirm that the cs2cs's +towgs84 parameters remain
>> > the same for a certain datum regardless of the direction of the
>> > transformation. Some people (e.g.
>> > http://n2.nabble.com/Datum-Shift-WGS84-to-Arc1960-not-working-propberl
>> > y-td3410479.html)
>> > <http://n2.nabble.com/Datum-Shift-WGS84-to-Arc1960-not-working-propber
>> > ly-td3410479.html>seem to suggest that the values need to be negated
>> > when going FROM wgs84 TO something else, but this seems a bit strange
>> > to me. Unfortunately all the examples in the man pages seem to be
>> > about changing the coordinates TO wgs84...
>>
>> Ilumas,
>>
>> The parameters are always expressed in terms of what it takes to
>> transform to WGS84.  If PROJ is transforming from WGS84 it will
>> internally reverse the sense of the transformation.
>>
>> Keep in mind there is also an issue with the handedness of the
>> parameters.
>> Some folks, mostly european, express the parameters with a different
>> handedness.  The scripts that generate the "epsg" init file already know
>> about this case and convert to the usual PROJ.4/North American
>> handedness but if you are researching parameters yourself from various
>> sources you may need to be aware of it.
>>
>> 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<http://pobox.com/%7Ewarmerdam>
>> and watch the world go round - Rush    | Geospatial Programmer for Rent
>>
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