[Proj] How to use +towgs84

[Jose Gonçalves]

Any coordinate system must have a "towgs84" or any other form of conversion
to a global datum. In particular WGS84 itself has "+towgs84=0,0,0". You
should use:

  cs2cs +proj=latlong +ellps=mod_airy +towgs84=xxxxx +to +proj=latlong
+ellps=WGS84 +towgs84=0,0,0
where xxxxx is the set of 7 parameters you have. They should have signs
according to the conversion from local datum to WGS84 and rotations must
agree with the "position vector rotation" method (not "coordinate frame
rotation"). For the conversion from WGS84 to local datum just change the two
coordinate system strings, not the signs.

Instead of  using
    +proj=latlong +ellps=WGS84 +towgs84=0,0,0
you can use
    +proj=latlong +datum=WGS84
which is equivalent.

Regards

Jose' Gonçalves

2009/2/27 Aaron Friesen <aaron at cartopac.com>

> Everyone,
>
> I'm trying to familiarize myself with proj.
>
> I'm trying to use cs2cs to do a datum shift where a 7-parm
> transformation is used.  I want it to do the shift in
> both directions.
>
> "proj -ld" seems to provide a possible example.  The listing
> for ire65 has example towgs84 settings.
>
> Datum:      ire65
> Ellipse:    modif_airy (sic)
> Comment:        towgs84=482.530,-130.596,564.557,-1.042,-0.214,-0.631,8.15
>
> What do I need to specify for cs2cs for both directions
> of the transform?
>
> +proj=latlong +ellps=mod_airy
>
> +to
>
> +proj=latlong +ellps=WGS84
>
> seems like a resonable start.  Where does the +towgs84 argument go?
> How do things change in the reverse direction?
>
> Thanks for your assistance,
>
> Aaron
>
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