[GRASS-dev] v.proj transforming z co-ordinates

[Brad Douglas]

On Sun, 2006-08-27 at 00:00 +0100, Paul Kelly wrote:
> I suspected v.proj was doing this for years (since the introduction of the 
> new vector format) but never got round to verifying the behaviour until 
> yesterday. When re-projecting a 3-D vector file, if datum transformation 
> parameters are specified using towgs84= notation (rather than nadgrids=), 
> v.proj will assume the z co-ordinates are ellipsoidal heights and 
> transform them accordingly.
> 
> I think there are probably very few circumstances in which this behaviour 
> would be desirable. It is much more common for height data to be specified 
> relative to a vertical datum, e.g. mean sea level, than relative to the 
> ellipsoid (which is normally only used for horizontal distances).

This is definitely desirable.  I would almost consider not being able to
project Z data a bug.

> GPS data is more likely to include ellipsoidal height, but again it is 
> (IMHO) of questionable usefulness to transform it to another ellipsoid: 
> the geoid models that exist to transform GPS data to the local vertical 
> datum in regular use (OSGM02 for Great Britain and Northern Ireland is the 
> one I'm familiar with) convert directly from WGS84 ellipsoidal height data 
> to height above mean sea level.

I would reject projections involving different geoids.  Otherwise, Z
data may be corrupted.  At a minimum, a big fat disclaimer, but I would
prefer checking geoids and error if needed.

> I propose the attached patch to add a specific flag to v.proj to enable 
> the current behaviour, otherwise the z co-ordinates are always left 
> untouched. I thought it was worth explaining and discussing properly 
> because it changes the current behaviour and will give different results 
> in certain circumstances, but IMHO the current behaviour is wrong.

Does your patch lacks output of Z data (I see it does calculate values)?

Other than the comments above, I'd like to see it applied.  Great job!


-- 
Brad Douglas <rez touchofmadness com>                      KB8UYR
Address: 37.493,-121.924 / WGS84    National Map Corps #TNMC-3785