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

[David Finlayson]

I agree with your proposal.

On 8/26/06, Paul Kelly <paul-grass at stjohnspoint.co.uk> 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).
>
> 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 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.
>
> Any comments?
>
> Paul
>
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>


-- 
David Finlayson
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