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

[Paul Kelly]

On Tue, 5 Dec 2006, Maciej Sieczka wrote:

> Paul Kelly wrote:
>> Hello Maciek
>> We're talking about your example at
>> http://grass.itc.it/pipermail/grass5/2006-September/025668.html, right?
>
> Right.
>
>> In that case both your co-ordinate systems are based on the same
>> ellipsoid (WGS84) so it is normal and expected for the ellipsoidal
>> height to stay the same.
>
> Sorry, but I'm quite a dummy as to ellipsoids vs height. So I should
> expect a difference whether using the '-z' switch only when
> reprojecting between 2 different ellipsoids, correct?

Yes. It is really not a very useful feature at all, quite confusing 
actually, and that's why I thought it was important to make it optional. 
Ellipsoidal height is not very useful because you can't use it to work out 
e.g. if a ball is going to roll down a hill. In theory the ellipsoidal 
height of the bottom of a hill could be higher than the top of the hill - 
what is more important in a practical situation is the height above the 
geoid, which is what most vertical datums use. And that is totally 
separate from the 2-D re-projection that the [rv].proj modules do. We've 
talked about better vertical datum support in GRASS but it is so 
complicated and would just open up a whole can of worms that I don't want 
to get into right now...

>> I'm not clear on what output you were expecting?
>
> Well, you wrote yourself "That is very strange. Can you post the output
> from v.proj please to see if there are any clues?". So I did, and
> waited for your response.

Yes well I didn't quite pick up on that then either (the ellipsoids being 
the same). It is quite hard to get your head around it and understand it 
and I only saw that was the problem when looking at it afresh!

Paul