[GRASSLIST:6965] Re: Ellipsoid definition

[Paul Kelly]

On Tue, 31 May 2005, Luca Penasa wrote:

> Listing the various ellipsoids this is what i see:
>
> ...
> rd18    Amersfoort
>                       (bessel ellipsoid)
> ---
> rome40  Monte_Mario
>                       (international ellipsoid)
> ---
> S-42    Pulkovo_1942
>                       (krassovsky ellipsoid)
> ...
>
>

That list is what you get when you list datums. rome40 is the datum and 
international is the ellipsoid. I don't understand datums enough to be 
able to explain it better than that. The information about the ellipsoid 
used for that datum is just printed there to make it easier for you to 
make a decision about which datum to use.

> I cant understand whats the meaning for the ellipsoid into the brackets. Like 
> (international ellipsoid) near to rome40  Monte_Mario. I know that the rome40 
> ellipsoid is based on the international one, but i cant understand why have I 
> to specify at this point that im going to use the rome40 ellipsoid, when 
> after i'll have to specify the towgs84 parameter. why grass need this two 
> different definitions?

Datum transformation parameters (like towgs84) are specfic to a localised 
area and there is no one correct set for each datum. The way GRASS prompts 
you for datum and transformation parameters etc. is designed, hopefully, 
to force you to think about what you are doing and not just blindly accept 
the defaults!

Paul