[GRASSLIST:3058] Understanding projections under grass.

[Antonio]

Hi.

After many weeks of trying to understand how grass works I think I have finally understood.
The main problems I had was in answering the questions that are made when setting up a new location.
Right, this is what I have figured out. Please correct me if I'm mistaken. :-)

When grass askes the coordinates and I reply UTM I'm only telling grass which coordinate system to use. He then asks me to which elipsoid must the coordinates be refered to and I answer
'international' as the maps I'm using are the official Italian cartography which are refered to
this elipsoid. Then grass asks me the datum and this was the question I just could figure out.
Now I think I realize it means 'to which point on the earth is the elisoid referred to' or
something like that.
So the 'eur50' datum means 'the internaltional elipsoid refered to the city of Bonn in Germany'.
Am I right?
The problem is that before I understood this I set up a database with the wrong datum (wsg84, the
default) and I don't know to which point it's refered to. I then imported a tiff map and
rectified it using i.points. However I gave points for rectification using the coordinates of
another datum, eur50. So now my vector maps are giving me the coordinates of the eur50 datum but
grass thinks they are wsg84 coordinates. This is rather difficult to explain ............ hope
you understand...
The problem is that I would like to recover my vector maps and transport them into a correct
location-prjection-datum grass database. I would create a grass DB with international elipsoid
and eur50 datum and then import the vector maps (but obviously they would be using the wsg84
datum and I don't want that)
Can anyone help me out?

Thanks

-- 
                 Antonio Gennarini
                Crecchio, CH - ITALY
	-----------------------------------
Earth Science student at University "G. D'Annunzio".
     Join th GNU generation, start from LINUX.