[GRASSLIST:6828] Re: PROJ trasformation parametrs for datums

[Frank Warmerdam]

On 5/20/05, Luca Penasa <luca.penasa at email.it> wrote:
> Thanks for your help... Now i want to know more about it, do you know
> where i can find other material?

Luca, 

I see a bunch of my old links are dead, but I would suggest you to review
some of the material at:

  http://earth-info.nima.mil/GandG/pubs.html

> I see that in my language there is a strange confusion of words (maybe
> the confusion is only in my mind, couse im a newbie), but i cant really
> understand what do you mean (what everyone means) with the word "datum".
> I think that a datum is a whole reference system (with an ellipsoid and
> a coordinate system), or have i to understand only "ellipsoid" when you
> say "datum"?
> ... thanks a lot for your time

To be honest I get quite confused about datums myself.  My understanding
is that a datum implies a particular ellipsoid, but the datum also takes into
account various kinds of survey error, and other systematic deviation
from the ellipsoid.   I can't give a good explanation though, since I don't
really understand it well.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent