[GRASSLIST:6827] Re: PROJ trasformation parametrs for datums

[Luca Penasa]

Frank Warmerdam wrote:

>On 5/19/05, Luca Penasa <luca.penasa at email.it> wrote:
>  
>
>>Im working with Grass to georeference some maps. iv created a new
>>location where to put georeferenced data. Its a Gauss-Boaga loaction and
>>into the poj_info's file i see:
>>towgs84=-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68
>>I know this is the correct towgs84 parameter but i cant understand how
>>does it work. I know that it's used for the datum trasformation, but i
>>also know that the gauss-boaga system is based on the internation
>>ellipsoid, so...the wgs84 parameter is used for the trasformation from
>>which ellipsoid?
>>    
>>
>
>Luca, 
>
>The parameters are used to apply a "datum shift" - somewhat more than
>just the conversion between ellipsoids is going on.  The transformation
>is how to get from the datum+ellpisoid in question to the WGS84 datum.
>
>The actually parameters are x/y/z shifts, x/y/z rotations and an overall
>scaling value.  Note that this transformation is only used if you convert
>between datums. 
>
>Best regards,
>  
>
Frank,
Thanks for your help... Now i want to know more about it, do you know 
where i can find other material?
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
 
 
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