[GRASSLIST:4632] Re: datums transormation and dX dY dZ values

[Crudeli]

>Hi all
>I'm not a very expert in datums shift and projections so, 
>eventually, I'm sorry for my basic question. 
>I have a map where are drawed the grids in two different datums 
>Gauss Boaga (Rome 40 datum) and UTM (eur50 datum). I'm not sure that 
>parameters (dx dy dz) for the datum shift indicated in the internal 
>datum tables or epsg or NIMA  tables are very good for my little 
>area of work so I would like know if is it possible desume (and in 
>which way) my user defined dx dy dz for the molodensky datum

It's quite easy to tell what your params are: GaussBoaga has a 
FalseEasting of 1500Km and 2520 Km respectively for the two fuses. If 
your translations parameters are that large, they refer to a direct 
transformation UTM/ED50->GaussBoaga.
if they are something in the 100-m range, probably they are for datum shifts.
I think that what's there depends on the different italian regions.
In every map you should also be able to find an average rotation parameter.

Anyway, if you need something accurate, don't even bother to loose 
time with the process:

transforming GB to GB-geographic
shift between the data
project back to UTM50

since the two systems are far from precise.
Actually they both have great differences between their mathematical 
definition and their practical implementation.  (soon I'll post more 
on the topic, I promise)

One accurate way would be to buy from IGM (Istituto Geografico 
Militare) some points of the IGM95 network; with them you should 
acheive a cm-precision.

Of course, if you are working at 1/25000, you don't have the 
aforementioned problems .

Regards,

RafDouglas CTC
--
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RafDouglas Candidi Tommasi Crudeli

"To know, to will, to dare, to be silent."