[GRASS5] Re: [Proj] Re: [kergis_bugs] Re: What proj library version is best?

[Michel Wurtz]

Gerald Evenden wrote:
> This discussion reminds me of the email interaction Frank Warmerdam and I
> had about what should be included in a projection library.  My position was,
> and still is, that *only* the projection (conversion between geographic and
> cartesian space) is the domain of the library.  All non-projection elements like
> datum shifts and  x-y-z transformations are separate and unrelated problems
> and belong in their own library systems.
> 
> The reason for this is twofold: 1) projections have uses that do not involve
> datums and their is no reason to burden these uses with elements that are
> not involved and 2) developers often only have expertise in only one of the
> areas and should not have to burden themselves with library maintenance
> involving unfamiliar elements.
[...]

I don't know how to interpret this : Datum is an important part of coordinate's
transformation.  The same projection on two diferent datums can lead to errors
of 500 m (at least in France, between the NTF and WGS84 Datum).  Projection
and Datum transformation are two different steps (and hence may be done in
different functions), but you cannot forget Datum transformation.  For exemple,
the correct way to tranform UTM/WGS84 to NTF/Lambert coordinate is :

(E,N) UTM -> lat/long WGS84 -> X,Y,Z (tridimentional) -> Datum shift, rotation
and scale from WGS84 to NTF -> lat/long NTF -> (E,N) Lambert.

Even with this formula you have an accuracy of 5 m in some places : You have to
use a correction matrix to improve the precision to a geodetic one (5 cm).

You can only forget datum when you work at very small scale (1:2,000,000 and under)

-- 
Michel Wurtz - Auzeville-Tolosane