[Proj] towgs84 values for EPSG:27700

[Martin Desruisseaux]

Just for information purpose, EPSG make an interesting note about "/early 
binding/" and "/late binding/" coordinate transformation softwares. Quoting:

    Some software resolves the so-called 'multiplicity' problem by associating
    one coordinate transformation only with each CRS. The relevant coordinate
    transformation is even treated as part of the definition of the CRS. This
    solution to the problem of multiplicity is referred to as 'early binding',
    as it binds a coordinate transformation to the CRS before it is associated
    with any geospatial data.

    'Late-binding' software solutions to the multiplicity problem will only
    require the specification of a coordinate transformation at the time the
    coordinates are required to be transformed, i.e. after the association of
    geospatial data and CRS has taken place.

    The solution that has been chosen for any given geoscience software package
    is fundamental to its behaviour with respect to geospatial integrity. Both
    solutions have advantages and disadvantages and both are valid ways of
    controlling geospatial integrity. In this guidance note preference is
    expressed for the late-binding solution, provided the specific disadvantage
    of this solution, is resolved in the software.


Source: http://www.epsg.org/gigs.html (Geospatial Integrity of Geoscience Software)
Document: 'Part 1 -- Guidelines' (OGP report number 430-1), describing the GIGS 
process
Section 3.4 at page 11.


Since the EPSG database does not associate TOWGS84 values to any particular CRS 
(instead, it associates TOWGS84 values to different pair of "sourceCRS to 
targetCRS" transformations), the EPSG database is a "/late binding/" model. 
However some (not all) coordinate transformation libraries convert the EPSG 
model into some form of "/early binding/" model. I would said that the "TOWGS84 
multiplicity" issue come from this conversion...

     Martin

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