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Just for information purpose, EPSG make an interesting note about "<i>early
binding</i>" and "<i>late binding</i>" coordinate transformation
softwares. Quoting:<br>
<br>
<blockquote><font color="#3333ff">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.<br>
<br>
‘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.<br>
<br>
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.</font><br>
</blockquote>
<br>
Source: <a class="moz-txt-link-freetext" href="http://www.epsg.org/gigs.html">http://www.epsg.org/gigs.html</a> (Geospatial Integrity of
Geoscience Software)<br>
Document: ‘Part 1 – Guidelines’ (OGP report number 430-1),
describing the GIGS process<br>
Section 3.4 at page 11.<br>
<br>
<br>
Since the EPSG database does not associate TOWGS84 values to any
particular CRS (instead, it associates TOWGS84 values to different
pair of "<tt>sourceCRS</tt> to <tt>targetCRS</tt>"
transformations), the EPSG database is a "<i>late binding</i>"
model. However some (not all) coordinate transformation libraries
convert the EPSG model into some form of "<i>early binding</i>"
model. I would said that the "TOWGS84 multiplicity" issue come from
this conversion...<br>
<br>
Martin<br>
<br>
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