Because of the WGS84 ubiquity, it is tempting to use that system as a hub or a pivot system
for all coordinate transformations.
The use of an “universal” system as a pivot simplifies the design of coordinate transformations libraries.
For example transformations from datum A to datum B can be done by first transforming
from A to WGS84, then from WGS84 to B.
With such approach, a coordinate transformations library would only need to associate each
GeodeticDatum instance with the transformation parameters from that datum to WGS84.
This approach was encouraged in version 1 of WKT format, since that format specified a
TOWGS84[…] element (removed in WKT version 2) precisely for that purpose.
This approach is known in EPSG guidance notes as “early binding” implementations
since information about coordinate transformations are associated early in geodetic object definitions,
usually right at GeographicCRS creation time.
While EPSG acknowledges that this approach is commonly used,
this is not a recommended strategy for the following reasons:
Example:
the EPSG geodetic dataset defines about 50 transformations from NAD27 to NAD83.
In an early binding approach, the same geographic CRS (namely “NAD27”) in the WKT 1
format would need to be defined with a TOWGS84[-8, 160, 176] element for coordinates in USA
or with a TOWGS84[-10, 158, 187] element for coordinates in Canada.
Different parameter values exist for other regions like Cuba, so it is not possible to represent such diversity
with a single TOWGS84[…] element associated to a CRS.
But even when restricting CRS usage to the domain of validity of its single TOWGS84[…] element,
those transformations are still approximative with a 10 metres accuracy in the USA case.
More accurate transformations exist in the form of NADCON grid shift files,
but those transformations are from NAD27 to NAD83 (which move together on the same continental plate),
not to WGS84 (which move independently).
The difference was often ignored when NAD83 and WGS84 were considered as practically equivalent,
but that assumption is subject to more caution today.
EPSG rather recommends the use of “late binding” approach,
in which coordinate transformation methods and parameters are defined for
“A to B” pairs of systems (eventually completed with domain of validity)
rather than associated to standalone datums.
Apache SIS is a “late binding” implementation,
while some reminiscences of “early binding” approach still exist in the form of the
DefaultGeodeticDatum.getBursaWolfParameters() property.
The later is used only if SIS fails to apply the late binding approach for given reference systems.