Martin Desruisseaux created SIS-390:
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Summary: When datum shift information are missing, still apply
ellipsoid change
Key: SIS-390
URL: https://issues.apache.org/jira/browse/SIS-390
Project: Spatial Information Systems
Issue Type: Improvement
Components: Referencing
Affects Versions: 0.7
Environment: For some pairs of CRS, the datum is not the same but
Apache SIS is nevertheless unable to apply the datum change, either because we
didn't found explicit information in EPSG geodetic database, or because the
datum shift depends on a grid file that we don't have. The problem is what to
do in such case? Apache SIS 0.7 did nothing. Apache SIS 0.8 takes at least the
change of ellipsoid in account.
Example: EPSG:4609 — _NAD27(CGQ77)_ uses the Clarke 1866 ellipsoid, which is
different than the WGS 84 ellipsoid. The coordinate operation from that CRS to
EPSG:4326 is
[EPSG:1691|http://epsg-registry.org/?display=entity&urn=urn:ogc:def:coordinateOperation:EPSG::1691].
That operation requires the {{CGQ77-98.gsb}} datum shift file, which may not
be installed. In absence of datum shift information, an ellipsoid change can
still be applied using a Molodensky operation with all geocentric translation
parameters left to zero. This can be seen in the following fragment in the
{{MathTransform}} WKT (this fragment is absent if the transform does nothing
about the datum/ellipsoid change). Note there is a change of 69 meters in the
semi-major axis length and 169 meters in the semi-minor axis length:
{noformat}
Param_MT["Abridged Molodensky",
Parameter["dim", 2],
Parameter["src_semi_major", 6378137.0, Unit["metre", 1]],
Parameter["src_semi_minor", 6356752.314245179, Unit["metre", 1]],
Parameter["tgt_semi_major", 6378206.4, Unit["metre", 1]],
Parameter["tgt_semi_minor", 6356583.8, Unit["metre", 1]],
Parameter["X-axis translation", 0.0, Unit["metre", 1]],
Parameter["Y-axis translation", 0.0, Unit["metre", 1]],
Parameter["Z-axis translation", 0.0, Unit["metre", 1]],
Parameter["Semi-major axis length difference", 69.4, Unit["metre", 1]],
Parameter["Flattening difference", 3.726463918114448E-5, Unit["unity", 1]]]
{noformat}
Apache SIS conservatively reports an "accuracy" of 3 km when there is no datum
shift operation, no matter if we nevertheless applied an ellipsoid change or
not.
Reporter: Martin Desruisseaux
Assignee: Martin Desruisseaux
Fix For: 0.8
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