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https://issues.apache.org/jira/browse/SIS-390?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Martin Desruisseaux closed SIS-390.
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> 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
> Reporter: Martin Desruisseaux
> Assignee: Martin Desruisseaux
> Priority: Major
> Fix For: 0.8
>
>
> 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.
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