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From "Martin Desruisseaux (JIRA)" <j...@apache.org>
Subject [jira] [Created] (SIS-390) When datum shift information are missing, still apply ellipsoid change
Date Sun, 28 Jan 2018 17:52:00 GMT
Martin Desruisseaux created SIS-390:
---------------------------------------

             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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