Martin Desruisseaux created SIS-434:
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Summary: Comparison of MathTransforms should use
ContextualParameter
Key: SIS-434
URL: https://issues.apache.org/jira/browse/SIS-434
Project: Spatial Information Systems
Issue Type: Task
Components: Referencing
Reporter: Martin Desruisseaux
Use case (assuming SIS-433 is not yet fixed): Let _A_ =
{{CRS.forCode("EPSG::29871)}} and _B_ = _A_ formatted in WKT and parsed back.
Compare those two CRS. Because of the slight difference in _"British chain
(Sears 1922)"_ unit definition described in SIS-433, we get slightly different
_"Easting at projection centre"_ parameter value after their conversion to
metres (parameter value before conversion is 29352.4763 chSe):
{noformat}
A easting = 590476.8714630401 metres
B easting = 590476.8714630402 metres
{noformat}
Apache SIS implementation splits map projections in _normalization_ →
_projection on normalized ellipsoid_ → _denormalization_ steps. The
denormalization matrices are:
{noformat}
A:
┌ ┐
│ 3813171.507859623 5084228.678431943 -0.003883839275807552 │
│ -5084228.678431943 3813171.507859623 -0.003303801804941493 │
│ 0 0 1 │
└ ┘
B:
┌ ┐
│ 3813171.507859623 5084228.678431943 -0.003883839175807552 │
│ -5084228.678431943 3813171.507859623 -0.003303801804941493 │
│ 0 0 1 │
└ ┘
{noformat}
Those matrices are almost identical. They differ only in their _m02_ terms,
copied below. The easting parameter is also shown for comparison purpose
(reminder: this parameter differs in the last digit between the two CRS):
{noformat}
A: -0.003883839275807552
B: -0.003883839175807552
Easting = 590476.8714630401 metres
{noformat}
The problem is that while the difference has its origin in the last digit of a
map projection parameter, its effect in the denormalization matrix appears in
the middle of a term. The relative error (A-B)/A is about 2.6E-8, which well
above the 1E-13 tolerance threshold currently used for matrix comparisons. We
can not just relax the tolerance threshold since whether the difference is
relevant or not depends on the operations performed before to add this term.
For fixing this issue, comparison of {{MathTransform}} instances should use the
information provided in {{ContextualParameters}} when those information are
available. It would allow to compare the original easting parameters (with a
difference only in the last digit) instead than the transformed value in
denormalization matrix.
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