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From "Martin Desruisseaux (JIRA)" <>
Subject [jira] [Created] (SIS-434) Comparison of MathTransforms should use ContextualParameter
Date Tue, 16 Oct 2018 16:11:00 GMT
Martin Desruisseaux created SIS-434:

             Summary: Comparison of MathTransforms should use ContextualParameter
                 Key: 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):

A easting = 590476.8714630401 metres
B easting = 590476.8714630402 metres

Apache SIS implementation splits map projections in _normalization_ → _projection on normalized
ellipsoid_ → _denormalization_ steps. The denormalization matrices are:

┌                                                              ┐
│  3813171.507859623  5084228.678431943  -0.003883839275807552 │
│ -5084228.678431943  3813171.507859623  -0.003303801804941493 │
│        0                  0             1                    │
└                                                              ┘
┌                                                              ┐
│  3813171.507859623  5084228.678431943  -0.003883839175807552 │
│ -5084228.678431943  3813171.507859623  -0.003303801804941493 │
│        0                  0             1                    │
└                                                              ┘

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):

A:            -0.003883839275807552
B:            -0.003883839175807552
Easting = 590476.8714630401 metres

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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