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From "Martin Desruisseaux (JIRA)" <>
Subject [jira] [Created] (SIS-433) Replace unit parsed from WKT by unit declared in EPSG database
Date Tue, 16 Oct 2018 14:49:00 GMT
Martin Desruisseaux created SIS-433:

             Summary: Replace unit parsed from WKT by unit declared in EPSG database
                 Key: SIS-433
             Project: Spatial Information Systems
          Issue Type: Task
          Components: Referencing
            Reporter: Martin Desruisseaux

In the EPSG geodetic database, units of measurement are defined by a _b_/_c_ ratio.
But when parsing a WKT, the {{UNIT}} element is defined only by a scale factor which is the
result of above division. This result is a slight lost of precision. Example with EPSG:9042

British chain (Sears 1922) = (792 / 39.370147) metres ≈ 20.116765121552632 metres

Unit conversions in Apache SIS internally use ratios for better accuracy. Consequently CRS
objects created from the EPSG database get slightly different unit conversion factors than
equivalent CRS objects parsed from WKT. This very small difference cascades in non-linear
computations up to a point where two CRS objects are considered not equal, even in {{ComparisonMode.APPROXIMATIVE}}
way. The proposed fix is to use the EPSG code of coordinate parameter values for fetching
the unit of measurement from EPSG database. This work is described in SIS-210; fixing that
later issue should help fixing this issue.

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