Martin Desruisseaux created SIS-433:
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Summary: Replace unit parsed from WKT by unit declared in EPSG
database
Key: SIS-433
URL: https://issues.apache.org/jira/browse/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 unit:
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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