[ https://issues.apache.org/jira/browse/SIS433?page=com.atlassian.jira.plugin.system.issuetabpanels:alltabpanel
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Martin Desruisseaux updated SIS433:

Description:
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:
{quote}
British chain (Sears 1922) = (792 / 39.370147) metres ≈ 20.116765121552632 metres
{quote}
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 nonlinear
computations up to a point where two CRS objects are considered not equal, even in {{ComparisonMode.APPROXIMATIVE}}
way (see SIS434 for more indepth description of that problem). The proposed improvement
is to use the EPSG code of coordinate parameter values for fetching the unit of measurement
from EPSG database. This work is described in SIS210; fixing that later issue should help
fixing this issue.
was:
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 nonlinear
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 SIS210; fixing that
later issue should help fixing this issue.
> Replace unit parsed from WKT by unit declared in EPSG database
> 
>
> Key: SIS433
> URL: https://issues.apache.org/jira/browse/SIS433
> Project: Spatial Information Systems
> Issue Type: Task
> Components: Referencing
> Reporter: Martin Desruisseaux
> Priority: Minor
>
> 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:
> {quote}
> British chain (Sears 1922) = (792 / 39.370147) metres ≈ 20.116765121552632 metres
> {quote}
> 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 nonlinear
computations up to a point where two CRS objects are considered not equal, even in {{ComparisonMode.APPROXIMATIVE}}
way (see SIS434 for more indepth description of that problem). The proposed improvement
is to use the EPSG code of coordinate parameter values for fetching the unit of measurement
from EPSG database. This work is described in SIS210; fixing that later issue should help
fixing this issue.

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