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If the length of a specific runway was provided in foot then the runway length value could be converted to meters by simply multiplying the original value with 0.30479976 a
foot being 12 inches and an inch being 0.02539998 m as defined by UCUM. Likewise, a quantity given as comparison value may be given in meters. Let us say that a client is
interested in finding airports with runways with a length bigger than or equal to 3000 meters e.g. for landing. If a given runway had a length of 8000 feet then the naive
number comparison without UoM conversion would result in a match, although the runway is in fact shorter than 2500 meters. With automatic UoM conversion, the service
would know that runway length had to be bigger than or equal to ~9842.53 feet to produce a match.
10.3.2 Enabling automated Unit of Measure Conversion
Querying features at a WFS is performed via a GetFeature request which supports query elements that include filter expressions according to the OGC Filter Encoding
Specification FES. Comparison operators in FES filter expressions reference the feature property that the operator shall be applied to and either provide the comparison value as
literal directly or reference it. The following paragraphs discuss how automatic UoM conversion can be enabled at a WFS
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. First of all, we need to identify how a service can know the UoM that a feature property
and comparison value is given in. There are different options: UoM is constant - this would be defined in the GML Application Schema of a
given feature type; note that in case that the comparison value is given by reference, the service would need to determine the feature type that the referenced
property belongs to
UoM is known to be given in base units - this would be a requirement for the given domain, but is probably hard to enforce; in addition, the domain would need
to agree on the base units though following the definition of base units provided by UCUM appears to be beneficial
UoM is explicitly provided - either in the dataset e.g. as additional column in the database of the service or in the filter expression.
Explicitly providing the UoM of a comparison value in a filter expression was discussed during the testbed. The participants agreed that explicitly adding the UoM information to
the literal representation of the comparison value is the best option.
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WFS 2.0 explicitly states that it “does not define any support for handling conversions between unit of measure” - see section 7.9.2.5.3.5 “Units of measure handling” in OGC
09-025r1 ISODIS 19142
