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is seldom necessary. However, the structure of these requests allows for more detailed parameterization of the request, thus essentially increasing the size of the
core request. In such a case, the ratio of core request size to SOAP wrapper size changes. The size of a complex request is often multiple times bigger than the
SOAP envelope that surrounds it.
The application of compression techniques that were investigated in the testbed can significantly decrease the size of a request, so that the size increase of a
SOAP request is diminished. As outlined before, SOAP enables common web service functionality that is
orthogonal to the core operations of the given OWS. The advantage of being able to support this functionality in a common way for any OWS operation appears to
outweigh the disadvantage of potentially doubling the request size.
10.2.6 Conclusion
Using SOAP based OWS interactions is more complex than using a KVP andor plain XML based operation encoding. At the same time, SOAP can be useful to avoid re-
inventing the wheel to enable common web service functionality such as secure and reliable message exchanges. Service infrastructures may not have a need for such
functionality. If that is the case, SOAP should not be used.
10.3 Unit of Measure Handling in Filter Expressions
10.3.1 Background
Clients often query AIXM features by filtering on feature properties that are quantities, i.e. which have a value provided in a certain unit of measure UoM. For example, the
length of a runway can be stated in meter, kilometer but also in foot or yards.
The numerical value of runway length changes based upon UoM it is provided in. Filtering AIXM data based upon simple value comparison of quantities against given
numbers therefore only makes sense if the client knows the “native” UoM of the property that the comparison operator is applied upon. In that case, the client can ensure that the
comparison value provided in the query is given in the same UoM. However, if the UoM for a given feature property is not defined as a constant in a GML Application Schema
such as AIXM then clients would need to determine the actual UoM of a given AIXM feature property first before being able to perform a meaningful query that involves a
comparison operator on this property.
A much more convenient solution for this issue would be to have the queried service automatically convert quantities - both in feature properties and ad-hoc queries with filter
expressions contained in client requests - into their base units and then perform comparison operators. Such a conversion is of course not necessary if the UoM of both
values is the same.
Example:
