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Time in GML

Geography is not only concerned with space and neither is GML. The temporal.xsd schema (see http://schemas.opengis.net/gml/3.1.1/base/temporal.xsd [1] ) contains a set of useful definitions for time related objects and properties.  The simplest of these is TimeInstant.

A TimeInstant is what it says – an instant in time. It has a single child (property) called timePosition.  A TimeInstant is thus written:

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    <TimeInstant>
      <timePosition>1876-08-01</timePosition>
    </TimeInstant>

Of course not all times are instants – some are TimePeriods. For these we can write:

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    <TimePeriod>
      <begin>
        <TimeInstant>
          <timePosition>1876-08-01</timePosition>
        </TimeInstant>
      </begin>
      <end>
        <TimeInstant>
          <timePosition>1876-08-02</timePosition>
        </TimeInstant>
      </end>
    </TimePeriod>

Of course GML can provide a number of ways of expressing time durations and time lengths as well and can deal with non-ISO times and dates (need a Geological Calendar?) but that is beyond our scope for now.