Summary Extract - Extending the Snapshot Concept .1 Introduction

84 Copyright © 2011 Open Geospatial Consortium, Inc. All Rights Reserved.

10.1.4 Reconsider Rules for Handling Changes to Multi Occurring Properties

Section 3.5 in the AIXM Temporality Model explains how multi occurring properties should be handled when one or more of their values changes permanently or temporarily. The issues that the section addresses is that simple UML objects are not identifiable in the XML encoding they do not have a gml:identifier. Recognizing which value of a multi-occurring property changed is not possible. Therefore, the Temporality Model states that “in a PERMDELTA or TEMPDELTA Time Slice, multi- occurring properties shall be provided with all occurrences included”. This statement does not exactly define which value occurrences shall be included, which is likely due to the simplicity of the chosen example. Section 3.5 appears to only cover properties without schedule. Furthermore, it does not appear to take into account a specific edge case. This is problematic as will be explained in the following paragraphs.

10.1.4.1 Delta for multi-occurring property with schedule

Let us consider the case of an Airspace which can be activated at certain times. An “activation” is a multi occurring property of an Airspace that has a schedule and thus is valid for certain times. Furthermore, it is a simple object, not a feature. Therefore, strictly following the rule from the Temporality Model section 3.5, whenever a change in the activation schedule either changing the regular schedule or performing an exceptional activation - such as when some military activity is taking place of the Airspace occurs, the delta timeslice would need to incorporate all activation objects from the applicable static information, even those whose schedule is before or after the valid time of the new activation. This is not necessarily an issue in situations where a permanent change is communicated. In such a situation, all activations that were applicable in the past can be omitted in the new PermdeltaBaseline. These activations would still be stored in the history of timeslices of the Airspace feature and thus can be queried if necessary. Furthermore, permanent changes in Airspace activations usually concern a change of the regular schedule and do not incorporate exceptional activations. Exceptional activations are communicated via Tempdeltas see the Digital NOTAM Event Specification for further details. For exceptional activations, however, the rule for multi-occurring properties as it is currently defined in the Temporality Model is not ideal. If the status of an exceptional activation is constant throughout the valid time of a Tempdelta then that Tempdelta would only need a single activation value. However, according to the current rule, it looks like the Tempdelta needs to include all other Airspaceactivation values that are contained in PermdeltasBaselines that are not before and not after the valid time of the Tempdelta. This definitely would be unnecessary overhead as in this situation a single activation object suffices to fully define the state of the activation property throughout the valid time of the Tempdelta.

10.1.4.2 Temporary change of multi-occurring property that overlaps a permanent change

A Permdelta that changes the value of a multi occurring AIXM feature property may have a valid time that is during the valid time of the Tempdelta - see the following figure. Copyright © 2011 Open Geospatial Consortium, Inc. All Rights Reserved. 85 Figure 32 – Permanent change of a multi occurring feature property occurring during the valid time of a temporary change of that property Here, property P2 is multi occurring. Its static set of values {1,2,3} in Baseline 1 is changed permanently via Permdelta 2 to {1,4,5}. However, a temporary change is applied to the property via Tempdelta 1 – adding value ‘6’ and its valid time contains the valid time of the permanent change. This situation is not explicitly described in the AIXM Temporality Model. As it is not forbidden, it may occur 8 . In such a situation it does not matter whether the multi occurring property P2 does or does not have a schedule - either way it is unclear how the Tempdelta should incorporate the unchanged property values from the static information Baseline 1 and 2. Should all unchanged values be incorporated? As the set of values for P2 changes permanently via Permdelta 2, the values that would be incorporated from the Baselines 1 and 2 in Tempdelta 1 following the current rule from 8 If this situation is not allowed to happen then the AIXM-TM should explicitly state this.