Visualisation of Temporal Data

Visualisation of temporal and spatio-temporal data is an important aspect of AWRIS.

Observational data sets such as precipitation, dam levels, stream flow or water quality measures are all time-series data. Even feature data sets such as gauging stations or monitoring points have a temporal component when considered as a set of features whose existence, identity or attributes change over time. Consideration should therefore be given to appropriate ways to present temporal information over the web.

Temporal information is inherently associated with change over time. In a spatial context, changes may be characterised as (Andrienko 2003):

1. Existential changes, i.e. appearance and disappearance.

2. Changes of spatial properties: location, shape or/and size, orientation, altitude, height, gradient and volume.

3. Changes of thematic properties expressed through values of attributes: qualitative changes and changes of ordinal or numeric characteristics (increase and decrease).

VISUALISATION COMPONENT REQUIREMENTS

Different presentation techniques are suited to particular types of changing data. In this section, several general techniques for spatio-temporal presentation are described, including information on the types of changes they are useful at representing.

3.5.1 Map Iteration

Map iteration is a traditional approach for presenting information on change. Change is visualised by showing images or maps at different stages of change side by side or in a sequence so that changes can be seen and compared. Map iteration is a general presentation technique, as existential, spatial and thematic changes can all be portrayed. This technique can be useful for presenting temporal changes, but can also be used to compare different result sets from, for example, a set of scenarios. The obvious disadvantage of this technique is that a limited number of images can be displayed simultaneously, thus limiting the resolution of change that can be shown.

3.5.2 Animation

Animation of web components exploits the ability of computers to deliver dynamic content. In contrast with the static images of paper reports and documents, the web allows for dynamic, interactive maps, charts and images to be presented. This is especially useful when presenting temporal information although animation can also be used for non-temporal information where time is used to reveal connected features and put them in a logic sequence (Oberholzer 2000, Dransch 1995).

Animation can be used to show changes in spatial or thematic attributes of objects. If the location, size or shape of a feature changes over time, this can be represented either directly in the images or maps, or indirectly through symbology changes. Thematic changes (such as changing water quality measures) can be shown using symbols, colours or movement.

Ideally, components that provide animated content should also provide control over the animation. In particular, the following features are useful to control, either during creation of the animation, or by the user (adapted from (Andrienko 2003) ):

• Speed of the animation;

• Direction – forth or back;

• Extent (start and finish points);

• Moments/intervals to include in animation:

ƒ Step (i.e. the interval between time moments successive animation frames refer to);

ƒ Moments or periods within a cycle;

ƒ Arbitrary selection.

• Smoothness (including interpolation between frames).

Fading and morphing are sometimes used in association with animation to show changes progressively in a sequence of images so that changing elements may be visible in more than one image at a time.

Animation can be useful in portraying changes including existential changes, spatial changes and thematic change. A detailed analysis of data is, however, difficult with animations, and periodic or cyclical trends can be difficult to identify.

3.5.3 Linked Views

Often one particular view of data (for example a 2D map view) is insufficient to allow proper understanding of a data set. Linked views refer to interfaces with several windows displaying different perspectives, dimensions, or thematic parameters of a data set, all of which are

dynamically linked. This might provide, for example, a navigable 2D spatial representation with thematic parameters represented in colour, and an associated profile view showing sub-surface structures, both linked in the temporal dimension so that moving a time-slider changes the state of both views simultaneously.

Figure 16 A set of dynamically linked views, source: (Uhlenkuken 2000)

Timelines are a commonly linked component – a timeline is displayed along with the map view and current time indicated as the thematic and spatial content changes on the map.

3.5.4 Time in the Third Dimension

The use of the third dimension as a temporal axis allows thematic changes to be displayed over particular locations. In these examples, a 2D map is displayed, and the third (vertical) dimension is used to display details of parameters over time. Variations in colour or size of indicators can represent the characteristics of events or parameter changes. Figure 17 shows an example of this

‘space-time cube’ approach where events are displayed by circles in the vertical axis. A similar

VISUALISATION COMPONENT REQUIREMENTS

approach is given in (Tominski 2005) and shown in Figure 18. In this instance, multiple attributes are displayed at once using each side of the pencil (left image) for a different parameter. In the right image, cyclical patterns in the data are emphasised through displays using helix icons.

Figure 17 "Space-Time Cube" showing events at different times (vertical axis), source: (Gatalsky 2004)

Figure 18 Temporal changes using the vertical axis and pencil (left) or helix (right) icons, source: (Tominski 2005)

3.5.5 Embedded Graphs

Embedding graphs and other diagrams within map images can also be useful in displaying temporal aspects of a data set. This is particularly common in applications such as GoogleMaps where extra content can be linked to marked sites in the map display. Selecting a site can then display a time-series graph (see Figure 19). A disadvantage of this approach is that the graph can temporarily obscure significant regions of the map, and graphs from multiple sites can not be compared.

Figure 19 GoogleMaps interface showing an embedded graph, source:

http://www.wron.net.au/DemosII/DamData/DamMap.aspx