Beyond the Straight Line Diagram

The one-dimensional nature of Straight Line Diagrams (SLD) makes them ideal for viewing the relationship of many LRS features along a common dimension, the route measure. However, even with the additional forms introduced in a previous Gistic blog post, Reinventing the SLD, SLDs are ill-fitted for in-depth exploration of individual LRS feature layers in multiple dimensions. This is where Layer Attribute Diagrams (LADs) and Time Space Diagrams (TSDs) come into play.

Layer Attribute Diagram

An LAD is drawn in a two-dimensional Cartesian space with route measure on the X-axis and a user-selected attribute on the Y-axis. The Y-axis attribute can be numeric values, either continuous or discrete. It can also be a string type that represents a category, such as pavement types.

Figure 1 shows an LAD of Crashes on a stretch of highway. After the user defines route segment, picks the Crashes layer and the Severity attribute for symbology, attributes from the Crashes layer that qualify for the Y-axis dimension are populated in the Attribute list box. The LAD is drawn as the user selects the Y-axis attribute, such as Num of Vehicles in this case.

With symbolized points drawn on two-dimensional space, the LAD allows its consumers to visualize three dimensions: Location, Num of Vehicles and Severity.

Figure 1. Crashes data drawn on dynamic LAD

Time Space Diagrams

The Time Space Diagram (TSD) can be considered a special case of LAD where the Y-axis represents time. The TSD has two interesting aspects: One is that both the X and Y axes are common dimensions to most, if not all, LRS feature layers. Therefore, two or more LRS feature layers can be represented in the space, allowing us to cross-relate them. The other is that a temporal LRS feature manifests in one of the four shapes in the TSD space shown in Figure 2 – Point Instant, Point Period, Linear Instant and Linear Period.

Figure 2. Temporal LRS segment types in TS illustration

Figure 3 shows a TSD example. Crashes are represented as Point Instant events, symbolized based on the crash severity, while the Speed Limit zones are represented as Linear Period, color-coded based on different speed limit ranges. Consumers of this TSD can relate crash frequency and severity with the posted speed limit. The TSD seems to suggest that the change of speed limit from mileposts 310.5 to 312.5 in mid 2014 was an effort to mitigate the high number of accidents on the stretch.

Figure 3. TSD with Crashes symbolized by Severity on the backdrop of Speed Limit zones

Conclusion

LADs and TSDs are nifty when slicing and dicing individual LRS feature layers. Why not pack them in our LRS toolbox as we tackle LRS data changes?!