In most DOTs, various types of boundaries are expressed and managed as LRS events. The following table shows how ADOT records maintenance district boundaries.
|District Name||Organization Name||Route||From MP||To MP|
|Southwest||Gila Bend||I 008||79.87||147.60|
|Southcentral||Casa Grande||I 008||147.61||178.33|
However, polygonal representation is often preferred for presentation and for ease of integration with other GIS resources. To this end, ADOT had developed a mostly manual polygonization process every time a change in district/unit events occurred.
Was it possible to fully automate the LRS Event to Polygon conversion, thus ensuring data fidelity and consistency while increasing productivity?
The ADOT Dataviz & Review team was challenged!
Karol Francis took the research to heart and saw the potential in Thiessen Polygons. Thiessen Polygons are a basic form of Voronoi Tessellation, where a plane with a given set of objects is partitioned such that every point in each partition is closer to its own object than any other objects.
Karol further translated the concept into a workflow:
a) geocode route events from a spreadsheet to generate linear features,
b) break line geometries into points,
c) partition the state of AZ by all the points via Voronoi Tessellation, and
d) dissolve the resulting partitions by district/organization attributes.
Most advanced GIS software, including ArcGIS, QGIS, and FME, have Thiessen or Voronoi processing built-in. The team used FME for its powerful integration and automation capabilities for the implementation. The portion of the FME workspace conducting Voronoi Tessellation is shown in Figure 1. Figure 2 is the outcome of ADOT’s maintenance district/organization boundary map.