Route reference does NOT represent a new data model nor is it a new Linear Referencing Method (LRM); it is an addon object to any existing LRS model. Governed by specific rules, maintained by trigger conditions, and accessible via APIs, route reference serves to dynamically translate the reference-based LRMs that describe LRS events to the measure-based LRM for dynamic segmentation processes (Figure 1). 

Let us call the route reference a “layer”, taking a page from the NCHRP model.

Figure 1. Route Reference Explained

Here is the secret sauce that makes route reference magical:

Content of Route Reference Layer

  • Include all intersecting routes, but exclude un-stable, non-visible boundaries or markers. 
  • Keep content schema lean and mean.  Some suggested columns are seen in Table 1.
  • The route reference layer should have a high degree of temporal consistency.  Table 1 shows some route reference rows after the extension is open to traffic in the fictitious LRS infrastructure illustrated in the previous article. 

Table 1. Route Reference Table After SR 001 Extension

Route ID Meas  XType XName Qualifier Begin Date End Date
SR 001 0 Node     1/23/2020  
SR 001 2,112 Route Corona Blvd   1/23/2020  
SR 001 0 Node     1/1/2001 1/23/2020
SR 001 4,752 Route Maple Ln   1/23/2020  
SR 001 2,640 Route Maple Ln   1/1/2001 1/23/2020
SR 001 4,752 Route A St   1/23/2020  
SR 001 2,640 Route A St   1/1/2001 1/23/2020
SR 001
SR 001 90,816 Node     1/23/2020  
SR 001 88,704 Node     1/1/2001 1/23/2020
  • Not all changes to routes are of historical significance.  Separate error corrections vs route updates that reflect the network change.

Maintenance of Route Reference Layer

  • The initial building of the route reference table should contain all referents used by all events.
  • Ensure the incremental update to the route reference table is efficient and the trigger mechanisms are robust.
  • Development tools with geospatial process capabilities can be used to build and maintain the route reference layer, which includes Esri tools, FME, and direct SQL. Route reference service APIs can be coded in any backend languages to support web-based event editing applications.

Use of Route Reference Layer in Event Definition

  • Not all referents are the same, so be choosey in picking the referent when defining your event.  Here is the hierarchy: 
    1. Planted posts
    2. Route intersections
    3. Canals or rail lines
    4. Stable boundaries, such as country/state boundaries with well-marked posts
    5. Start / end of route segment
  • Avoid using duplicate referents (US 19) and use qualifiers sparingly in your event definition.
  •  Do NOT include measure columns in your business event tables. 

This concludes the three-part series on the whys and hows of route reference layers.  The moral of the article series is that the natural way humans communicate linear event locations, lost in the frenzied dash to technology adoption, can be restored with smart and strategic tweaks to your existing LRS implementations.