GIS for Transportation: Principles, Data and Applications

6.1 Linear Referencing Systems


Last week, we explored the purpose and components of an LRS. This week, you’ll have an opportunity to solidify your understanding by completing some hands-on exercises to create routes, calibrating linear measures along a route, and using dynamic segmentation to convert event tables into event features. You’ll also see how GIS software can be used to reduce the burden associated with the creation and management of an LRS and event data.

Assignment 6-1 (25 points)

Complete ESRI’s ArcMap tutorial on linear referencing. Submit an M.S. Word document to Assignment 6-1 in Canvas which addresses the following questions:

  1. In Exercise 2, you created a route feature class from the “base_roads” feature class. Why do you think some of the features in the “base_roads” feature class are lacking a route identifier? (2 points)
  2. In Exercise 2, you created a route feature class. In creating the route, you defined how measures along the route would be determined. You had three choices: LENGTH, ONE_FIELD, and TWO_FIELDS. In your own words, describe how you think the measures along the route are determined using each of these methods. (3 points)
  3. How does a route feature class differ from a general linear feature class? (2 points)
  4. How does roadway grade (i.e., hills and valleys) affect the measures along a route? (2 points)
  5. In Exercise 3, you learned how to locate a point or range along a route using the "Find" tool. Using this tool, locate the section of route 20000264 between milepost 8.45 miles and 14.93 miles. Using the functions available on the context menu, which is visible when you right-click the identified section roadway in the "Find" dialog, draw, label, and zoom to this location on the map. Include a screenshot of the resulting map display. (3 points)
  6. Include a screenshot of the ArcMap window, showing both the table of contents and the map displaying the accident and pavement events you created in Exercise 4. (3 points)
  7. In completing the tutorial on linear referencing, did you perform dynamic segmentation? If so, identify the exercise(s) and step(s) where dynamic segmentation took place. (2 points)
  8. What is the difference between an event table and an event layer? (2 points)
  9. Include a screenshot of the map you created at the end of Exercise 4 showing the injury accidents which occurred on roadways with a pavement rating below 50. (3 points)
  10. Briefly, compare and contrast the route calibration in Exercise 2 to that in Exercise 5. (3 points)

In the past few years, ESRI has been promoting their Roads and Highways product which is designed to provide a fuller featured set of tools for transportation organizations to manage their LRS and roadway event data. A 57-minute introduction to this tool was provided at the 2013 ESRI users’ conference.

Straight Line Diagrams

Transportation organizations capture and maintain a large number of linearly referenced roadway events including:

  • Intersections
  • Bridges
  • Rail Crossings Locations
  • Legal Boundaries
  • Accidents
  • Signs
  • Speed Limits
  • Average Annual Daily Traffic (AADT)
  • Width of the Right of Way
  • Width of the Roadway Shoulders
  • Functional Class
  • Curves
  • Grades
  • Projects

Displaying more than a few of these on a single map can begin to clutter the map and make it difficult for the user to understand.

One tool which transportation organizations have used for many years to visualize road attribute information is a Straight Line Diagram (SLD). In an SLD, a roadway section of interest is presented as a straight line along with various roadway attributes or events. Often these roadway attributes are maintained by separate groups within the transportation organization, and sometimes they are linearly referenced using different LRMs. An SLD brings many attributes together with a uniform referencing method to facilitate visualization of the data and the potential identification of relationships between different data elements.

The specific layout of SLDs varies from one organization to another. One common layout for an SLD includes three components: a map component, a schematic component, and an attribute component. The map component often appears at the top of the SLD and presents the alignment of the route of interest. The schematic component, sometimes referred to as a stick diagram, presents the route as a straight line and can incorporate roadway features such as intersections, bridges, ramp entrances, and exits and legal boundaries. The attribute component includes roadway event data presented along the same horizontal axis. Linear events such as speed limit are displayed as a series of horizontal bars with the extent of each bar corresponding to the region over which the attribute has a constant value. Point events are displayed as point symbols positioned according to their location along the route.

Example layout of SLD with speed limits from 55 to 40, AADT, and where the crashes took place.
Figure 1 - Example SLD Layout

Historically, SLDs were manually created and assembled into books for reference across the organization. Given the effort required to generate SLDs in this manner, the books often reflected data which was somewhat dated. Today, most SLDs are created dynamically from current event data using sophisticated GIS software applications.

As an example, the Massachusetts Department of Transportation uses a web-based tool called Massachusetts Route Log to generate Straight Line Diagrams (SLDs). Here is an example SLD generated from this application for a portion of State Route 9.

Massachusetts Route Log tool displaying a portion of State Route 9.
Figure 2 - Massachusetts Route Log
Credit: MassDOT

Vermont’s Agency of Transportation (VTrans) has a similar SLD tool called VTrans Routelogs.

VTrans Route Log and Progress Chart
Figure 3 - VTrans Routelogs SLD
Credit: VTrans

Assignment 6-2 (10 points)

In this assignment, you will manually create an SLD for route 40001200 in Pitt County, North Carolina (the same county you worked with in Assignment 6-1). Submit the SLD as an M.S. Word document to Assignment 6-2 in Canvas. In your SLD, the positions of the events should be reasonable, but you can eyeball them. There is no need to be super accurate for the purposes of this assignment. The SLD should cover the portion of the route between milepost 8.5 and 11.5 and should include the following elements:

  1. The top portion of the SLD should include a map of the corresponding roadway alignment. (2 points)
  2. The central portion should present a linear representation of the route and all interesting routes. Only include routes maintained by the state. (2 points)
  3. The bottom portion of the diagram should display the following event data:
    1. Accident data (2 points)
      Use the accident event data from Assignment 6-1 and limit the accidents to those with injuries (Hint: Use an attribute query to see which accidents you’re interested in. Limit the route, measures, and the number of injuries accordingly).
    2. Pavement rating data (see Assignment 6-1) (2 points)
    3. Speed limit data based on the following table: (2 points)
      Route Speed Limit Data
      Route Speed Limit Beginning MP Ending MP
      40001200 25 0.0 3.0
      40001200 55 3.0 9.0
      40001200 35 9.0 10.0
      40001200 55 10.0 15.0

Note: You can use whatever tools you’d like to create the diagram as long as it is submitted as an M.S. Word document. If you’d like, you can hand sketch the diagram. Assuming you don’t create the SLD directly in Word, you would insert an image or paste an image of your SLD into Word. My personal recommendation is to use a combination of M.S. PowerPoint with its drawing tools along with the Snipper utility in Windows.