Linear Referencing Dynamic Segmentation and Caltrans’ Implementation

1. Linear ReferencingDynamic Segmentationand Caltrans’ Implementation

2. Overview • Purpose • Linear Referencing Systems • The underlying reference data • Dynamic Segmentation • The process of extracting from the LRS • Complications • Caltrans Solutions

3. Purpose • The purpose of Linear Referencing Systems and Dynamic Segmentation is to convert text descriptions of locations along linear features into points or lines • Linear features can be roads, rail, streams, others • Location description includes name of linear feature and a measure (or measures)

4. Purpose

5. Linear Referencing Systems • LRS • The underlying reference data that supports Dynamic Segmentation • Consists linework that comprises a set of “Routes” • Each has a unique ID • Each has “Measures”

6. Route ID Measures Linear Referencing Systems

7. Clean linework is a must Watch gaps, self-intersects, zero-length segments Take advantage of Geodatabase Topology Rules Generalize (simplify linework) LRS - Linework

8. Create Routes tool Route ID (Rte) Initial From, To Measures (bpm, epm) Use a Model to enforce consistency of the process LRS – Building Routes

9. Calibration points adjust uniform measures to known points Bridges, intersections, etc. LRS – Calibrating Routes

10. Calibrate Routes Tool Route ID (Rte) Measure Field (Measure) Use a Model to enforce consistency LRS – Calibrating Routes

11. As each route is completed it is added to an LRS Feature Class in a Geodatabase Use a Model to enforce consistency LRS - Append Route to GDB

12. “Dynamically segmenting an LRS” Identifying points or lengths along an LRS Requires tabular data with Route ID Measure (point), or Begin and End Measures (line) Called an “Event” layer Can be left as an Event layer Events will change as underlying table changes Or can be converted to Shapefile, Feature Class, etc. Better performance Dynamic Segmentation

13. Dynamic Segmentation

14. Reality Steps In…..The Caltrans Experience • Introduction of gaps and overlaps • Historical routes • Meandering routes • Route breaks • Centerlines vs Carriageways • Confusion

15. Gaps and Overlaps • In the beginning the postmile system was a true linear reference system • Began at 0 at county line • Incremented sequentially to end of county • Gaps and Overlaps are introduced by • Bypasses that tend to increase the length of a section of highway • Realignments that tend to shorten the length of a highway • Can no longer reliably calculate distances using postmiles

16. Identify different postmiles that are the same location Resulting from gaps and overlaps SBt 156 R13.760 = R15.012 In this case the Hollister bypass resulted in a shorter segment of highway 156 Equation Points

17. Historical Routes • Historical routes are created by • Route adoption, where a route moves from one set of roads to another • Check out cahighways.org • Bypasses • Data are associated with historical routes • Historical routes should be retained in route databases, but generally are not • Date should be a component of a Route ID • The route on which date?

18. Meandering Routes • Postmiles go back to zero at the beginning of each county • Therefore a unique Route ID includes the county • Some routes cross back and forth between two counties • Postmiles generally continue to increment

19. Route Breaks • Where a route temporarily ends at another highway • Resumes many miles away • Postmile system considers the distance of the break to be 0 miles • Problematic when using postmiles to calculate distances • Ambiguous location – which 8.927?

20. Most of the SHS is treated as a centerline With various independent alignments The SHS is comprised of both single and dual carriageways For small scale mapping, treating as a centerline is best Centerline vs Carriageways

21. Gaps, overlaps, extensions, other issues are handled with postmile prefixes Independent right and left alignments are handled by “R” and “L” suffixes SB 1 R0 SB 1 R29 SB 1 M29 SB 135 M10.75 SB 135 R10.75 SB 135 10.75 MON 101 R42.4L MON 101 R42.4R Prefixes and Suffixes

22. R – Realignment M – Realignment of a realignment D – Duplication due to meandering county line H – Realignment of duplication T – Temporary connection L – Overlap due to correction or change N – Realignment of M mileage C – Commercial lanes paralleling main highway Postmile Prefixes

23. Many Caltrans staff don’t understand the postmile system Missing prefixes Postmiles rounded to non existent values Wrong county/route combinations Any tabular data with County, Route, Postmile references are wrong about 25% of the time Confusion

24. Caltrans Dynamic Segmentation Applications • A separate application is needed to convert Caltrans postmiles into true measures • Makes use of a Section table • Cross references postmiles to measures • Underlying LRS ignores counties, gaps, overlaps • Accumulates segment lengths from beginning of the route • It has a true linear measure system • And logic to determine corresponding measure for a postmile

25. Section Table - 227 Unique Route ID (Right and Left) Begin and End Postmiles Begin and End Measures 0.001 postmile gap (equation point) 0.227 postmile gap (equation point)

26. Section Table - 156 Measures are zero at Route begin, increment to route end Postmiles zero out at each county Equation at end of bypass

27. HQ Postmiler Tool

28. D5 Assign Measures Tool ArcMap

29. Results