Linear Referencing Dynamic Segmentation and Caltrans Implementation Overview

Linear Referencing Dynamic Segmentation and Caltrans’ Implementation

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

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)

Purpose

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”

Linear Referencing Systems • Route ID • Measures

LRS - Linework • Clean linework is a must • Watch gaps, selfintersects, zerolength segments • Take advantage of Geodatabase Topology Rules • Generalize (simplify linework)

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

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

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

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

Dynamic Segmentation • “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

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

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

Equation Points • Identify different postmiles that are the same location • Resulting from gaps and overlaps • SBt 156 R 13. 760 = R 15. 012 • In this case the Hollister bypass resulted in a shorter segment of highway 156

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?

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

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?

Centerline vs Carriageways • 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

Prefixes and Suffixes • Gaps, overlaps, extensions, other issues are handled with postmile prefixes • Independent right and left alignments are handled by “R” and “L” suffixes • • SB 1 R 0 SB 1 R 29 SB 1 M 29 SB 135 M 10. 75 SB 135 R 10. 75 SB 135 10. 75 MON 101 R 42. 4 L MON 101 R 42. 4 R

Postmile Prefixes • 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

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

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

Section Table - 227 Unique Route ID (Right and Left) Begin and End Postmiles 0. 001 postmile gap (equation point) Begin and End Measures 0. 227 postmile gap (equation point)

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

HQ Postmiler Tool

D 5 Assign Measures Tool Arc. Map

Results