Linear ReferencingDynamic 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, self-intersects, zero-length 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 R13.760 = R15.012• 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 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

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• 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

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

0.001 postmile gap (equation point)

Unique Route ID(Right and Left)

Begin and EndPostmiles

Begin and EndMeasures

0.227 postmile gap (equation point)

Section Table - 156

Equation at end of bypass

Postmiles zero out at each county

Measures are zero atRoute begin, incrementto route end

HQ Postmiler Tool

D5 Assign Measures Tool

ArcMap

Results