Linear Referencing Dynamic Segmentation and Caltrans Implementation

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Linear ReferencingDynamic Segmentationand
Caltrans Implementation
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Overview

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

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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)

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Purpose
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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

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Linear Referencing Systems

• Route ID
• Measures

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LRS - Linework

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

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LRS Building Routes

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

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LRS Calibrating Routes

• Calibration points adjust uniform measures to
  known points
• Bridges, intersections, etc.

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LRS Calibrating Routes

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

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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

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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

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Dynamic Segmentation
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Reality Steps In..The Caltrans Experience

• Introduction of gaps and overlaps
• Historical routes
• Meandering routes
• Route breaks
• Centerlines vs Carriageways
• Confusion

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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

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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

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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?

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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

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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?

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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

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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

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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

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Confusion

• Many Caltrans staff dont 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

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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

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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)
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Section Table - 156
Measures are zero at Route begin, increment to
route end
Postmiles zero out at each county
Equation at end of bypass
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HQ Postmiler Tool
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D5 Assign Measures Tool
ArcMap
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Results