Linear Referencing Dynamic Segmentation and Caltrans’ Implementation
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Transcript of Linear Referencing Dynamic Segmentation and Caltrans’ Implementation
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