linear referencing dynamic segmentation and caltrans’ implementation
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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