Washington Metropolitan Area Transit Authority Linear Referencing System

Washington Metropolitan Area Transit Authority Linear Referencing System in WMATA GIS Voliya Arakkal Sr. GIS Analyst GIS in Public Transportation Conference September 15, 2011

Purpose • To explain Linear Referencing System • Importance in transit GIS • Building LRS with legacy data

What is Linear Referencing System? • Three ways to specify a location – X, Y Coordinates – Refer to grid or sphere – Address reference system – Linear Reference System – Measure along a linear feature from a known point

Importance of LRS • Great method to specify assets along transit line. Legacy data from different sources LRS Database for analysis and query • Translate Legacy data into useful database.

- s Fr Di om le scr diff ga ep er cy an en da cy t s ta in ou rc e - Issues with WMATA Legacy data • WMTA Legacy Data

LRS in WMATA GIS • LRS is used in WMATA primarily to specify locations along rail lines – WMATA rail lines have no address. – Half the system in tunnels. – Most of the track lines built prior to GPS technology. – 100 feet chain marker along all rail lines for location reference.

Build Linear Reference System • Create Route • Calibrate route • Post events

Create Route • A linear feature will convert to route with direction and measurement. • Unique route identifier • Line Measure field – Length – Start value – Start and End Values

Calibrate Route • Set of well known calibration points. - 100 feet chain marker points • Recalculate and adjust route measure.

Post Legacy data • Event table - A table containing route events - Point events - Line events • Yields Feature class

Feature class from events data • Track segment location • Asset location

Validate location

ont rac tru t sect ctio ion n. nin g atio us st s C ase -1 tio n lica mp

Complications Case - 2 • Deliberate alignment of stationing between tracks - Reduce the impact of different curves in outbound and inbound track

Calibration Method 1 - Ratio • Equation Point - Difference > 1 feet - Difference < 50 feet Equation caused 1 - Track linear measure between 66+00 and 65+00 = 65 feet 2 – 35. 14 feet track buried in. 14 feet Survey - 1 65+15. 29 Survey point Survey - 2 65+50. 15 Survey point

Solution • Two additional calibration points for each equation values - 6515 and 6550 New Calibration points are, 1. 2. 15 feet 6515 - Measure 15 feet from 6500 towards 6600 6550 - Measure 50 feet from 6600 towards 6500 50 feet After calibration points between 6515 and 6550 will posted inside. 14 feet line . 14 feet

Calibration Method 2 - Approximation • Equation Point - Difference • Solution < 1 feet - Mean value - New calibration point 6389. 925

Calibration method 3 – Line Break • Equation difference > 50 feet - Solution Split line at equation point. Line 1 - End point – Calibration point 1 - Measure value - 48500 feet Line 2 -Begin point –Calibration point 2 - Measure value - 97950 feet

Summary • Linear referencing System is the best method for, - Spatial legacy data - Yields GIS benefits -Analysis -Query

Questions and Discussion