Washington Metropolitan Area Transit Authority

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

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 – 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. • Translate Legacy data into useful database. Database for analysis and query Legacy data from different sources LRS

Issues with WMATA Legacy data - From different sources - Discrepancy in 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

Complications Case - 1 • Discontinuous stationing • - Multiple contract section • during construction.

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, 6515 - Measure 15 feet from 6500 towards 6600 2. 6550 - Measure 50 feet from 6600 towards 6500 15 feet 50 feet After calibration points between 6515 and 6550 will posted inside .14 feet line .14 feet

Calibration Method 2- Approximation • Equation Point • - Difference < 1 feet • Solution • - 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

Washington Metropolitan Area Transit Authority