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

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

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 What is Linear Referencing System?

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

WMTA Legacy Data Issues with WMATA Legacy data - From different sources - Discrepancy in 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 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 Survey Survey point Survey Survey point Equation caused 1 - Track linear measure between and = 65 feet 2 – feet track buried in.14 feet

15 feet50 feet.14 feet Solution Two additional calibration points for each equation values and 6550 New Calibration points are, Measure 15 feet from 6500 towards Measure 50 feet from 6600 towards 6500 After calibration points between 6515 and 6550 will posted inside.14 feet line

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

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

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

Questions and Discussion