Washington Metropolitan Area Transit Authority

Voliya ArakkalSr.GIS Analyst

GIS in Public Transportation ConferenceSeptember 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

LRSDatabase 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.

- 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

G G G G G G G GG G_̂ _̂_̂ _̂ _̂ _̂ _̂700200 900800600500400300100 1000

_̂ Points Located after Calibration

GKnown Chain Marker Values Used for Calibration

Calibrated Route

Track Segment between 300 and 1200 Chainmarker after calibration

• Track segment location• Asset location

G G G G G G G GG G_̂ _̂_̂ _̂ _̂ _̂ _̂700200 900800600500400300100 1000

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 - 1 65+15.29 Survey point

Survey - 2 65+50.15 Survey point

Equation caused 1 - Track linear measure between 66+00

and 65+00 = 65 feet 2 – 35.14 feet track buried in .14 feet

")

") ")") ")")") ") ") ")

6560

6520

6590

65506515

6600

6500

")

")

")

")

") ")

")

")

")

")

6520

6550

6515

15 feet 50 feet

.14 feet

Solution

• Two additional calibration points for each equation values- 6515 and 6550

New Calibration points are, 1. 6515 - Measure 15 feet from 6500 towards 66002. 6550 - 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 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