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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 – 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.
Issues with WMATA Legacy data
• WMTA 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
• •
Track segment location
G
Asset location
E RSE C TI ON /1 1S T NW
300 and 1200 Chainmarker after calibration
N/ G AND
Calibrated Route
ST INT NT LI
Known Chain Marker Values
NE M ET ROCE FAN SHA ER PL AT TI FORM ON END FT L 0 OCA 21 0 EM ERGEN NW CY EX IT 30 0 T O GST EM ERGEN CY PH ONE LOCA 10 TH ST 60 NW 0 TI O N 80 0
G
100 200 300 400 500 600
G
700
G G G
800 900 1000 PL AT FORM BE GIN POINT 12 00
Validate location E RSE C TI ON /1 1S T NW LI NE BEGI N/ G AND 12 TH ST INT END NT E R PL A TF TI ORM ON M E TROCE FAN SHA FT L 0 OCA 21 0 EM ERGEN CY EX 30 0 IT T O GST EM NW ERGEN 10 TH ST 60 NW 0 CY PH ONE LOCA TI O N 80 0
G
100 200 300 400 500 600
G
700 800 900 1000 PL A TF ORM BE GIN P OINT 12 00
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, 1.
2.
6515 - Measure 15 feet from 6500 towards 6600
6550 - Measure 50 feet from 6600
towards 6500
) "
5 6 0 0
) " ) " ) " ) "
5 6 0 6
) "
5 6 0 9
) "
6 6 0 0 15 feet 50 feet
) "
5 6 5 1
) "
5 6 0 2
) "
5 6 0 5
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