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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. - PowerPoint PPT Presentation
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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
G Known 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