USING SENSECAM TO PURSUE “GROUND TRUTH” FOR GPS TRAVEL SURVEY

Ohio, May 201314th TRB Transportation Planning Applications Conference

Li Shen and Peter StopherInstitute of Transport and Logistics StudiesThe University of Sydney

Background

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GPS Travel Survey

Global Positioning System (GPS) technology has been used in travel surveys

since the late 1990s. GPS devices could correct the trip misreporting issue

caused by respondents and improve the accuracy of travel data.

GPS ≠ Ground Truth Signal loss Signal noise

Ground Truth

Ground Truth in Travel Surveys

- Reflect real trips

- Validate GPS data processing results (i.e., accuracy)

- Used for learning system for GPS data processing

Approaches to obtaining ground truth

- Prompted Recall (PR) survey

- Experiments

- Visual data (images, videos, etc.)

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About SenseCam

SenseCam (supported by Microsoft) is a passive digital camera which contains a

number of different electronic sensors.

• Takes about 3000 photos per day

• Time stamped and battery lasts for 18 hours/day

• Stores 8‐10 days of data

Certain changes in sensor readings can be used to

automatically trigger a photograph to be taken. If nothing

changes, it takes photos every 50 s.

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Travel Mode

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Activity (Trip purpose)

Data

› Purposes of This Study

Use SenseCam and GPS devices to pursue ground truth

Understand GPS devices’ performance based on the ground truth

› Data Collection

12 volunteers in Oxford (3-day survey in Oxford)

7 volunteers in Sydney (5-day survey in Sydney)

Volunteers were asked to carry both SenseCam cameras (provided by University of Oxford) and GPS devices (provided by the University of Sydney)

› Data Processing

GPS: G-TO-MAP + Manual map editing

SenseCam: SenseCam Browser + Manual image editing

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Comparison for Sydney Survey (1)

Number of Trips %

Match Segment match 155 59.4%

Split by SenseCam 29 11.1% 74.7%

Split by GPS 11 4.2%

GPS map editing 14 5.4%

Not in GPS 50 19.2%

Not in SenseCam 2 0.8%

Total 261 100%

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Trip Comparison (based on start/end time and travel duration)

Reasons for GPS data missing

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Reasons Number of Trips %

Cold start 28 43.8%

Short duration trips (<2mins) 21 32.8%

Travelling in special areas 9 14.1%

Unknown 6 9.4%

Total 64 100%

Comparison for Sydney Survey (2)

Reasons for failing to split trips

Reasons

Short duration trips (<2mins) 22 75.9%

Short duration activities (<2mins) 7 24.1%

Total 29 100%

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Comparison for Sydney Survey (3)

for Sydney Survey (2)

Number of trips %

Match 139 89.7%

Not match 16 10.3%

Total 155 100%

Travel Mode Comparison

Trip Purpose Comparison

Number of trips %

Match 131 84.5%

Not match 24 15.5%

Total 155 100%

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Comparison for Sydney Survey (4)

Mode All trips % Missing in GPS %

Walk 158 60.5% 52 81.3%Bike 12 4.6% 0 0.0%Car 57 21.8% 7 10.9%Bus 4 1.5% 0 0.0%Train 28 10.7% 4 6.3%Boat 2 0.8% 1 1.6%Total 261 100% 64 100%

Mode distributions of all trips and missing trips in GPS

Summary

› SenseCam can help find the “ground truth”.

› SenseCam can help understand the GPS device’s performance in travel data collection.

› GPS data missing is more likely to happen

• at the beginning of a trip

• for short duration trips

› Trips recorded in the GPS devices may be split

• when a short duration trip occurs at the beginning/at the end of the whole journey

• when a short duration activity occurs during the whole journey

› Missing trips are more likely to be walking trips.

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Questions and Discussion

Li Shen, Institute of Transport and Logistics Studies, The University of SydneyE-mail: li.shen@sydney.edu.au

Peter R. Stopher, Professor Institute of Transport and Logistics Studies, The University of SydneyEmail: peter.stopher@sydney.edu.au

Ohio, May 201314th TRB Transportation Planning Applications Conference