USING SENSECAM TO PURSUE “GROUND TRUTH” FOR GPS TRAVEL SURVEY Ohio, May th TRB Transportation Planning Applications Conference Li Shen and Peter Stopher Institute of Transport and Logistics Studies The University of Sydney

Background 2 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.) 3

4 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.

5 Travel Mode

6 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 7

Comparison for Sydney Survey (1) Number of Trips% MatchSegment match % Split by SenseCam2911.1% 74.7% Split by GPS114.2% GPS map editing145.4% Not in GPS5019.2% Not in SenseCam20.8% Total261100% 8 Trip Comparison ( based on start/end time and travel duration)

Reasons for GPS data missing 9 ReasonsNumber of Trips% Cold start2843.8% Short duration trips (<2mins)2132.8% Travelling in special areas914.1% Unknown69.4% Total64100% Comparison for Sydney Survey (2) Reasons for failing to split trips Reasons Short duration trips (<2mins)2275.9% Short duration activities (<2mins)724.1% Total29100%

10 Comparison for Sydney Survey (3) for Sydney Survey (2) Number of trips% Match % Not match1610.3% Total155100% Travel Mode Comparison Trip Purpose Comparison Number of trips% Match % Not match2415.5% Total155100%

11 Comparison for Sydney Survey (4) ModeAll trips%Missing in GPS% Walk % % Bike % 0 0.0% Car % % Bus 4 1.5% 0 0.0% Train % 4 6.3% Boat 2 0.8% 1 1.6% Total % % 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. 12

Questions and Discussion Li Shen, Institute of Transport and Logistics Studies, The University of Sydney Peter R. Stopher, Professor Institute of Transport and Logistics Studies, The University of Sydney Ohio, May th TRB Transportation Planning Applications Conference