Accuracy issues in the utilization of mobile Accuracy issues in the utilization of mobile yyphones for realphones for real‐‐time VGI data collectiontime VGI data collection
Paul A. ZandbergenPaul A. ZandbergenDepartment of GeographDepartment of GeographDepartment of GeographyDepartment of GeographyUniversity of New MexicoUniversity of New Mexico
OutlineOutline
• VGI data collection methods• Sensors on mobile phonesSensors on mobile phones• Examples of using mobile phones for VGI• Accuracy of mobile phone positioning• Implications of VGI• Implications of VGI
VGI Data CollectionVGI Data CollectionDedicated GPS receivers Mobile phones Other mobile devices
Geocode addresses Geotagging on imagery
Sensors on Mobile PhonesSensors on Mobile Phones
Video CameraLocation
Camera
Microphone
Compass
Accelerometer 3rd party add‐onsAccelerometer 3 party add ons
3rd Party Sensors3rd Party Sensors
Woodrow Wilson InternationalInternational
Center for Scholars, May 2009
Nature,April 23 2009April 23, 2009
WIRED,February 2009February 2009
Air Quality MonitoringAir Quality Monitoring
Visibility Android ApVisibility Android Ap
Running Pace and SpeedRunning Pace and Speed
Nike+ system
Human Activity and MobilityHuman Activity and Mobility
CitySense iPhone Ap
Traffic ConditionsTraffic Conditions
Waze iPhone Ap
Watercourse ConditionsWatercourse Conditions
Creek Watch iPhone Ap
Public TransitPublic Transit
Subway Arrival iPhone Ap
Accuracy of Sensors?Accuracy of Sensors?
Video CameraLocation
Camera
Microphone
Compass
Accelerometer 3rd party add‐onsAccelerometer 3 party add ons
Positioning on Mobile PhonesPositioning on Mobile Phones
A‐GPS WiFi CellularA GPS WiFi Cellular
Comparison of Positioning TechnologiesComparison of Positioning Technologies
Technology Autonomous GPS Assisted GPS WiFi Cellular
Beacons Satellites Satellites WiFi access pointsBeacons Satellites Satellites WiFi access points
Time to first fix Several seconds <1 sec <1 sec <1 sec
Positioning Horizontal, vertical, Horizontal, Horizontal only Horizontal onlyPositioning speed vertical, speed Horizontal only Horizontal only
Accuracy 3 to 5 m 5 to 10 m 50 to 100 m 100 m to > 1 km
Conditions Outdoors Outdoors Outdoor and Outdoors and Conditions Outdoors Outdoors indoors indoors
Coverage Anywhere Requires cellular network coverage
Requires cellular network coverage Requires cellular
network coveragenetwork coverage and WiFi coverage network coverage
ExampleExample
Positional AccuracyPositional Accuracy
H d M d M di ( )Hardware Mode Median error (m)
Garmin GPS GPS 3 m
iPhone A-GPS 8 m
iPhone WiFi 74 m
iPhone Cellular 600 m
Source: Zandbergen, P.A. 2009. Accuracy of iPhone locations: A comparison of assisted‐GPS, WiFi and cellular positioning. Transactions in GIS, 13(s1): 5‐25.
Cellular service areas in Rome, Italy. Source: Sevtsuk and Ratti, 2010
ChallengesChallenges
1. Availability of positioning technologies– Varies by phone and conditions
2. Switching between positioning modesg p g– Seamless for users, often not clear
3. Lack of metadata– hardware positioning mode accuracyhardware, positioning mode, accuracy
Implications for VGIImplications for VGI
1. Mobile phones have become the dominant platform for context‐rich VGI data collection
2. Some information may be very inaccurate2. Some information may be very inaccurate
3 M t d t i l ki3. Metadata remains lacking
VGI #5: A little bit of metadata in my lifeVGI #5: A little bit of metadata in my life….
• Type of phone e.g. iPhone• Operating System e.g. iOS 4.2• Application e.g. Nike+• Positioning mode e.g. A‐GPS• Positioning details e.g. 6 satellites, 3.0 DOP• Estimated precision e.g. +/‐ 10 m horiz. +/‐ 15 m vert.
Quality of Metadata?Quality of Metadata?
