title: Development of Time-location Weighted Spatial Measures Using Global Positioning System Data.
abstract: OBJECTIVE
  Despite increasing availability of global positioning system (GPS), no research has been conducted to analyze GPS data for exposure opportunities associated with time at indoor and outdoor microenvironments. We developed location-based and time-weighted spatial measures that incorporate indoor and outdoor time-location data collected by GPS.
  
  
  METHODS
  Time-location data were drawn from 38 female subjects in California who wore a GPS device for seven days. Ambient standard deviational ellipse was determined based on outdoor locations and time duration, while indoor time weighted standard deviational ellipse (SDE) was developed to incorporate indoor and outdoor times and locations data into the ellipse measure.
  
  
  RESULTS
  Our findings indicated that there was considerable difference in the sizes of exposure potential measures when indoor time was taken into consideration, and that they were associated with day type (weekday/weekend) and employment status.
  
  
  CONCLUSIONS
  This study provides evidence that time-location weighted measure may provide better accuracy in assessing exposure opportunities at different microenvironments. The use of GPS likely improves the geographical details and accuracy of time-location data, and further development of such location-time weighted spatial measure is encouraged.
authors.affiliation: Department of Epidemiology and Biostatistics, Texas A&M School of Rural Public Health, College Station, TX, USA.
authors.forename: Daikwon
authors.initials: D
authors.lastname: Han
authors.forename: Kiyoung
authors.initials: K
authors.lastname: Lee
authors.forename: Jongyun
authors.initials: J
authors.lastname: Kim
authors.forename: Deborah H
authors.initials: DH
authors.lastname: Bennett
authors.forename: Diana
authors.initials: D
authors.lastname: Cassady
authors.forename: Irva
authors.initials: I
authors.lastname: Hertz-Picciotto
keywords: Global positioning system; Indoor time-location weighted spatial measure; Time-location data