title: Accuracy of a low-cost global positioning system receiver for estimating grade during outdoor walking.
abstract: The aim of this study was to assess, for the first time, the accuracy of a low-cost global positioning system (GPS) receiver for estimating grade during outdoor walking. Thirty subjects completed outdoor walks (2.0, 3.5 and 5.0 km · h-1) in three randomized conditions: 1/level walking on a 0.0% grade; 2/graded (uphill and downhill) walking on a 3.4% grade; and 3/on a 10.4% grade. Subjects were equipped with a GPS receiver (DG100, GlobalSat Technology Corp., Taiwan; ~US$75). The GPS receiver was set to record at 1 Hz and its antenna was placed on the right shoulder. Grade was calculated from GPS speed and altitude data (grade  =  altitude variation/travelled distance  ×  100). Two methods were used for the grade calculation: one using uncorrected altitude data given by the GPS receiver and another one using corrected altitude data obtained using map projection software (CartoExploreur, version 3.11.0, build 2.6.6.22, Bayo Ltd, Appoigny, France, ~US$35). Linear regression of GPS-estimated versus actual grade with R2coefficients, bias with 95% limits of agreement (±95% LoA), and typical error of the estimate with 95% confidence interval (TEE (95% CI)) were computed to assess the accuracy of the GPS receiver. 444 walking periods were performed. Using uncorrected altitude data, we obtained: R2=  0.88 (p  <  0.001), bias  =  0.0  ±  6.6%, TEE between 1.9 (1.7-2.2)% and 4.2 (3.6-4.9)% according to the grade level. Using corrected altitude data, we obtained: R2=  0.98 (p  <  0.001), bias  =  0.2  ±  1.9%, TEE between 0.2 (0.2-0.3)% and 1.0 (0.9-1.2)% according to the grade level. The low-cost GPS receiver used was weakly accurate for estimating grade during outdoor walking when using uncorrected altitude data. However, the accuracy was greatly improved when using corrected altitude data. This study supports the potential interest of using GPS for estimating energy expenditure during outdoor walking.
authors.affiliation: Movement, Sport and Health Laboratory (EA 1274), Faculty of Sport Sciences, University of Rennes 2, F-35000 Rennes, France.
authors.forename: Pierre-Yves
authors.initials: PY
authors.lastname: de Müllenheim
authors.forename: Ségolène
authors.initials: S
authors.lastname: Chaudru
authors.forename: Marie
authors.initials: M
authors.lastname: Gernigon
authors.forename: Guillaume
authors.initials: G
authors.lastname: Mahé
authors.forename: Sandrine
authors.initials: S
authors.lastname: Bickert
authors.forename: Jacques
authors.initials: J
authors.lastname: Prioux
authors.forename: Bénédicte
authors.initials: B
authors.lastname: Noury-Desvaux
authors.forename: Alexis
authors.initials: A
authors.lastname: Le Faucheur