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2016 ; 30
(2
): 316-333
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Why GPS makes distances bigger than they are
#MMPMID27019610
Ranacher P
; Brunauer R
; Trutschnig W
; Van der Spek S
; Reich S
Int J Geogr Inf Sci
2016[Feb]; 30
(2
): 316-333
PMID27019610
show ga
Global navigation satellite systems such as the Global Positioning System (GPS)
is one of the most important sensors for movement analysis. GPS is widely used to
record the trajectories of vehicles, animals and human beings. However, all GPS
movement data are affected by both measurement and interpolation errors. In this
article we show that measurement error causes a systematic bias in distances
recorded with a GPS; the distance between two points recorded with a GPS is - on
average - bigger than the true distance between these points. This systematic
'overestimation of distance' becomes relevant if the influence of interpolation
error can be neglected, which in practice is the case for movement sampled at
high frequencies. We provide a mathematical explanation of this phenomenon and
illustrate that it functionally depends on the autocorrelation of GPS measurement
error (C). We argue that C can be interpreted as a quality measure for movement
data recorded with a GPS. If there is a strong autocorrelation between any two
consecutive position estimates, they have very similar error. This error cancels
out when average speed, distance or direction is calculated along the trajectory.
Based on our theoretical findings we introduce a novel approach to determine C in
real-world GPS movement data sampled at high frequencies. We apply our approach
to pedestrian trajectories and car trajectories. We found that the measurement
error in the data was strongly spatially and temporally autocorrelated and give a
quality estimate of the data. Most importantly, our findings are not limited to
GPS alone. The systematic bias and its implications are bound to occur in any
movement data collected with absolute positioning if interpolation error can be
neglected.