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(a)
(b)
α
2
α
1
n
t+
1
n
t
Roa
d 2
Road 1
Roa
d 2
Road 1
Fig. 2 Spatial pattern of log points in cross-sections. a Changing direction. b No turns
connecting the two nodes has to be in the same direction to con
rm the angle of
α
rather than 180
. For the sake of simplicity, in practice, we restrict the angle as to
− α
be less than 90
°
.
2.1.3 Connectivity
Apart from the distance and direction information, one of the most important factors
is the connectivity, which means the topology information of the road and cross
sections in the geographical representation. This variable is quite important to
lter
out the un-reasonable roads and increase search efficiency. In practice, only these
roads, which are connected at least at one node with the previously con
rmed road
segment, are treated as connected.
2.1.4 Other Factors
The accuracy of GPS data depends on signal strength, sensitivity of the sensor and
the spatial context of the travel environment. In principle, the more satellites, the
less noise in the measurements. In addition, the difference in complexity of trav-
eling environment results into different travel patterns in the sense that travel speed
in the high density city centres and suburban areas differ signi
cantly. Therefore, it
is also necessary to incorporate velocity information and accuracy of the GPS
measurements.
2.2 The BBN Model
The method we adopted here is the Bayesian Belief Network (BBN), which
replaces ad hoc rules with a dynamic structure, leading to improved classi
cation if
consistent evidence is obtained over time from more samples (more traces).
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