Global Positioning System Reference
In-Depth Information
Figure 13 presents the percentage of solved spatial ambiguities after applying the map-
matching algorithm for Columbia, Portage, and Polk counties. This chart indicates that over
90% of incorrectly snapped data points collected in Columbia County were solved by the
algorithm when employing a 30-foot buffer size. Whereas, solved cases reached their
maximum values (68% and 64%) for Portage and Polk counties with 50 and 60-foot buffers,
respectively. As mentioned earlier, Polk County data was tested for buffer sizes every 20
feet, thus, there is no data for buffer sizes equal to 30 and 50 feet.
4.3.2 Speed
The map-matching algorithm determines the correct roadway centerline on which a vehicle
is traveling by computing feasible shortest paths between snapped data points. This
feasibility is sensitive to the allowable range utilized when comparing computed and
recorded speeds. The analysis of this variable examines the effect that it has on the
performance of the map-matching algorithm.
The average recorded speed (v) is computed using the recorded speeds (v 1 and v 2 ), as
shown in Equation 1. Equation 2 presents the computed speed calculation (s) given the
shortest distance traveled (D) and timestamps (t 1 and t 2 ) between a pair of snapped data
points. Subsequently, the algorithm accepts a tested path as feasible if the average recorded
speed is within the equally distributed speed range shown in Equation 3.
vv
+
v
=
1
2
(1)
2
D
s
=
(t
(2)
t )
2
1
SpeedRange
v
∈±
s
(3)
2
FN curves were computed for various buffer sizes and different speed range tolerances from
5 to 35 mi/h with increments of 5 mi/h for the three counties. Analysis results for this
variable show that feasible paths are rejected when small speed ranges are employed
leaving FN data points not snapped to any roadway centerline. On the contrary, as speed
range increases, FN percentages diminish since feasible paths are found during the speed
comparison process. Figure 14 shows FN curves for Columbia County with data collected
every 2 seconds. These curves are approximately parallel as the speed range varies, and
stabilize for speed ranges greater than 15 mi/h. Speed ranges equal to or greater than 25
mi/h are needed to minimize FN percentages in Portage and Polk counties. Further speed
range increase does not improve the results because all feasible paths are accepted. In
general, FN curves are steeper for small buffer sizes, and approach near-zero slope for
buffer sizes equal to or greater than 40 feet.
Analysis results for this variable indicate that the percentage of solved cases increases as
speed range also increases. The percentage of solved cases has the highest value of
approximately 90% when the algorithm employs speed ranges equal to or greater than 20
mi/hr and a 30-foot buffer for Columbia County data. Conversely, there is no considerable
Search WWH ::




Custom Search