Global Positioning System Reference
In-Depth Information
increase in the percentage of solved cases remaining at 68% for Portage County data when
speed range values equal to or greater than 25 mi/h are employed. The percentage of solved
cases for Polk County remained constant at 50% for speed ranges equal to or greater than 15
mi/h, independent of buffer size. Thus, the map-matching algorithm is sensitive to speed
range values, particularly when small speed ranges are employed since feasible paths are
rejected.
10
9
8
7
6
5
4
3
2
1
0
20
30
40
50
60
Buffer Size (ft)
5 mi/hr
10 mi/hr
15 mi/hr
20 mi/hr
25 mi/hr
30 mi/hr
35 mi/hr
Fig. 14. FN Percentages After Applying Algorithm for Different Speed Ranges by Buffer Size
for Columbia County
4.3.3 Number of consecutive GPS data points
If no feasible paths are obtained between a pair of snapped data points, then the algorithm
tests for viable routes between preceding and subsequent data points, as described in Figure
5. If a small buffer size is utilized, several successive data points do not snap to any roadway
centerline generating FN data points. Thus, the number of consecutive data points used by
the algorithm needs to be incremented to consider adjacent data points that are correctly
snapped and minimize FN percentages.
Although the map-matching algorithm may employ any number of consecutive data points,
the performance of the map-matching algorithm was analyzed with a number of
consecutive data points between three and eight. A previous test determined that this range
of consecutive data points is suffice for solving spatial ambiguities with the spatial and
temporal data employed in this study.
Similar to the FN curve behavior due to speed range variations, FN curves for different
number of consecutive data points are parallel for the three counties and converge to constant
values as the buffer size increases. Figure 15 shows the percentage of FN data points as the
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