The Minimal Time / Minimal Distance Method [16] is
the simplest method to save coordinates. The user sets two
parameters: a value for the “minimal time” and a value for
the “minimal distance”. The first value determines when to
check the second value. If the second value is exceeded,
then the incoming coordinate will be saved. For example, if
minimal time is set to 5 sec and minimal distance is set to
12 meters, the application checks every 5 sec whether the
user is 12 meters away from the previously stored point. If
yes, the point will be saved. Otherwise, it is omitted.
This method is very sensitive to the threshold values
chosen by the user. To demonstrate this, we recorded the
test path twice by varying the threshold values. For the first
track it is 3 sec and 10 meters while for the second it is 10
sec and 80 meters. Fig. 5 illustrates the results. It is obvious
that the larger the values the less the coordinates stored,
which affects the quality of the stored track. On the other
hand, if we choose small values to avoid this problem we
store many (perhaps redundant) points. The ‘optimal’ values
of course depend on the mode of movement (walking,
running, driving, etc.) and may be difficult to be known in
advance by the user.
Figure 5: Compressed tracks with Minimal time/distance method.
Left: Minimal Time = 3 sec / Minimal Distance = 10 m;
Right: Minimal Time = 10 sec / Minimal Distance = 80 m
Figure 6: Recorded tracks with the BOPW-algorithm.
Left: ED = 8 m; Right: ED = 15 m
Figure 7: Recorded tracks with the Threshold-method.
Left: threshold = 8 m; Right: threshold = 15 m
The two methods proposed in [17], Before Opening
Window (BOPW) and Normal Opening Window (NOPW),
achieve a very good compression rate. The user gives as
parameter a distance threshold (Euclidean Distance - ED)
which is a bound on how much the original path can deviate
from the compressed one. However, a high ED value may
lead to loss of data which are crucial to represent well the