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Figure 5: Difference between accuracy and precision.
GPS systems can locate an object within a radius of 10 m for approximately
95% of the measurements. Other more expensive units based on differential-
GPS do much better, reaching an accuracy of 1-3 m in 99% of cases.
Distances characterize the accuracy of a system, whereas percentages
indicate the precision , i.e. how often we can expect to achieve a certain level
of accuracy. Figure 5 shows the difference between accuracy and precision:
six shots have been fired on four targets. In A, the precision is good but the
accuracy is poor. In B, the accuracy is quite good but the precision is poor. In
C, both the accuracy and the precision are good. In D, both are poor. When
comparing different positioning systems, both attributes have to be taken into
account.
It is worth noticing that the accuracy itself is a relative attribute, so a
system with a given accuracy can be not suitable for some application and
suitable for another. For example, the accuracy required by a motion-capture
system for computer animation is 1 cm or less, whereas the GPS-based
systems used by biologists to locate flocks of migratory birds may have an
accuracy of 1 km 2 .
3.4 The range
A positioning system must be able to locate objects anywhere it is designed
to operate: a single room, a particular building, a campus, a metropolitan area
or the whole Earth. Moreover, the number of objects that a system can locate
at the same time could be limited. For example, a GPS system can meet the
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