Database Reference
In-Depth Information
approaches. The distance of a Bluetooth device from a specific receiver can be
calculated using techniques that consider signal levels.
The disadvantage of this technique is that it can be mainly used for indoor
tracking of objects as Bluetooth receivers cover a limited area and they cannot
really be used for outdoor object tracking.
RFID Data
The purpose of an RFID system is to enable data to be transmitted by a portable
device, called a tag, which is read by an RFID reader and processed according to
the needs of a particular application. A typical RFID tag consists of a microchip
attached to a radio antenna mounted on a substrate. A typical chip can store as
much as 2 kilobytes of data. A reader is needed to retrieve the data stored on
an RFID tag. A typical reader is a device that has one or more antennas that
emit radio waves and receive signals back from the tag. The data transmitted
by the tag may provide identification or location information, or specifics about
the product tagged, such as price, color, and date of purchase. As in Bluetooth
technology, RFID readers can locate tags within a limited area so it is hard to
apply this technology for outdoor tracking of moving objects.
2.3 Handling Trajectory Data
Real-life trajectory data, collected using the technologies previously presented,
are not really readily used for analysis purposes. In this section, we elaborate
on various approaches for handling trajectory as a necessary step for identifying
clean
(i.e., without noise),
accurate
(i.e., map-matched), and
compressed
(i.e.,
compact) trajectories, from the original sequence of spatio-temporal positions
(e.g., GPS records) of the moving objects.
2.3.1 Data Cleaning
Data sets collected by mobile sensors are often imprecise either unintentionally,
due to limitations of positioning systems (e.g., inaccurate GPS measurement
and sampling errors, signal loss, battery running out), or intentionally, so as to
protect individuals' privacy (e.g., people may expose an approximation of their
positions).
In case of unintentional (GPS) errors, trajectory cleaning (i.e., removing
errors) is an important step in the procedure of constructing meaningful raw
trajectories from the GPS feeds. Generally speaking, two types of GPS errors
can be identified:
systematic
errors, due to a system's limitations, and
random
errors, due to external reasons. Systematic errors can be caused by horizontal
dilution of position (HDOP) due to the low number of available satellites, while
