Database Reference
In-Depth Information
Foursquare), the GPS data are used to infer higher-level information, such as
the address of a place, and the passed coordinates are hidden. Another source
of uncertainty is the location system used by the device. Figure
16.1
shows
an example of this error. On the left bottom corner of the trajectory there is a
point located in Portoferraio, Elba island. The user never went to the island. She
set off the GPS for energy saving and the data were retrieved using the GSM
antenna. Her mobile phone was attached to the Portoferraio antenna but she was
physically on the cost, some kilometers north. There is no way to extract this
information from the generated data.
Uncertainty about Credibility
In some cases we can witness the presence of “spammer” users, who bombard
the system with tweets and/or randomly change the GPS coordinates relative to
their geographic positions to cheat the anti-spam system. In other cases people
can voluntarily publish different locations just for fun or to protect their privacy.
Uncertainty Due to Privacy Settings
At the social network level, the geographic information can be filtered and
modified for privacy reasons using a less detailed geographic level, although the
device transmits coordinates.
Uncertainty Due to Multiple Data
Tweet example: “Two very large forest fires in the mountains behind Funchal
clouds of smoke covered the sun turn sunlight deep yellow ash coming down.”
4
It is possible to associate this Tweet to three locations spatially not coincident.
Referring to the conceptual model described in Figure
16.3
, the Feature of
Interest described by the content is the forest fire that has itself a spatial location
(the forest location). The content has also implicit geographic information with
the toponym Funchal, or more precisely with the mountains behind the city. It
is worth noticing that there is a certain level of uncertainty both in the definition
of the forest on the mountain and in the toponym Funchal, which represent
two different locations. The third location is the user/device location. Suppose
the Tweet message itself has the coordinates originating from the source GPS
device, we can suppose that the user was sending the message a safe distance
away from the forest fire.
Uncertainty Due to User and Content Location
Photo example: Let us consider a person taking a picture with a camera or a
smartphone with GPS-integrated system. The person and the device coordinate
overlap, while the subject of the photo coordinate has a distance from the camera.
4
Twitter, posted by user “Kevin bulmer” on Fri., Aug. 13, 2010 h20:21.
