Information Technology Reference
In-Depth Information
exist. The devices you work with (and the examples we'll use) typically have access to
one or more of the following capabilities:
Global Positioning System (GPS
)—hardware in the device that
receives and decodes GPS satellite signals (or similar competing
services like Galileo or Glonass). Accuracy can be as fine-grained as 1
meter, although this can degrade with caching and other effects.
WiFi triangulation
—your device uses WiFi for network connectivity,
but it can also use a combination of known SSID names for WiFi
networks, together with signal strength in dBm, to triangulate location.
This tends to be cruder than GPS, and suffers in less built-up areas or
areas with many transient hot-spots. Note that your users may disable
WiFi for power consumption or conservation reasons.
Cell Tower triangulation
—similar to the WiFi technique, your device
knows with which cell it has registered at any given time, and also
knows what other cells are detectable (along with signal strengths and
related timing information). Even with only one cell tower nearby, your
device's location can still be known to the coarse level of that tower's
service area.
IP Address allocation
—While your device most likely receives a
dynamic IP address when connecting to WiFi or mobile networks, your
provider usually has a known set of IP address blocks, and known
patterns of distribution in various geographic areas. While this won't
locate your device to the nearest meter, it can do a pretty good job of
determining whether you're in Kansas or Kathmandu.
As you can see, multiple methods are available that support the actual mechanics of
determining your device's location. Regardless of the methods supported by the device,
you don't have to concern yourself with selecting a given mechanism when writing your
web applications. What you do have to consider are the other two governing constraints
on your geolocation application.
The first such constraint is the user's choice of settings for device-wide location
features. Quite literally, has the user enabled or disabled use of GPS, and for that
matter, have they allowed or prohibited other location-determining mechanisms from
pinpointing the device? Figure 9-1 shows the normal settings over which the user has
control
