Geolocation Methods
Geolocation on mobile
devices has grown from being used solely for emergency and law
enforcement purposes to being an integral component of consumer mobile
applications. Once only performed by triangulation of cell towers,
modern mobile OSes have expanded to support retrieval of positional data
via wireless survey or GPS systems, giving an enhanced degree of
precision and faster update times. Different methods have their own
strengths and weaknesses, along with variations in accuracy.
Tower Triangulation
Accuracy: 50m–1,000m
Tower triangulation is the
oldest widely used method of geolocation via cell phone. This method
uses the relative power levels of radio signals between a cell phone and
a cell tower of a known location—this of course requires at least two
cell towers to be within range of the user. This service is used for the
E911 system in the United States, transmitting location data when
emergency calls are made. With user permission, however, the phone can
be instructed to transmit tower triangulation data to phone
applications.
Because this requires that the
user be near to multiple cells, and because signal strength can be
affected by many factors, tower triangulation is a fairly inexact method
of positioning (see Figure 12-1).
GPS
Accuracy: 5m–15m
Using
satellite signals instead of cell phone or wireless infrastructure, GPS
service is often available at times when other methods are not.
However, satellite acquisition is generally impaired when the user is
indoors, making the use of GPS alone inadequate for some mobile
applications. Additionally, initial GPS location information can take
several minutes to acquire.
An advantage of GPS is that it
can provide continuous tracking updates, useful for real-time
applications, instead of just one-time lookups.
Assisted GPS works by
providing an initial location obtained via another means (either tower
triangulation or 802.11) to the GPS receiver, to reduce satellite
acquisition time and correct for signal noise. This makes GPS somewhat
more viable for indoor use; however, acquiring positional data this way
still takes upwards of 10 seconds, still making it a relatively slow
method.
802.11
Accuracy: 10m–200m (but potentially erroneous)
The iPhone was the first
smartphone to add this additional method for geolocation, using an API
made available by Skyhook Wireless. This location method works by doing a
survey of any nearby 802.11 (Wi-Fi) wireless access points and then
submitting data about
them (presumably MAC address and SSID) to a web service, which returns
coordinates from what is essentially a very large “wardriving” database.
This allows for devices without GPS to provide potentially highly
accurate location data.
This approach has the
advantage of being both faster and much more accurate than cell tower
triangulation, but has a couple of drawbacks. Because location data
relies on specific wireless APs, if those APs move, location data can be
drastically wrong. Because the wireless
APs were listed in the Skyhook database, any attempt to use location
services near the offices reported the company as being in the previous
location, making it difficult to find places to go to lunch. A more
extreme example is when attending a security conference in Tokyo, one of
the authors’ iPhone 2G reported being in Vancouver, B.C. (the last
place the conference APs were used).
The Skyhook software
development kit (SDK) has also recently become available for Android,
but is not yet integrated in an official capacity. More recently,
however, Google launched its “Latitude” service, which provides a newer
implementation of Skyhook’s technology, combining all of the preceding
methods.
A more extensive evaluation of the strengths and weaknesses of this method can be found at www.techcrunch.com/2008/06/04/location-technologies-primer/.
Geolocation Implementation
Each platform treats
geolocation services differently, with different methods of requesting
user permission, ranging from asking every launch of the application to
leaving notification up to the developer.
Android
As with most services on
Android, permission to use the geolocation features is requested via the
program manifest and is granted by the user at install time. Either
coarse or fine precision can be requested, using the
ACCESS_COARSE_LOCATION (for cell triangulation or Wi-Fi) or
ACCESS_FINE_LOCATION (GPS) permission (see Figures 1 and 2). These permissions are requested and controlled separately.
The
android.location package provides the LocationManager service, which can
be called to return both geographic location and current bearing, using
the internal compass (if available). Listing 1 provides an example of using the LocationManager service in Android.
Listing 1. Using the LocationManager in Android
locationManager=(LocationManager)
getSystemService(Context.LOCATION_SERVICE);
Criteria mycriteria=new Criteria();
mycriteria.setAccuracy(Criteria.ACCURACY_FINE);
mycriteria.setBearingRequired(true);
String myprovider=locationManager.getBestProvider(mycriteria, true);
Location mylocation=locationManager.getLastKnownLocation(myprovider);
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In
addition to this, the LocationManager can be used to register for
positional update notifications as well as for an intent to be triggered
when a device comes within a specified proximity of a set of geographic
coordinates. See the locationManager.requestUpdates and
locationManager.addProximityAlert methods for more information. It is
worth noting that on some platforms geolocation is guaranteed to be
available, but there is no such mandate on Android-powered devices.
More information on
the LocationManager can be found on the Android developer site at
developer.android.com/guide/topics/location/index.html.
iPhone
Geolocation on the iPhone requires user approval every time an application that uses geolocation APIs is launched (see Figure 3).
The CLLocationManager returns a CLLocation object. There are several
constants developers can choose from when requesting locational data:
const CLLocationAccuracy kCLLocationAccuracyBest;
const CLLocationAccuracy kCLLocationAccuracyNearestTenMeters;
const CLLocationAccuracy kCLLocationAccuracyHundredMeters;
const CLLocationAccuracy kCLLocationAccuracyThreeKilometers;
Use the least
precise measurement that will meet the functionality requirements. For
example, to merely determine what city a user is in, you should use
either the kCLLocationAccuracyKilometer or
kCLLocationAccuracyThreeKilometers constant.
The method used for
geolocation is abstracted and not controllable by the developer, but any
combination of Wi-Fi, tower triangulation, and GPS (on post-2G devices)
may be used.
Windows Mobile
Windows
Mobile has no mechanism for a user to control geolocation API access on
an application-by-application basis—all applications are allowed to
access this data if location services are enabled on the device, via the
GPS Intermediate Driver API’s GPSOpenDevice and GPDGetPosition.
