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Geolocation
Before we explore sensors more generally, I’ll call out the geolocation capabilities for Windows Store
apps separately because its API is structured differently from other sensors. We’ve already used this
since Chapter 2, “Quickstart” in the Here My Am! app, but we need the more complete story of this
highly useful capability.
Unlike all other sensors, in fact, geolocation is the only one that has an associated capability you
must declare in the manifest. Where you are on the earth is an absolute measure, if you will, and is
therefore classified as a piece of personal information. So, users must give their consent before an app
can obtain that information, and your app must also provide a Privacy Statement in the Windows Store.
Other sensor data, in contrast, is relative—you cannot, for example, really know anything about a person
from how a device is tilted, how it’s moving, or how much light is shining on it. Accordingly, you can use
those others sensors without declaring any specific capabilities.
As you might know, geolocation can be obtained in two different ways. The primary and most
precise way, of course, is to get a reading from an actual GPS radio that is talking to geosynchronous
satellites some hundreds of miles up in orbit. The other reasonably useful means, though not always
accurate, is to attempt to find one’s position through the IP address of a wired network connection or to
triangulate from the position of available WiFi hotspots. Whatever the case, WinRT will do its best to
give you a decent reading.
To access geolocation readings, you must first create an instance of the WinRT geolocator,
Windows.Devices.Geolocation.Geolocator. With that in hand, you can then call its
getGeopositionAsync method, whose results (delivered to your completed handler) is a Geoposition
object (in the same Windows.Devices.Geolocation namespace, as everything here is unless noted).
Here’s the code as it appears in Here My Am!:
var gl = new Windows.Devices.Geolocation.Geolocator();
gl.getGeopositionAsync().done(function (position) {
//Save for share
lastPosition = { latitude: position.coordinate.latitude,
longitude: position.coordinate.longitude };
}
The getGeopositionAsync method also has a variation where you can specify two parameters: a
maximum age for a cached reading (which is to say, how stale you can allow a reading to be) and a
timeout value for how long you’re willing to wait for a response. Both values are in milliseconds.
A Geoposition contains two properties. First, its coordinate property is a Geocoodinate object that
provides accuracy (meters), altitude (meters), altitudeAccuracy (meters), heading (degrees relative to
true north), latitude (degrees), longitude (degrees), speed (meters/sec), and a timestamp (Date). The
second part of a Geoposition is a CivicAddress object named—what else!—civicAddress, which might
contain city (string), country (string, a two-letter ISO-3166 country code), postalCode (string), state
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