Peter Lubbers - Pro HTML 5 Programming

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CHAPTER 4 ■ USING THE HTML5 GEOLOCATION API
Latitude and Longitude Coordinates
The location information consists primarily of a pair of latitude and longitude coordinates like the ones
shown in the following example, which shows the coordinates for beautiful Tahoe City, located on the
shore of Lake Tahoe, America’s most beautiful mountain lake:
Latitude: 39.17222, Longitude: -120.13778
In the preceding example, the latitude (the numerical value indicating distance north or south of the
equator is 39.17222) and the longitude (the numerical value indicating distance east or west of
Greenwich, England) is -120.13778.
Latitude and longitude coordinates can be expressed in different ways:
• Decimal format (for example, 39.17222)
• Degree Minute Second (DMS) format (for example, 39° 10' 20')
■ Note When you use the HTML5 Geolocation API, coordinates are always returned in decimal degrees.
In addition to latitude and longitude coordinates, HTML5 Geolocation always provides the accuracy
of the location coordinates. Additional metadata may also be provided, depending on the device that
your browser is running on. These include altitude, altitudeAccuracy, heading, and speed. If this
additional metadata is not available it will be returned as a null value.
Where Does Location Information Come From?
The HTML5 Geolocation API does not specify which underlying technology a device has to use to locate
the application's user. Instead, it simply exposes an API for retrieving location information. What is
exposed, however, is the level of accuracy with which the location was pinpointed. There is no guarantee
that the device's actual location returns an accurate location.
LOCATION, LOCATION
Peter says: “Here is a funny example of that. At home, I use a wireless network. I opened the HTML5
Geolocation example application shown in this chapter in Firefox and it figured out that I was in
Sacramento (about 75 miles from my actual physical location). Wrong, but not too surprising, because my
Internet Service Provider (ISP) is located in downtown Sacramento.
Then, I asked my sons, Sean and Rocky, to browse to the same page on their iPod Touch (using the same
Wi-Fi network). In Safari, it looked like they were located in Marysville, California—a town that is located
30 miles from Sacramento. Go figure!”