Ordnance Survey – Great Britain's national mapping agency
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6.6: Mobile GIS (1)
Positioning
Everyone has experienced the feeling of being lost. Positioning is the process of gaining information about our location. This can take many different forms and often it is enough to know which town, street, house number or room. Our location might also be specified in terms of latitude and longitude, or in metres north and east of a false origin, as in the case of a national grid.
A Cartesian coordinate system such as in Chapter 2 Section 3 is a grid, with one corner being the arbitrary "false origin" (0,0) and all positions on the grid measured as distances north and east of it. British National Grid is one example.
For example, the position of Southampton in British National Grid (BNG) coordinates is 440,000 metres east of the false origin and 100,000 metres north.
Although coordinates are essential, most people are usually more interested in which house and street to find someone. Coordinates are therefore often linked to a computerised map showing information that can be interpreted by a user to allow functions such as route planning and querying the user’s current location.
Many technologies are available for positioning, but perhaps the best known is the US Global Positioning System (GPS) (See Chapter 2, section 2.2). GPS receivers provide location information as a set of coordinates in latitude and longitude format.
24 GPS satellites orbit the Earth, providing constant position information to commercially available receivers on the ground.
The receiver calculates its location from distances measured to satellites orbiting the Earth. The receiver picks up a digital signal transmitted by the satellites and also measures the time taken for the signal to arrive. Since the signals travel at the speed of light, the receiver can calculate how far away the satellite is by calculating distances from at least four satellites and simultaneously knowing their exact positions in the sky. This gives the position of the receiver accurate to within about 100 metres anywhere on Earth; however, signals from more satellites and various techniques can achieve accuracy below one centimetre.
Originally GPS was intended to provide position information for the US military, but today GPS has a multitude of civilian uses, from surveying to the provision of LBS, which is what will be discussed in the next section.