Writing Your Own GPS Applications: Part 2

Determining Precision Needs

Now that the mechanics of precision have been explained, the next step is to figure out how to determine the actual precision needs of an application. As a general rule of thumb, an HDOP value of six or less is recommended for any application which makes suggestions to the user based on the current location. For example, in-car navigation programs which tell the user to “turn left now” should ignore positional measurements when HDOP is greater than six. But is six really good enough? How can developers figure out which HDOP values to use for their own applications? To answer these kinds of questions, I like to use a simple formula:

Accuracy of GPS Device * DOP = Maximum Allowable Error

This formula uses DOP as a factor of error which, when combined with the accuracy of the GPS device being used, yields the maximum error allowed by a level of DOP in the form of a specific, measurable distance. Another general rule of thumb is that typical consumer GPS devices are capable of between 5-7 meters of accuracy without enhancements like DGPS or WAAS, or an average of six meters. Using the in-car navigation HDOP of six and a typical GPS device, the maximum error allowed is 6m * 6 = 36 meters , or 118 feet. Given that a downtown city block is roughly 475 feet square, the maximum allowable error is about a quarter of a city block. This is precise enough to make sure that the driver turns at the correct road. On the other hand, an HDOP of twelve results in an allowable error of half a city block (237 feet), which could cause drivers to turn down an alley accidentally. So, using the formula, it is possible to use an HDOP greater than six for in-car navigation, but not by much.

The trick to using this formula is researching real-world distances, especially the smallest important distances. To demonstrate, take a look at golf. Does golf require more precision than in-car navigation? A golf program needs to tell the user which golf club to use in order to make the best shot. Some research into important golf distances finds that for most players, there is a regular distance interval between clubs of about 10-15 yards. Therefore, a golf program needs no more than 10 yards (9.1 meters) of allowable error to consistently suggest the right club. When 9.1 meters is put into the formula as Maximum Allowable Error , the maximum HDOP comes out to 3. So, golfing applications require about twice the precision as in-car navigation systems.

Why not skip the formulas and always enforce an HDOP of one? This looks like a reasonable practice, but greater precision requires greater satellite visibility. An in-car navigation system will probably not get an HDOP of one (or even three) downtown because signals are being obscured by buildings. If the enforced HDOP is too small, the application will throw out too many positional readings and just sit there while the driver loses patience. Golfing applications, on the other hand, can realistically enforce a small HDOP because they operate outdoors. The golfer's PDA is likely to have plenty of open sky enough to pick up several evenly-distributed satellites, unless their ball is in the middle of the woods, in which case they're on their own.

To summarize, successful GPS software developers will use the formula to determine the greatest possible DOP number. This will ensure that the application minimizes most problems due to inaccuracy while at the same time allowing the application to function in the poorest possible satellite visibility conditions. This practice will maximize the value and versatility of any GPS application.

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About the author

Jon Person

Jon Person United States

Jon Person is the author of the award-winning “GPS.NET” software now in use in over two thousand GPS applications in all industries, from geocaching to disease outbreak prevention. Jon runs his...

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