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.