GPS accuracy
Contents
Basic Calculation:
- E-W accuracy = (10^-2) * Width of graticule in feet
- N-S accuracy = (10^-2) * "Height" of graticule in feet
This means you can get within a circle of radius Sqrt(AccuracyEW * AccuracyNS / pi) feet of your target coordinates.
Example Error Calculation
For a "normally-sized graticule" (taking the Cincinnati Graticule, 39 -84, as our example), a GPS receiver of the following accuracy results in possible errors of:
Cincinnati Graticule's size :
- E-W (across the 39.5 latitude line): 53.46 miles * 5280 ft/mi = 282268.8 ft
- N-S: 69 miles * 5280 ft/mi = 364320 ft
3 decimal places
- E-W accuracy: (10^-2) * 282268.8 ft = 282.2 ft
- N-S accuracy: (10^-2) * 364320 ft = 364.3 ft
This means you can get within a circle of radius 180.8 feet of your target coordinates.
More sensitive receivers
| # Decimal Points | Radius (Ft) | Radius (m) |
|---|---|---|
| 2 | 1809 feet | 551 m |
| 3 | 180.9 feet | 55.1 m |
| 4 | 18.9 feet | 5.51 m |
| 5 | 1.809 feet | 55.1 cm |
| 6 | 0.1809 Ft (2.17") | 0.0551 m (5.51 cm) |
GPS System Accuracy
The factor contributing to most GPS inaccuracy, Selective Availability (SA), was turned off in May of 2000. SA limited the accuracy of GPS-generated points to an area with a radius of approximately 50 meters. With SA disabled, GPS is generally accurate to within 6 meters. See: [1]
Based on the calculations above, a GPS with 4 digits of accuracy will put you within 11.5 meters (6 meters + 5.51 meters) of your intended target.
