GPS accuracy

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.