GPS Accuracy

	Accuracy vs Precision
	Accuracy
	Precision

• Modern GNSS receivers are capable of providing accurate and precise position information.
• Both accuracy and precision are essential components for a successful identification of any given location.
• Accuracy without precision typically produces only a general idea of where something may be located.
• Precision without accuracy commonly results in detailed location information that is in the wrong area.
• The nuanced differences between accuracy and precision are explored below.

Accuracy vs Precision
The following examples exaggerate the results of saving multiple waypoints near the Eiffel Tower with varying degrees of accuracy and precison. Accuracy references how close measured values are to the true value. Precision references how close measured values are to each other.
Accuracy = Poor	Accuracy = Good	Accuracy = Poor	Accuracy = Good
Precision = Poor	Precision = Poor	Precision = Good	Precision = Good
Accuracy
Accuracy references how close measured values are to the true value. Modern Garmin consumer grade receivers report GNSS Accuracy as a two sigma (95th percentile) value. This means a receiver reporting an accuracy of 3.65m will be within 3.65m of the true location 95 percent of the time. It is not possible for the receiver to determine the actual difference between the reported position and true position. If the receiver could determine the exact distance from the true position, it would just provide the true position. The accuracy value being reported is not the literal accuracy but rather a probability of accuracy at 95%. For the example below, a Garmin GPSMAP 66sr was placed directly above Benchmark IEM1 (AJ3685). The published NGS Data Sheet Benchmark coordinates are N 33° 32' 32.55845"  W 111° 39' 41.53522". The GPSMAP 66sr indicated a current position of N 33° 32.5428'  W 111° 39.6939' with 6 ft. accuracy. Using the FizzyCalc coordinate calculator we found the actual distance between these two locations to be 8.4 ft.
Garmin GPSMAP 66sr	Expected Device Location	Reported Location	Reported Location
Satellite Page Indicates	is Within a 6 ft Radius of	vs	is 8.4 ft from published
GNSS Accuracy is 6 ft	Benchmark Coordinates	Benchmark Location	Benchmark Coordinates
Precision
Precision references how close measured values are to each other. Precision can be measured by the number of decimal places in the latitude and longitude values provided. The number of decimal places used correlates directly to the level of precision available. Tip: Longitude measurement precision improves as Latitude values are increased.
hddd°mm'ss.ss" hddd°mm.mmmm' hddd.dddddd° Distance Application 1° 00' 00.00" 1° 01.0000' 1.000000° 111 km Can be used to identify a country or large region 0° 06' 00.00" 0° 06.0000' 0.100000° 11.1 km Can be used to identify a city or town 0° 00' 36.00" 0° 00.6000' 0.010000° 1.11 km Can be used to identify a neighborhood 0° 00' 03.60" 0° 00.0600' 0.001000° 111 m Can be used to identify a parcel of land 0° 00' 00.36" 0° 00.0060' 0.000100° 11.1 m Can be used to identify a building or small structure 0° 00' 00.03" 0° 00.0006' 0.000010° 1.11 m Can be used to distinguish one tree from another 0° 00' 00.00" 0° 00.0000' 0.000001° 11.1 cm Can be used to distinguish individual persons or animals
The map images below illustrate the smallest possible area that can be identified when using the specified level of precision at a given latitude.
@ 0° Latitude, 1.0° Longitude = 111 km @ 48° Latitude, 1.0° Longitude = 74.6 km         @ 0° Latitude, 0.1° Longitude = 11.1 km @ 48° Latitude, 0.1° Longitude = 7.46 km         @ 0° Latitude, 0.01° Longitude = 1.11 km @ 48° Latitude, 0.01° Longitude = 746 m         @ 0° Latitude, 0.001° Longitude = 111 m @ 48° Latitude, 0.001° Longitude = 74.6 m         @ 0° Latitude, 0.0001° Longitude = 11.1 m @ 48° Latitude, 0.0001° Longitude = 7.46 m         @ 0° Latitude, 0.00001° Longitude = 1.11 m @ 48° Latitude, 0.00001° Longitude = .746 m