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A standard GPS receiver for civil use offers an accuracy down to a few meters. In praxis the number and geometry of the received satellites influences the accuracy considerably, and in daily use, accuracies of about 20 m can be expected. More sophisticated GPS Tracker receivers as they can be found for land survey cost several thousand dollars and achieve an accuracy of a few centimetres.
With selective availability activated, receivers achieve accuracies of approximately 100 m (these declarations are true for 95 % of all cases). After deactivation of the selective availability the accuracy rose to approximately 15 m, depending of the number and position of available satellites.
See also here for up-to-date Information about local accuracy in southern germany (page is in german but numbers are numbers).

Differential GPS (DGPS)

The technique called differential GPS (DGPS) enables civil receivers to achieve accuracies of 5 m or less. Therefore a second stationary GPS receiver is applied for correcting the measurements of the first receiver. If the position of the stationary receiver is known very accurately, by means of a long wave transmitter (283.5 – 325.0 kHz) a correction signal can be sent which is received and analyzed by a receiver connected to the mobile GPS. The correction signal is like the GPS signals themselves free of charge, the only costs of the long wave receiver arise. This receiver is connected to the GPS and transfers the correction data in a serial data format (RTCM SC-104). The transmission partially is restricted to coastal areas, as it is often sustained by the coastguard of a country.

Wide Area Augmentation System (WAAS)

WAAS (Wide Area Augmentation System) has been operating since 1999 in the United States and is available for portable GPS receivers since 2001. WAAS consists of approximately 25 ground stations controlling the GPS signals and two reference stations that collect the data of the ground stations and calculate correction data. These data contain corrections for the satellite orbits, clock drift and signal delay of the satellites caused by the ionosphere and troposphere. The data are sent to the receivers via to geostationary satellites.
WAAS has been working since December 1999 with nearly no interruption. It was developed for the aeronautical authority FAA to enable precise instrument approaches for landing. The WAAS signal is accessible for civil use and offers a better coverage on land and on sea than the land-based DGPS systems. Unlike DGPS, the reception of WAAS requires no additional receivers. Only the software of the GPS receiver must support the reception of WAAS correction signals. However it is important that one of the geostationary satellites is in view of the receiver. This is more problematic, if the receiver is positioned further north, as the altitude of geostationary satellites above the horizon decreases. Therefore WAAS is especially useful for navigation in open land, aviation and navigation on sea.
In Europe a system corresponding to WAAS is operated, called EGNOS (Euro Geostationary Navigation Overlay Service). In Asia, a japanese system called MSAS (Multi-Functional Satellite Augmentation System) is planned. As all those systems operate with the same principle, a GPS receiver supporting WAAS can also benefit from EGNOS and MSAS. More about the WAAS/EGNOS-System can be read here.

Overview of Typical Accuracies

Accuracy of GPS system with SA activated ± 100 Meter
Typical accuracy with SA deactivated ± 15 Meter
Typical accuracy of differential GPS (DGPS) ± 3 – 5 Meter
Typical accuracy with WAAS/EGNOS ± 1 – 3 Meter

Accuracy Values by Garmin Receivers

Accuracy of position determination
Accuracy of position determination
The declaration of the accuracy by Garmin GPS receivers often leads to confusion. What does it mean if the receiver states an accuracy of 4 m? This readout refers to the so-called 50 % CEP (Circular Error Probable). This means that 50 % of all measurements are within a radius of 4 m. On the other hand, 50 % of all measured positions are outside of this radius. Furthermore, 95 % of all measured positions are within a circle of twice this radius and 98.9 % of all positions are within a circle of 2.55 the radius. In the given example, nearly all positions are within circle with a radius of 10 m. The determined position is in the worst case accurate to 10 m.