GPS Position Accuracy
GPS trace points are locations defined by their geographical coordinates. Applications need to be able to determine the road link (the road section with homogeneous attributes) to which a point belongs, its position in relation to the map link geometry and the direction of travel along the link. This allows applications to establish whether the driver is on the expected route, whether he/she obeys the legal speed limit, the type of road and its characteristics (for example curvature, incline, road surface type, the number of lanes).
The accuracy of the GPS coordinates depends on many factors, with the result that the computed latitude and longitude can be from 0.5 meters to up to 40 meters away from the actual position. It is often impossible to determine a GPS position inside tunnels, inside buildings or in urban canyons. GPS receivers cannot provide a maximum deviation and thus, for example, are unable to indicate that the calculated location lies within 3.5 meters of the actual location with 95% probability. GPS receivers only provide an HDOP/VDOP/PDOP... value (horizontal/vertical dilution of precision) which indicates that the computed coordinates of a location cannot be more accurate than this value considering the number and/or position of satellites and the mathematical algorithms. This minimum error cannot be used to estimate the maximum error.
GPS heading and speed are computed by the receiver device based on the last several sets of GPS coordinates. The accuracy of the calculation depends on the actual speed of the vehicle and becomes unreliable if the vehicle speed drops below ~10 km/h. At low speeds, even the positional accuracy declines considerably, resulting in large random point clouds in some situations.