Accuracy Defined 
Accuracy specifications can be expressed in many ways
Circular Error of Probability (CEP) CEP is based on a 50% confidence level. If 100 positions are collected at one point, 50% of them will be within the stated accuracy and 50% will be outside the stated accuracy. Root Mean Square (RMS) error RMS error is based on a 63  68% confidence level. If 100 positions are collected at one point, 63  68 of them will be within the stated accuracy, and 32  37 will be outside the stated accuracy. 2D Root Mean Square (2D RMS) error Two Degree Root Mean Square (2D RMS) error is based on a 95% confidence level. If 100 positions are collected at one point, 95 of the positions will be within the stated accuracy, and 5 will be outside the stated accuracy. Vertical accuracyDepending on the geometry of the satellite constellation, the vertical accuracy of any GPS position may be from submeter to 3 times larger than the horizontal accuracy. The vertical component is difficult to calculate because SVs have a limited perspective in which to measure height. If the receiver could use signals from underneath it, the vertical component could be accurately measured, but the Earth blocks these signals. Accuracy versus precisionAccuracy describes how closely the calculated value compares to the known value. Precision is the measure of repeatability.
