When visible GPS satellites are close together in the sky, the geometry is said to be weak and the DOP value is high; when far apart, the geometry is strong and the DOP value is low. Thus a low DOP value represents a better GPS positional accuracy due to the wider angular separation between the satellites used to calculate a GPS unit's position. Other factors can increase the effective DOP are obstructions such as nearby mountains or buildings. DOP can be expressed as a number of separate measurements. HDOP, VDOP, PDOP and TDOP are respectively Horizontal, Vertical, Position (3-D) and Time Dilution of Precision. They follow mathematically from the positions of the usable satellites. GPS receivers allow the display of these positions ("skyplot") as well as the DOP values.
The term can also be applied to other location systems that employ several geographical spaced sites. It can occur in electronic-counter-counter-measures (electronic warfare) when computing the location of enemy emitters (radar jammers and radio communications devices). Using such an interferometry technique can provide certain geometric layout where there are degrees of freedom that cannot be accounted for due to inadequate configurations.
The effect of geometry of the satellites on position error is called Geometric Dilation of precision and it is roughly interpreted as ratio of position error to the range error. Now lets imagine that a tetrahedron is formed by lines joining the four satellites and receivers. The larger the volume of the tetrahedron, the better the value of GDOP; the smaller the volume of the tetrahedron, the worse the value of GDOP will be. Similarly, the greater the number of satellites, the better the value of GDOP.
|0||Ideal||This is the highest possible confidence level to be used for applications demanding the highest possible precision at all times|
|1-3||Excellent||At this confidence level, positional measurements are considered accurate enough to meet all but the most sensitive applications|
|4-6||Good||Represents a level that marks the minimum appropriate for making business decisions. Positional measurements could be used to make reliable in-route navigation suggestions to the user|
|7-8||Moderate||Positional measurements could be used for calculations, but the fix quality could still be improved. A more open view of the sky is recommended|
|9-20||Fair||Represents a low confidence level. Positional measurements should be discarded or used only to indicate a very rough estimate of the current location|
|21-50||Poor||At this level, measurements are inaccurate by as much as 50 metres and should be discarded|
The DOP factors are functions of the diagonal elements of the covariance matrix of the parameters, expressed either in a global or a local geodetic frame.