A position fix or simply a fix is a term used in position fixing in navigation to describe a position derived from measuring external reference points.
The term is generally used only with manual or visual techniques such as the use of intersecting visual or radio position lines rather than the use of more automated and accurate electronic methods such as GPS.
Usually, a fix is where two position lines intersect. If three position lines can be obtained, the resulting "cocked hat" gives the navigator an indication of the accuracy in the three separate position lines.
The most accurate fixes occur when the position lines are at right angles to each other.
Fixes are a necessary part of navigation by dead reckoning because dead reckoning relies on estimates of speed, course, wind and tidal currents. The fix confirms the actual position during a journey. The fix itself can introduce inaccuracies if the reference point is not correctly identified or is inaccurately measured.
In celestial navigation, navigators derive an estimated position by plotting the celestial circle of position on a special chart called a plotting sheet.
The estimated position lies on the point where the circle of position intersects the dead reckoning track.
The software calculates the craft’s position from data entered by the navigator, such as the name of the celestial body, its vertical angle in degrees, minutes, and seconds, and the time the measurement was taken.
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