An accuracy contour can be defined either in terms of a locus of constant radial error or a locus of constant probability density. A short comparison of the two concepts is given in reference 3.

The object of this note is to define contours of constant probability density in the case of a two-position-line running fix; the outcome is of interest in that it enables the navigator to plan his observations so that the accuracy is optimized. In Fig. 1 the object observed is S. The transferred position line corresponding to the first observation is drawn from the point S′, where SS′ is the vector corresponding to the ship's run during the time between the two observations. The fix is at O.