4. Estimation methods

4.1 Graphic methods: ruler with holes

The oldest means of estimation is a tracing on a sheet of tracing paper, where the operator tries to place a ruler with evenly-spaced marks against the sighting lines previously transferred to the paper. This ruler, equipped with several graduations each corresponding to a target speed, was known as a "hole ruler" (these holes enabled the operator to write comments without having to lift the ruler). In principle, this is a global method.

At present, this operation is performed using a graphical representation on a computer screen. The operator has the same possibilities for adjusting angle measurements to his candidate solution, by acting on the four parameters that define it: a reference azimuth, heading, speed and distance...