Dead Reckoning (D.R.) The term Dead Reckoning is used to cover all positions that are obtained from the course steered by the ship and her speed through the water since her last position and from no other factors. The D.R. position is therefore only an approximate position which in astro navigation is known as the assumed position. The D.R. position is shown on the chart as a small cross with the description and time marked alongside it thus: DR 1100
Estimated Position (E.P.) The E.P. is derived from the D.R. position adjusted for the estimated effects of wind, currents and tidal streams. However, it is still an approximate position because the exact influence on the ship’s course and speed of these factors cannot be accurately assessed.The E.P. is shown on the chart as a dot surrounded by a small triangle, with the description and time alongside it, thus: E.P. 1200
Assumed Position. In Astro Navigation, although we start from the approximate position, we use an ‘assumed position’ to help us to keep the number of interpolations to a minimum. We can do this because, when we calculate the intercept, we assume that the vessel is in the vicinity of the D.R. position which of course is not an accurate position. We can therefore, within reasonable limits, choose a position in the neighbourhood of the D.R. position that will not only save time and work but will also reduce the risk of making arithmetic errors.
A fuller explanation of this topic is given in the book ‘Astro Navigation Demystified’.
Home page: www.astronavigationdemystified.com