The computation of the position of a ship or an airplane without
the use of celestial sightings or electronic aids. Position is computed
instead from the position last determined, the compass course
followed, and the rate of movement.
Estimated Position (EP)
When the DR position is corrected for estimated drift it will be
called the estimated position and will be recorded on the chart or
plotting sheet and marked "EP" with the time written next to it.
Assumed Position (AP)
To simplify the calculations an assumed position in the vicinity
of the DR or EP may be used, by rounding off the latitude and
longitude to a whole number of degrees or to the nearest half
degree to simplify the entry into a calculator.
Any of the above positions can be used as a reference point for the
sight reduction procedure described below. The time used for determining
the the position of the DR or AP must agree with the time of measurement
within a few minutes. The reference point will determine the reference
latitude and the reference longitude as used below.
Using the time of the observation (to the second, of GMT), obtain from
the Nautical Almanac the Greenwich hour angle, and the declination of the
celestial object that is observed. (ref page E6)
From the reference position (using the reference longitude)
determine the local hour angle (LHA) (ref page E6)
Using the declination of the star, the LHA, and the Latitude of the
observer (reference latitude), calculate the Azimuth and elevation af the star.
(ref pages C1, C2, C3, C4, or E5)
Draw the azimuth line on the chart or plotting sheet from
the reference point.
Determine the difference between the calculated elevation
and the measured elevation: (ref page E4)
Draw the line of position (LOP) at right angle to the azimuth line
(as per instructions on page E4).
By performing a sextant measurement on another celestial object that is
in a different direction, a second LOP can be determined and the location
of the observer will be at the intersection of the lines of position.