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E7 - - Sight Reduction Summary - - 10/03/2004

Definitions

Dead Reckoning Position (DR).
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.

Sight Reduction

  1. 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)

  2. From the reference position (using the reference longitude) determine the local hour angle (LHA) (ref page E6)

  3. 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)

  4. Draw the azimuth line on the chart or plotting sheet from the reference point.

  5. Determine the difference between the calculated elevation and the measured elevation: (ref page E4)

  6. 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.

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