Lesson 2

Position on the chart

Chosen Position (CP)

That position nearest to the observer chosen so that the latitude is an integral degree and the longitude is such that the LHA of the body at the time of observation is also an integral degree.  Associated with the use of short method navigation tables.

Most Probable Position

The navigator's assessment of the ship's position based on information derived from all available sources.

Observed Position (OBS) or FIX

Could be taken to be any reliable position obtained by recognised navigational methods; this would normally involve making observations to obtain a minimum of Two lines of position which intersect at a reasonable angle (minimum 30°).  Such lines of position could be:

Visual bearings

Radio DF bearings

Position lines from celestial observations

Co-ordinates from radio navigational aids systems observations

Position circles from HSA or VSA observations, dipping ranges or radar ranges.

or various combinations of the above.

Marking of Positions

On Charts, the drawing of a small circle to indicate a position should only be used for an observed position (fix); other positions are indicated by a small cross + or in the case of an EP by D

Always mark positions with the appropriate abbreviation (DR, EP, etc) together with the time of observation and log reading (if given).

Position of Lighthouse

Indicated by the dot in the centre of the light star with purple teardrop.

Position of Light-Vessel

The position of a light-vessel, light-float, buoy or beacon is the centre of the base, and is indicated by a small circle, thus: -o-

Measurement of Distance

Distances are measured on the latitude scale, but never on the longitude scale.  On a Mercator chart the size of latitude scale graduations varies with latitude, therefore always measure distance in the latitude of the question.

Bearings

Any bearings, visual or radio DF, is named as the direction from the observer towards the object observed.  Radio DF bearings are normally given relative to the ship's head, and subsequently it is necessary to distinguish between the Great Circle bearing and the Mercator (rhumb line) bearing.

Wind Direction

Named as the direction from which the wind blows.

Current Direction

Named as the direction towards which the water flows.  When coasting in tidal water areas, current should be more correctly termed tidal stream.

Rate

The speed of a current/tidal stream in knots (kts).

Drift

The distance covered in a given time due solely to the movement of current,

Tidal stream and surface drift.

Set

The resultant direction towards which current, tidal stream and surface drift

flow.

Sea Position

The point at the termination of the water track.

Lesson 2.doc              Position on the Chart              DGR1999

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