What are various method of obtaining position circle?

Position Circle (distance by vertical sextant alt, horizontal angle or radar) :

Position circle means a circle of positions. It is a curve or a set of all probable positions of ship, equidistant from a point. Out at sea, it is very common to use principle of position circle to find ships position, in day to day navigation.

Example–

Plotting of ships position using two or three radar distances, where position is obtained at the, intersection of two or three such position circles. Also plotting a position using a bearing & a distance off, position & position is intersection of the position circle & position line.

Following are some of the common methods of obtaining position circles. :

  • Radar distance off ( Normally obtained from)
    1. Point of land or cape.
    2. Recon on radar reflection buoy.
    3. Nearest land which is just touching the VRM
    4. Any point of land or a target, which is positively identified on the radar.
  • Vertical sextant angle:
    1. Principle of night angle triangle is used. In a right angle , in a vertical plane the 900 angle is made at base of the light house or the target whose vertical angle is taken & the line joining observer  & the base of target from the base of the triangle.
    2. Height of eye is neglected.
      The height of the lighthouse etc is normally given above MHWS & the observer may be taking the VSA at low water. Variation in tide level must be considered for better accuracy.
Formula used for calculation :
  • Distance from the base of object in Nautical miles

(Height of object above sea level in meter)   X 1.854
(observed altitude  of object in minutes)

  • Distance from the base of object in nautical miles

 (Height of object above sea level in feet )   X  0.5645
(observed altitude  of object in minutes)

 Horizontal sextant (or Azimuth, compass, pelorus) angle :

The horizontal between any two identified charted objects may be found by each of the instruments mentioned above. In case of HSA the sextant angle will heed to be corrected only for index error. However this correction may be neglected for chart work purpose owing to its small value.
In case of horizontal compass or pelorus  angle, one need not worry about the compass error or pelorus set for wrong ship’s head. Difference of the two bearings may be straight away taken as horizontal angle. Suppose the horizontal angle between two charted objects A & B is θ.

AMIT KUMAR

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