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Position lines and Position fixing

Three point fix:

Position lines are obtained from usable navigational aids on land or sea.  Lines can be defined by:

• Two marks or features in line give a transit – very accurate
• A hand bearing compass reading to a mark, light or feature - moderate to good.
• A depth contour - moderate to good, very useful in fog.
• Sectored lights – change from one colour to another - accurate.
• A range from a dipping light (moderate), bearing. 
• Radar range is accurate, radar bearing is moderate.
• Compass bearing with range from vertical sextant angle - can be good with practice.

The intersection of two (preferably 3) or more position lines gives a fix, but you must allow for errors in measuring angles, identification and plotting.  Apply Variation to the compass bearing to get True bearing (subtract West Variation).  
A hand bearing compass is assumed to have no Deviation.

For maximum accuracy:

• Ensure the feature is identified accurately
• Features should be some distance away, not too close as the bearing will change rapidly.
• Lines should cut at 60° - 90° for maximum accuracy.
• Three lines are necessary.
• Take the bearing from ahead or astern first as this bearing will change least while you complete the other bearings.
• If the ‘cocked hat’ is large check again.
• Take the position as that closest to the nearest danger

Note that navigation buoys can be off station, or even completely missing.

Dipping Distance

Dipping distance M = 2.08 × (√eye height m + √height of light m)

• A light ‘dips’ when you can first see the ‘blink’ above the horizon.

• Before that you can see the ‘loom’ when it is still below the horizon (like the sun before dawn).

• Height of eye in a yacht is about 2 - 2.5 metres

• The table in the back of the Training Almanac tabulates dipping distances for various heights of eye and light.

• If the tide is below MHWS it increases the height of the light above sea level while the height of eye stays the same, so you can see the light from a greater distance.

posted 5th November 2009