Course / The assumed position and LHA

Part III · The line of position · Page 3 of 4

The assumed position

Sight reduction tables are printed for whole degrees of latitude and of LHA only. So instead of working from the DR, you work from a nearby assumed position (AP) chosen to make both whole:

  • AP latitude: the whole degree nearest your DR latitude.
  • AP longitude: within 30′ of your DR longitude, with minutes that make LHA a whole degree. In west longitude, use the same minutes as the GHA; in east longitude, 60′ minus the GHA's minutes.

Predict Is the assumed position your best estimate of where the boat is?

40°20′N40°40′N41°00′N41°20′N41°40′N42°00′N66°00′W65°20′W64°40′W64°00′W63°20′WDRAP20 nmN ↑
AP: N 41°00.0', W 064°23.4'. From the DR: 149°, 14.0 nm.
GHA
212°23.4'
AP latitude: a whole degree
N 41°00.0'
AP longitude
W 064°23.4'
LHA = GHA − AP longitude
148°00.0'
AP from the DR
14 nm

In west longitude the AP's minutes equal the GHA's minutes (23.4′), so they cancel.

The AP is not a guess at where you are: it is only a convenient point to measure from. It can be 30 miles or more from the DR, and the plot still comes out right, because the intercept is measured from the AP.