Choosing a-Lat and a-Lon / by Roger Kapsi

a-Lat

Take your DR-Lat and round it up or down to whole degree.

a-Lon

  1. Choose a-Lon° = DR-Lon° and a-Lon' = GHA'

  2. Look at the difference and call it Delta
  3. You're done if delta is less than or equal 30'. The AP is a-Lon° a-Lon' = DR-Lon° GHA'
  4. If Delta is greater than 30', then we need to change a-Lon° by up or down according to step 5 or 6
  5. Western Longitude: If GHA' is greater than 30' then decrease a-Lon° by and increase it if it's less than 30'
  6. Eastern Longitude: If GHA' is greater than 30' then increase a-Lon° by and decrease it if it's less than 30'.

Glossary

a-Lat = Assumed Latitude, a-Log = Assumed Longitude, DR = Dead Reckoning, GHA = Greenwich Hour Angle