Part III · The line of position · Page 6 of 8
Pub. 249, Pub. 229, or a calculator?
All three give the same line of position. They differ in precision, in what they cover, and in what you carry. Most navigators learn one well and know where the others differ.
| Pub. 249 | Pub. 229 | Formula | |
|---|---|---|---|
| Made for | Air navigation | Marine navigation | Any calculator |
| Hc, Z | to 1′, 1° | to 0.1′, 0.1° | exact |
| Declinations | 0°–29° (Vol. 1: selected stars) | 0°–90°: every body | every body |
| Interpolation | Table 5, whole minutes | Interpolation Table, tenths | none |
| Worked from | an assumed position | an assumed position | your DR |
| Books | 3 volumes | 6 volumes, 15° of latitude each | none, but needs a calculator |
How to choose
- On a yacht, for an ocean passage: Pub. 249 is quicker, lighter and accurate enough. Its whole-minute rounding puts the line within about a mile, about the size of the error in a sextant sight from a small boat anyway.
- On a ship, or heading for a professional license: learn Pub. 229. Ships usually carry it (some carry 249 too), and courses for professional mariners teach it. Its extra precision is worth having from a steady deck.
- Stars with a high declination (Dubhe, Kochab, Capella, Deneb): Pub. 249 Volumes 2 and 3 stop at 29°, so use Volume 1, Pub. 229 or the formula.
- A calculator is quickest of all, and a good check on the tables, but keep a set of tables for when the batteries go flat.
Learning one makes the others easy, but 229 to 249 is the easier direction: 249 is 229 with the decimals and most of the interpolation taken away. In the practice, choose the method above any sight; you can change it at any time.