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Guy wrote

> When using HO 249 The assumed latitude is rounded and the assumed Longitude is
> the number that will make the LHA an even number (within the range of +/- 30
> min. of the DR Longitude). When plotting the assumed Lat and Long are used as
> the starting point of the procedure. OK if I use the formulas:
> Sin-1Hc = sin(dec) x sin(lat) + cos(dec) x cos(lat) x cos(LHA) and Cos-1 (Zn)=
> sin(dec) - sin(lat) x sin(Hc) / cos(Hc) x cos(lat) to calculate Hc and Z or Zn
> and then plot my position
> wouldn't I be using the DR position as the start point?

Similar to 229 I suspect, where you use the nearest whole latitude to your
DR/EP position as AP and fudge AP longitude to make it whole degrees too (no
minutes or seconds). Odd/even is a semantics trap I have fallen into.
Literally 228d 00' 00" is even, 229d 00' 00" is odd). Rule of thumb, start
plotting from the AP you used as input for the calculations.

Past odd/even semantics your statement is correct, you use exactly where you
think you are (exact EP/DR--discounting some figure of speech that eludes me
for the moment) in your pocket-calculator calculations and as your anchor
point for any intercept(s).  Many benefits, including increased accuracy via
shorter intercepts, although the pure scientists on the list may argue that
carrying that many digits forward [to the right of the decimal point] in the
calculations is folly given the significant-digits input.  Yet many do it
privately anyway!

Bill

   

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