Great Circle route difficulty

[P.Rutherford]

I was brushing up on the procedure for figuring the great circle route, the one in the 50 year almanac by Kolbe and had some troubles. Maybe  I was just tired and the tv was on but 2 attempts at fairly close destinations, both under 400 NM just didn't work. Once I chose someplace more distant, about 2900 NM it worked fine. Is it possible to be too close to use? TIA

BigBill,

Are you trying to plot waypoints along a great circle route?

Or, are you trying to get the distance from departure to destination?  If so- this is the formula you need;

https://thenauticalalmanac.com/Formulas....estination

To get the initial bearing use this formula;

Azimuth to Destination

NOTE- LHA= Present Longitude – Destination Longitude

Z = tan-1(sin (LHA) ÷ (cos (LHA) x sin(Your present Latitude) – cos(Your present Latitude) x tan(Destination Latitude))

Then to put Z into the right quadrant, apply the following rules-

If answer is negative, add 180º to Z
LHA is NEGATIVE if it's less than 180º .


Yes, a very short distance route, Great circle or Rhumb line, aren't significantly different. Great Circle routes less than about 1,500 miles aren't much different than Rhumb line.

Using OpenCPN, a computer chart plotting program, you can create a Great Circle route, get multiple waypoints and visually see how it compares to a Rhumb line route.

Paul