Join Date: Jun 2011
Location: San Diego
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From the Seaclear help file:
"Calculations and accuracy
Heading and distance calculations are done using the rumbline. All calculations are done with double precision eliminating math errors. On greater distances rumbline calculation is less accurate. In normal use, such as in arcepelago navigation, the errors are not notable, as the GPS position and the charts are far less accurate.
SeaClear does not work with charts and routes spanning the dateline (180 E/W)."
Rhumbline Sailing may be OK for your requirements. Bowditch says don't use Rhumblines over a few hundred miles, what ever that is (300 maybe?). The accuracy depends on mid latitude and how much east west component there is to a route. The waypoints should be at the right position, but with Plane trig, the distance and direction may be a little off.
From San Francisco to Hawaii the difference is only about 20 NM out of 2000, but the course difference is about 8 degrees. That is huge. You will be way off course when you leave. As another example from Jacksonville Florida to Bermuda, I get a course difference of 4 degrees over a distance of 858 miles.
Another example at a higher latitude is from Hobart Tasmania to Resolution Island in New Zealand. Using Plane trig, it shows the distance and direction from Hobart to New Zealand (Resolution Island) as 828 miles and 106 degrees True. The actual distance and direction is 843 miles and 110 degrees True. That's about the same course descrepancy (4 degrees) as the Florida to Bermuda example, but the distance descrepancy is greater.
For an east-west course, you can estimate the descrepancy by using the cosine of the latitude. For example the cosine of 45 is about 0.7, so where a minute is a mile on a great circle, a minute on the 45th parallel is 0.7 miles. That is a big descrepancy.
If your waypoints are 200 mi or less apart then Plane Sailing would be OK, but for trans-ocean sailing you would probably only want 1 waypoint at your destination so Great Circle Sailing would be better; just depends on what your requirements are.