Mercator Sailing: Course and Distance

08312018, 05:46 AM
(This post was last modified: 09012018 05:45 AM by Dieter.)
Post: #15




RE: Mercator Sailing: Course and Distance
(08312018 02:15 AM)Gene222 Wrote: I see what I am doing wrong. The course is 270 (due west) on a Mercator map. I was looking a polar map. I guess starpath.com online Mercator calculator was right. For the record: my 35s program returns 1875,78 miles and a course exactly West, true course 270°. By the way, the result 1875,4003 nm of the starpath.com Mercator Calculator simply is 180°·60·cos(80°). This is a simplified formula that ignores (!) the ellipsoid's eccentricity. That's why there is another factor in my samelatitudeformula – which returns the correct result. (08312018 02:15 AM)Gene222 Wrote: I still have errors in my program. The Latitude difference for the above problem is 600' and the Meridional parts for point 1 and 2 are not the same. This really sounds like there are errors. ;) For 80° the meridional parts (WGS84) should be 8352,48. Your results of –3456,8203 and –4507,4040 are the meridional part values for latitudes of –50° and –60°, respectively. Maybe this helps finding the error. (08312018 02:15 AM)Gene222 Wrote: My procedure for EastWest courses needs to be cleaned up, which should be easy to do. I can't read your program (no Prime here), but from what I see when I open it in a text editor the program distinguishes various cases for different hemispheres. For instance in one case the absolute values of the meridional parts are added. I don't think this is required – the sign convention handles this automatically. Try it. Here is the way I do the calculation, in pseudocode. I hope I got it right. ;) Code: e=0,08181919... Finally the true course is calculated. I do it wih the 35s "ARG" command which returns something like ATAN2 in some programming languages, a quadrantadjusted angle (0...180 for Q1 and Q2, 0...180 for Q3 and Q4, in this case add 360°), and it also works for DMP=0. You'll know how to do it on the Prime. ;) Dieter 

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