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I am impostor here, and am by no means mathematically literate, so please bare with me.

I have posted a software question on StackOverflow, regarding an unexpected drift in latitude using Vincenty's Direct formula; https://stackoverflow.com/questions/31687723/vincenty-direct-formula-latitude-issue

I am trying to calculate a new location when travelling due east from a known location and distance.

What has surprised me is when calculating using a 90 degree bearing, the Latitude drifts south.

This is not the case at the equator, but certainly is closer to the pole.

Can anyone explain this effect? Are there any other formulas I could use in place of Vincenty direct to avoid this drift?

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TommyGuns21
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  • For the latitude to be the same would imply a rhumb line trajectory, but Vincenty's formulae deal with trajectories that follow geodesics (if they were using a spherical model of the Earth (but they aren't) these would be great circles) – Conrad Turner Jul 29 '15 at 11:05
  • Thank you Conrad, really helpful. Can you recommend a great circle formula using rhumb lines which I could use to plot a point based on bearing and distance? I assume a reworking of Haversine distance formula would achieve this? – TommyGuns21 Jul 29 '15 at 11:20
  • Rhumb lines and great circles are different things, great circles are geodesics on a spherical model of the Earth., and no I'm not aware of such formulae (Heversines will give you the same problem as Vincenty, it is based on great circles rather than geodesics on a spheroid). A google search for "rhumb line distance" turns up a number of hits that may help, including this – Conrad Turner Jul 29 '15 at 12:51
  • Other links for rhumb lines are the online calculator RhumbSolve and this page describing the underlying mathematics. (I am the author of both pages.) – cffk Aug 15 '15 at 15:38

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