LNAV follows Geodetic VS Great Circle

[mmrassi]

Hi,

Can someone explain to me what's the difference when LNAV follows a Geodetic line compare to great circle tracks?

From my understanding from the descriptions in the web Geodesic path and great circle path are the same in a sphere.

The question rises when I notice some of B737NGs have LNAV provide geodesic guidance and some provide great circle courses.

Regards

[pattern_is_full]

As to the difference - all great circles are geodesics/geodetic lines, but only apply to perfect spheres. All geodesics are not great circles, since they apply to non-spherical surfaces as well.(i.e. ellipsoids)

The Earth is NOT a perfect sphere, therefore technically it has no Great Circles except for the equator (maybe - even that is now technically counted as just a "geodesic"). But for a very long time, "assuming" Great Circles was close enough. They got you there.

If I go any further, I'd just be quoting or rephrasing from this - so you may as well read the original itself: https://en.wikipedia.org/wiki/Geodesics_on_an_ellipsoid

Note the point that the Equator itself may not be the shortest route connecting two places on the Equator (!!)

This may just be labels catching up to current geographical practice - is there a software difference between the 737s involved?

Or it may just be an option to follow tried-and-almost-true, known, quote-Great Circle-unquote routes, vs. more technically up-to-date (and shorter, fuel-saving) routes.

[mmrassi]

Thanks alot

[underfire]

For the FAA TERPS and other design programs, geodesic/geodetic calculations WGS84 spheroid are used. These take into account the oblate spheroid surface of the Earth.
These are good for point to point, but do not work well for curved flightpaths. For this, most other design programs use Vincenty's Algorithm which is pretty good.