http://ozreport.com/docs/Distancecalculations.pdf

There are different methods to calculate distances on the earth's
surface. In paragliding and hang gliding sports two systems are used, the
(older) FAI sphere and the WGS84 ellipsoid. The WGS84 ellipsoid needs more
calculation power than the FAI sphere. Therefore the FAI sphere method is better
suited for flight instruments; whereas, PC based flight evaluation software use
the WGS84 ellipsoid

http://www.flymicro.com/records/distcalc.cfm

Due to changes in the FAI web site I don't have access on their site to distance
calculations using the FAI sphere instead of the WGS 84 ellipsoid, but you can
find it here:
http://microguard.dk/bookmark/icons/great-circle-calculator.htm. I also
don't have access to the latest FAI/CIVL documents re distance measurements from
the FAI web site.

http://ozreport.com/21.47.2

http://www.fai.org/downloads/civl/CIVL_2017_Minutes

The last discussion was about changing the distance formula from FAI sphere to
WGS84, which is required by the FAI general section. This cannot be done on a
general basis today for hang gliding, as the majority of the pilots in the next
Cat 1 event will still be stuck with older FAI sphere only instruments (Flytec
6030's), while it is viable for paragliding where the access to newer
instruments makes this less of a problem. The time frame for changing to
SeeYou/GAP scoring from May 2018 would include the change to WGS84, and as such
it should be a natural point to introduce this change for both hang gliding and
paragliding in the rules regarding distance calculations. If possible, we could
implement WGS84 as an option in FS in addition the FAI Sphere from now on, and
this option could be used where appropriate in PG competitions with large
turnpoints.