Re: Sphere or Ellipsoid?

[Tim Newport-Peace]

X-no-archive: yes
In article >, Pat Russell
> writes
>Which distance calculation algorithm is used by FAI in the homologation of
>gliding world records?
>
>-Pat
>
Either the FAI sphere or the WGS84 ellipsoid. But where the distance is
critical, the WGS84 ellipsoid is used.

There is a script for calculating WGS84 distances at:
http://www.fai.org/distance_calculation/

For more info read the Sporting Code (General section).

Para 7.3.1.1, states: "For FAI distance calculations, the earth model
used may either be the WGS84 ellipsoid or a sphere of radius 6371 km
exactly. For accurate measurement and calculation of distance, Air Sport
Commissions shall choose which model is to be used in their area of
activity. The WGS84 ellipsoid is very close to the real shape of the
earth, the FAI sphere is an approximation."

Sporting code Section 3 (Gliding) Para 4.4.1 says:

"For the purpose of the calculation of FAI geodesic distances, the WGS84
earth model shall be used. See also Annex C, Appendix 2.

When calculation of the exact distance is not critical, less accurate
methods may be used. See Annex C , para 1.6b."

Best regards,

Tim Newport-Peace

"Indecision is the Key to Flexibility."

[[email protected]]

Thanks for looking up the rules for me, Tim.

However, I am still confused about which method the FAI would
actually use in a World Record claim.

The rules say that we (or they) must make a determination of
when "accurate measurement" is required:

>"For accurate measurement and calculation of distance, Air Sport
>Commissions shall choose which model is to be used in their area of
>activity."

Assuming that a World Record requires "accurate measurement,"
the rule quoted above seems to state that the method to be used,
in the case of gliding, would be the method mandated by SC3.
The SC3 rule is:

>"For the purpose of the calculation of FAI geodesic distances, the WGS84
>earth model shall be used."

Given all of the above, it seems that FAI must use the ellipsoid
(and no other method) to calculate distance for the homologation
of world gliding records.

I am suspicious that they don't actually do this. Does anyone
know for sure?

And can anyone give me an example of a "non-critical" distance
measurement?

[Denis Flament]

wrote:

> The SC3 rule is:
>
>
>>"For the purpose of the calculation of FAI geodesic distances, the WGS84
>>earth model shall be used."

Yes - since 1st Oct 2002 - WGS 84 is the only method to be used for
soaring records (sphere remains in use in hang gliding)

> And can anyone give me an example of a "non-critical" distance
> measurement?

50 km badge if distance is 51 km or more.... the difference between
sphere and ellipsoid is less than 1 %. You may event measure on the map,
like in the good old times, when there were no GPS ;-) (remember the FAI
SC has been written in these times - that's why there is such odd things
like the requirement of overtaking the old record by more than 10 km)

But every good flight planning software* makes now the exact calculation
in WGS 84, so why not use it ?

*at least SeeYou, CouTraCi, in last versions - and if yours don't, there
is a html WGS84 distance calculator on IGC site http://www.fai.org/gliding

--
Denis
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