So, to partially answer my question to ft, and to answer my question
Originally Posted by PBL
Is there anything else to say?
affirmatively,
Let me consider the earth to be a sphere with circumference 21,600 nm (i.e., 1nm = 1 minute of arc), giving a radius of 3437.75 nm. Flying level at 1,000 ft (about 1/6 nm) in a great circle, you are flying a circle radius 3437.916 nm. So your TAS is a factor of 0.00004829, that is, 0.004829%, higher than the speed of a vehicle on the ground vertically underneath (i.e. on the same radius to the center as) you. Generally, for every 1,000 ft higher you fly, your GS (Kayton defn) will increase by this factor.
Kayton suggests that the deflection of the vertical is "commonly less than 10 seconds of arc and rarely greater than 30 seconds of arc", as I quoted. Taking his 30 sec figure, that would yield a correction cos(30 sec) to TAS, therefore to GS. The correction factor is (1 - cos(30 sec)) = (1 - cos(0.0001454 rad)) = 1.057 x 10^(-8), which is about 1/4000th of the 1,000-ft-altitude factor. The correction factor for 10 sec of arc, (1 - cos(10 sec)), which is what Kayton suggests "commonly" is the case, is 1.1752 x 10^(-9), about 1/40,000th of the 1,000-ft-altitude difference.
That suggests that the difference in GS (Kayton defn) due to geoid undulations is negligible compared with a difference in GS (Kayton defn) due to an altitude difference of about 1,000 ft.
PBL