italia,
if you try to make definitions simplistic, you are often in for it. Definitions are, by definition (
), to be exact rather than simple. As has been made clearly apparent, there's a lot of room for different interpretations as to what ground speed is.
PBL,
looking at Kayton and Fried, I see an figure from an avionics text book (which I am grateful for the reference to!), illustrating varius concepts and terms used, rather than a strict definition. In other words, I'd say they made a for most purposes perfectly reasonable simplification, but a definition it is not.
Going for definitions, The United States Air Force Dictionary (Heflin) defines ground speed as:
"
The speed of an aircraft, esp. an airborne aircraft, measured by the distance it travels over the ground either by reference to check points on the ground, or by taking wind velocity into account."
The U.S. Navy Hydrographic Office (1955) define ground speed as:
"
the rate of motion of the aircraft relative to the earth's surface. It is the speed of the aircraft over the ground; hence the name."
Clearly these definitions do not agree with Kayton and Fried (with the possible exception, and slight ambiguity, introduced after the last comma by Heflin), as can be illustrated by taking it to the extreme of very high altitude flight. If we look at the primary purpose of ground speed, namely navigation, the definitions presented here remain useful while errors would creep into the navigation solution using the IMO simplified explanation in the figure in Kayton and Fried - which is intended for a different audience than navigators.
The definitions listed above still leave it up to the reader to decide what being above a place on the surface of the earth actually means, but it is to the bet of my knowledge widely accepted in geodesy that being above something is referenced to the geoid and not the ellipsoid. In other words, you are above a place on the surface of the earth when a dropped penny will hit that place. Using the ellipsoid as the level gives interesting effects such as streams flowing uphill in places, so it is best avoided.
I think the hobgoblin of this discussion is that for all normal practical purposes, a simplified definition by far exceeds the required precision and hence, many more or less authorative publications will go with the simplification - without explicitly stating so.
References
Heflin, W. A. (Ed.). Publication year unknown, but probably 1950s or 1960s.
The United States Air Force Dictionary. D. Van Nostrand Company, Inc., Princeton, NJ, U.S.A.
The U.S. Navy Hydrographic Office, H.O. Pub. No. 216. (1955).
Air Navigation. U.S. Navy Hydrographic Office, United States Government Printing Office, Washington, U.S.A.