Full Idea
You could imagine a person using the angle from a theodolite to decide a suitable spot to cut the height of the flagpole, …but since such circumstances would be very unusual we naturally say the flagpole subtends the angle because of its height.
Gist of Idea
The height of a flagpole could be fixed by its angle of shadow, but that would be very unusual
Source
J.J.C. Smart (Explanation - Opening Address [1990], p.14)
Book Reference
'Explanation and Its Limits', ed/tr. Knowles,Dudley [CUP 1990], p.14
A Reaction
[compressed; he mentions Van Fraassen 1980:132-3 for a similar point] As a response this seems a bit lame, if the direction is fixed by what is 'usual'. I think the key point is that the direction of explanation is one way or the other, not both.