(1) The purpose of marking an obstruction which presents a hazard to aeronautics is to warn airmen during the hours of daylight of the presence of such an obstruction. To accomplish this objective, it may be necessary to color such an obstruction so that it will be visible from aircraft at any normal angle of approach, or to indicate the general definition and location of the obstruction by use of suitable markers or flags.

(2) When the upper part of only a portion of a structure or similar extensive obstruction projects above an obstruction determining surface, as described in state standards, that portion only need be obstruction marked and the point or edge of it highest in relation to the obstruction determining surface should be regarded as the "top of the obstruction." In certain cases, however, such as when the obstruction determining surface concerned is an approach or transition surface (i.e., sloping) this point or edge highest in relation to the obstruction determining surface may not be the highest point or edge above a horizontal plane passing through the base of the object. In such cases, those portions of the object, the upper parts of which are higher above a horizontal plane passing through the base of the object than the upper part considered as the "top of the obstruction," should also be obstruction marked.