Abstract: Apparatus for determining whether an intermediate terrain feature blocks the line of sight between a pair of spaced terrain points, comprising a linearly extending base, at least three spaced posts extending perpendicularly to the base and having cursors on them, the distances between the posts being selectively adjustable. In one embodiment, the base is of three telescopic sections, with a post mounted on each of the sections. In another embodiment, the base is fixed and has three sliders, on each of which a post is mounted. The posts are pivotally mounted on the base, so as to be stored substantially parallel to the base.