  In this paper, we consider the 1.5-dimensional orthogonal terrain guarding
problem. In this problem, we assign an x-monotone chain T because each edge is
either horizontal or vertical, and determine the minimal number of vertex
guards for all vertices of T. A vertex vi sees a point p on T if the line
segment connecting vi to p is on or above T. We provide an optimal algorithm
with O(n) for a subproblem of the orthogonal terrain guarding problem. In this
subproblem, we determine the minimal number of vertex guards for all
right(left) convex verteices of T. Finally, we provide a 2-approximation
algorithm that solves the 1.5-dimensional orthogonal terrain guarding problem
in O(n).
