  Given is a 1.5D terrain $\mathcal{T}$, i.e., an $x$-monotone polygonal chain
in $\mathbb{R}^2$. For a given $2\le k\le n$, our objective is to approximate
the largest area or perimeter convex polygon of exactly or at most $k$ vertices
inside $\mathcal{T}$. For a constant $k>3$, we design an FPTAS that efficiently
approximates the largest convex polygons with at most $k$ vertices, within a
factor $(1-\epsilon)$. For the case where $k=2$, we design an $O(n)$ time exact
algorithm for computing the longest line segment in $\mathcal{T}$, and for
$k=3$, we design an $O(n \log n)$ time exact algorithm for computing the
largest-perimeter triangle that lies within $\mathcal{T}$.
