  Let $G$ be a connected graph and $u,v$ and $w$ vertices of $G$. Then $w$ is
said to {\em strongly resolve} $u$ and $v$, if there is either a shortest
$u$-$w$ path that contains $v$ or a shortest $v$-$w$ path that contains $u$. A
set $W$ of vertices of $G$ is a {\em strong resolving set} if every pair of
vertices of $G$ is strongly resolved by some vertex of $W$. A smallest strong
resolving set of a graph is called a {\em strong basis} and its cardinality,
denoted $\beta_s(G)$, the {\em strong dimension} of $G$. The {\em threshold
strong dimension} of a graph $G$, denoted $\tau_s(G)$, is the smallest strong
dimension among all graphs having $G$ as spanning subgraph. A graph whose
strong dimension equals its threshold strong dimension is called $\beta_s$-{\em
irreducible}. In this paper we establish a geometric characterization for the
threshold strong dimension of a graph $G$ that is expressed in terms of the
smallest number of paths (each of sufficiently large order) whose strong
product admits a certain type of embedding of $G$. We demonstrate that the
threshold strong dimension of a graph is not equal to the previously studied
threshold dimension of a graph. Graphs with strong dimension $1$ and $2$ are
necessarily $\beta_s$-irreducible. It is well-known that the only graphs with
strong dimension $1$ are the paths. We completely describe graphs with strong
dimension $2$ in terms of the strong resolving graphs introduced by Oellermann
and Peters-Fransen. We obtain sharp upper bounds for the threshold strong
dimension of general graphs and determine exact values for this invariant for
certain subclasses of trees.
