  This letter studies measurement partitioning and equivalence in state
estimation based on graph-theoretic principles. We show that a set of critical
measurements (required to ensure LTI state-space observability) can be further
partitioned into two types:~$\alpha$ and~$\beta$. This partitioning is driven
by different graphical (or algebraic) methods used to define the corresponding
measurements. Subsequently, we describe observational equivalence, i.e. given
an~$\alpha$ (or~$\beta$) measurement, say~$y_i$, what is the set of
measurements equivalent to~$y_i$, such that only one measurement in this set is
required to ensure observability? Since~$\alpha$ and~$\beta$ measurements are
cast using different algebraic and graphical characteristics, their equivalence
sets are also derived using different algebraic and graph-theoretic principles.
We illustrate the related concepts on an appropriate system digraph.
