  We introduce the notion of a positive spectral measure on a $\sigma$-algebra,
taking values in the positive projections on a Banach lattice. Such a measure
generates a bounded positive representation of the bounded measurable
functions. If $X$ is a locally compact Hausdorff space, and $\pi$ is a positive
representation of $C_0(X)$ on a KB-space, then $\pi$ is the restriction to
$C_0(X)$ of such a representation generated by a unique regular positive
spectral measure on the Borel $\sigma$-algebra of $X$. The relation between a
positive representation of $C_0(X)$ on a Banach lattice and -- if it exists --
a generating positive spectral measure on the Borel $\sigma$-algebra is further
investigated; here and elsewhere phenomena occur that are specific for the
ordered context.
