  Localization of a set of nodes is an important and a thoroughly researched
problem in robotics and sensor networks. This paper is concerned with the
theory of localization from inner-angle measurements. We focus on the
challenging case where no anchor locations are known. Inspired by Euclidean
distance matrices, we investigate when a set of inner angles corresponds to a
realizable point set. In particular, we find linear and non-linear constraints
that are provably necessary, and we conjecture also sufficient for
characterizing realizable angle sets. We confirm this in extensive numerical
simulations, and we illustrate the use of these constraints for denoising angle
measurements along with the reconstruction of a valid point set.
