Geographic wayfinders and space-time algebra

Time Geography is a framework for describing reachable points in a (static) spatio-temporal environment. While originally devised to facilitate reasoning about an individual's or a population's living conditions, it was later adapted to many other applications. A wayfinder is an entity that moves through a space-time continuum with possible obstacles. We show how to model the pertinent notions in relational algebra (and, more abstractly, in modal semirings) with box and diamond operators. Admissible or undesired regions can be described as Boolean combinations of primitive regions such as the set of all points reachable by forward or backward movement from a given region or starting point. To derive results about the region blocked by the union of two regions we introduce an abstract algebraic view of coordinates that is largely independent of dimensional and metric aspects and thus very general. Moreover, the approach lends itself quite well to machine-supported proofs. (C) 2019 Elsevier Inc. All rights reserved.