A robot can explore an unknown environment by incrementally constructing a roadmap of that environment using line of sight sensor information. Recall that a roadmap is a one-dimensional representation of a robot's environment. This paper addresses one problem that occurs while generating a roadmap: what happens when sonar sensors cannot detect sharp objects because of the specularities? To do this, a new sensor model is combined with an already existing incremental construction procedure for a roadmap. Experiments on a mobile robot validate the results of this paper.