Existing ad hoc location systems measure locations proactively by continuous measurements and communication among all nodes in an ad hoc network. However, in certain location-based applications only a few nodes need the location of a few other nodes, in which case the proactive approach results in an unnecessarily large overhead. We propose a scheme for a reactive ad hoc location system, which avoids the overhead of a proactive scheme by limiting multi-lateration and coordination of coordinate systems to nodes centered around the route between the source node requesting the location and the target node whose location is being requested. Simulations show that our scheme reduces overheads substantially compared to an existing proactive scheme.