Many applications of wireless sensor networks require the sensor nodes to obtain their locations. The main idea in most localization methods has been that some nodes with known coordinates (e.g., GPS-equipped nodes) transmit beacons with their coordinates in order to help other nodes to localize themselves. A promising method that significantly reduces the deployment cost is to replace the set of statically deployed GPS-enhanced sensors with one mobile landmark equipped with a GPS unit. In this case, a fundamental research issue is the planning of the path that the mobile landmark should travel along in order to minimize the localization error. In this paper we first study the localization error of three different trajectories for the mobile landmark, namely SCAN, DOUBLE SCAN, and HILBERT. We further study the tradeoffs between the trajectory resolution and the localization accuracy in the presence of 2-hop localization, in which sensors that have already obtained an estimate of their positions help to localize other sensors. Our trajectories are practical and can be easily implemented in mobile robot platforms.