Location discovery is a fundamental building block for many mobile applications. Yet dedicated infrastructure for determining node locations is expensive, energy consuming, and simply unavailable under certain development scenarios. This paper presents an accurate,cheap and scalable protocol for location discovery. Called Zoom, this protocol operates by setting up and solving a system of geographic constraints based on connectivity information from the underlying communication network. Zoom achieves high accuracy by aggressively extracting constraints from the link layer, by propagating this information across multiple network hops and by explicitly tracking the set of possible locations for any given node instead of a single position estimate. Physical experiments with motes show that a large number(98%)of the nodes in a network can determine their positions based on a small number(30%)of landmark nodes with high accuracy(median error less than 30% of transmission range).