1. A method comprising: receiving measurement data for a wireless transmitter at a server; and producing, by the server, a quasi-fingerprint of the wireless transmitter using the received measurement data, the quasi-fingerprint comprising a window of applicability that circumscribes positions at which the measurement data was taken, the quasi-fingerprint further comprising a parametric representation that models received signal strength within the window of applicability, wherein the quasi-fingerprint consists essentially of a nominal position of the wireless transmitter, a set of signal power radii that respectively define positional contours at least partially around the nominal position of the wireless transmitter for a plurality of different signal strength values, and the window of applicability.