1. A method comprising: receiving measurement data for a wireless transmitter at a server; calculating geographic contours of a window of applicability that circumscribes positions at which the measurement data was taken, the geographic contours defining a shape of the window of applicability; calculating a set of signal power radii that respectively define positional contours at least partially around a nominal position of the wireless transmitter for a plurality of different signal strength values; and producing a quasi-fingerprint for the wireless transmitter comprising the window of applicability and the set of signal power radii.