1. A method for determining an error estimate concerning a target device's location, wherein the target device is operable to move in a wireless environment and to communicate with the wireless environment using signals each of which has at least one measurable signal value; the method comprising: maintaining a probabilistic model of the wireless environment, the probabilistic model indicating a probability distribution for signal values at several sample points in the wireless environment; making a set of observations of signal values in the wireless environment at the target device's location; estimating the target device's location based on the probabilistic model and the set of observations; determining the error estimate as a combination of products over several sample points, each product comprising: a probability for the sample point in question being the target device's location; and a distance function between the sample point in question and the target device's estimated location.