A method of detecting the structural elements within a scene sensed by at least one sensor within a locale, the method comprising: a) capturing data from the sensor, which data provides a first representation of the sensed scene at the current time; b) generating a second representation of the sensed scene where the second representation is generated from a prior model of the locale; and c) comparing the first and second representations with one another to determine which parts of the first representation represent structural elements of the locale.