Frequency transformed radiomap data set

A method of transforming a radio map data set (RMDS) is disclosed whereby the original radio map has a discrete frequency transform applied to it; the discrete frequency transform may be a Discrete Fourier Transform (DFT), Discrete Cosine Transform (DCT) a short time Fourier Transform (STFT), a Z Transform, a Fast Cosine Transform (FCT) or a Fast Fourier Transform (FFT). The transform may be inversed following being sent to another apparatus and may be used to aid in the determination of the location of the apparatus. The radio map may comprise a discrete coordinate grid, the size of the grid units and a reference location of the grid. The method may also incorporate generating the original radio map data set which may be done via interpolating a radio measurement value for each discrete coordinate set of the grid.