A number of methods have been proposed for the determination of quiet day curves of riometer recordings of cosmic radio noise in polar regions. Most of these methods are based on the distribution of the recorded intensities, in a given siderial time period, over a number of consecutive days. The method proposed in this paper is based on the filtering, using a low-pass filter in the Fourier domain, of a discrete two-dimensional function or matrix. The two dimensions of the matrix are siderial time and day number, with each row of the matrix representing one siderial day. With a low-pass filter the low frequency Fourier coefficients are retained while the high frequency Fourier coefficients are discarded. The highest Fourier coefficient that is retained is the cutoff frequency of the filter. The proposed method was compared to two existing methods using data obtained from a single broad beam antenna. It was found that the proposed method is less sensitive to noise than the existing methods.