1 A method for classifying a survey area according to an attribute comprising the steps of: (a) repeatedly transmitting sonar pings of a uniform duration and known frequency from an echo sounder located at a reference origin on a survey vessel towards the survey area; (b) measuring the amplitudes of echoes received at the reference origin, each echo comprising sound scattered back from an insonified footprint within the survey area, said insonified footprint being associated with a survey point having a known position measured from the reference origin; (c) calculating the depth to each survey point from the reference origin; (d) forming a time series for each echo as a selected number of amplitude values sampled from the echo at a frequency determined as a function of the depth to the survey point for each said echo, said number being selected so that the likelihood of an echo enduring longer than said number of samples is small; (e) assigning a value for the attribute at each survey point based on the time series sampled from the echo scattered back from the insonified footprint associated with each said survey point.