In a method for ascertaining and monitoring fill level of a medium in a container by means of a field device using a travel time measuring method, wherein transmission signals are transmitted toward the medium and reflection signals are received; the received reflection signals are registered as echo signals in an echo function, or envelope curve, dependent on travel time or travel distance; by means of an echo search algorithm, at least one wanted echo signal is ascertained in the echo function, or envelope curve; by means of at least one filter with filter parameters the echo function, or the envelope curve, is preprocessed; at least a first filter range with a first width is predetermined at the position of the wanted echo signal in the echo function, or in the envelope curve, and at least three filter ranges are formed in the echo function, or in the envelope curve.