A method for ascertaining and monitoring fill level of a medium in a container a field device using a travel time, measuring method, wherein transmission signals are transmitted and reflection signals received. The received reflection signals are registered as echo signals in an echo function, wherein masking curve, evaluation curves and/or echo parameters of the echo signals in the echo function are ascertained or predetermined, as well as stored in a first measuring cycle. By a static echo search algorithm, through the masking curve, the evaluation curve, the echo parameters, position and/or amplitude of at least one wanted echo signal are/is ascertained, wherein, a dynamic echo search algorithm, a continuous echo tracking of positional changes and/or amplitude changes of individual echo signals and/or the wanted echo signal in the echo function is performed. On the basis of positional changes and/or amplitude changes of individual wanted echo signals, masking curve, evaluation curve and/or echo parameters of the static echo search algorithm are adjusted.