In mesotidal environments, the monitoring of shoreline changes is hampered by the determination of this line on aerial photographs. This paper discusses and evaluates the importance of taking into account the margin of error associated in determining the state of the shoreline. To demonstrate this effect, three cases were selected and studied using aerial photographs from 1946 and 2016 (70-year period). The sites chosen on the Moroccan Atantic coast are very close to each other but evolve differently. We proved statically that the impact of taking into account the margin of error will not have the same effect on the interpretation of the results. By taking these errors into consideration, the interpretation of the results becomes more consistent and more suitable for the decision-makers.