The data that inputs to these graphs is from NOAA, the National Oceanic & Atmospheric Administration in America who use a lot of sophisticated science, satellite data and buoys to arrive at a 7 day swell prediction for offshore waters. There is a formula for the expected swell on the beaches, which is usually smaller and it has been factored in. However, there is no guarantee the prediction will be accurate as it is impossible to allow for local conditions and geography. These charts are therefore nothing more than a reasonable starting point for planning surf trips.