A tidal atlas or a tidal stream atlas is used to predict the direction and speed of tidal currents. A tidal atlas usually consists of a set of 12 or 13 diagrams, one for each hour of the tidal cycle, for a coastal region. Each diagram uses arrows to indicate the direction of the flow at that time. A spring tide is a tide just after a new or full moon, when there is the greatest difference between high and low water. A neap tide is a tide just after the first or third quarters of the moon when there is least difference between high and low water. This tidal atlas covers Galway Bay which is situated on the west coast of Ireland. The data used to create the Galway Bay tidal atlas was derived from the output of an operational hydrodynamic model of Galway Bay. The Galway Bay tidal atlas is managed by Oceanographic Services within Ocean Science and Information Services section of the Marine Institute (Ireland).