A tidal atlas or a tidal stream atlas is used to predict the direction and speed of tidal

Tide

Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

 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. The speed of the flow may be indicated by numbers on each arrow or by the length of the arrow. Areas of slack water

Slack water

Slack water, which used to be known as 'The stand of the tide', is a short period in a body of tidal water either side of high water or low water essentially when the water is completely unstressed, and therefore with no rise or fall of the tide and no movement either way in the tidal stream, and...

 may be indicated by no arrows or the words "slack water".

An alternative to a tidal atlas is a nautical chart

Nautical chart

A nautical chart is a graphic representation of a maritime area and adjacent coastal regions. Depending on the scale of the chart, it may show depths of water and heights of land , natural features of the seabed, details of the coastline, navigational hazards, locations of natural and man-made aids...

 that provides tidal diamond

Tidal diamond

Tidal diamonds are symbols on British admiralty charts that indicate the direction and speed of tidal streams.The symbols consist of a letter of the Roman alphabet in a rhombus, printed in purple ink...

s.