A Tidal Atlas is an essential tool for navigators to determine the direction (set) and speed (drift) of tidal streams at various locations and times. It provides a visual representation of tidal flow, crucial for planning safe and efficient voyages.
What is a Tidal Atlas?
A Tidal Atlas is typically a book or series of charts showing the predicted direction and speed of tidal streams for a specific area at hourly intervals relative to high or low water at a designated standard port. Unlike tidal diamonds on a navigation chart which give average data for a specific point, a tidal atlas provides a broader picture of the flow over a larger area.
Key Components of a Tidal Atlas
Understanding the elements within a tidal atlas is vital for effective use:
- Chart Pages: Each page represents a specific time step, usually marked in hours relative to high water (HW) or low water (LW) at a standard port (e.g., HW -3, HW, HW +2).
- Location Diamonds: These indicate specific points where tidal stream data is provided.
- Arrows: These visually show the direction of the tidal stream at the location diamonds for that specific time step.
- Numbers: Typically printed near the arrows, these numbers represent the speed of the tidal stream, usually given in knots for both spring and neap tides.
Step-by-Step Guide to Using a Tidal Atlas
Using a tidal atlas involves a few key steps to extract the relevant tidal stream information for your position and planned time:
- Identify Your Location and Time: First, pinpoint your geographical position on the chart and determine the exact time for which you need the tidal stream information.
- Determine the Tidal State: Find the time of High Water (HW) or Low Water (LW) at the standard port referenced by your tidal atlas. Calculate the time difference between the actual time you need the data and the time of HW or LW. This gives you the "tidal state" (e.g., 3 hours before HW, 2 hours after LW).
- Select the Correct Atlas Page: Open the tidal atlas to the page that corresponds to the tidal state you calculated in the previous step (e.g., the page marked "HW - 3 hours").
- Locate Your Position on the Page: Find your vessel's position on the selected atlas page. Observe the tidal stream arrows and numbers nearest to your position.
- Orient Your Plotter and Read the Direction: This step is crucial for determining the precise direction of the tidal stream. As seen in the reference, you orientate your plotter like so and in the same way as we would on a normal chart. Align your parallel rule or plotter with the direction shown by the tidal stream arrow near your position on the atlas page. We then go and we can read off that tidal Direction. Use the plotter's compass rose or transfer the line to a meridian to determine the true bearing (set) of the tidal stream.
- Read the Tidal Speed: Note the numbers printed near the arrow. These usually provide two values: the speed at spring tides and the speed at neap tides. These are typically given in knots.
- Interpolate (If Necessary):
- Time: If your required time falls between the times shown on two different atlas pages (e.g., 2.5 hours after HW, but the atlas only shows HW+2 and HW+3), you may need to estimate the direction and speed by looking at both pages. The stream will gradually change between the two states.
- Speed (Tidal Range): The speed given is for springs and neaps. If the actual tidal range for the day is between springs and neaps, you'll need to interpolate between the spring and neap speeds to get a more accurate estimate of the drift. Tidal stream prediction tables (like Admiralty TotalTide or tables in almanacs) or interpolation formulas can help with this.
- Apply the Information: Use the determined tidal stream (set and drift) when calculating your course to steer and estimated time of arrival, adjusting for its effect on your vessel's movement.
Why is Using a Tidal Atlas Important?
Ignoring tidal streams can lead to significant errors in navigation. A strong stream pushing you off course or slowing you down can result in:
- Ending up in a dangerous location.
- Missing a narrow entrance or channel.
- Taking much longer than expected for a passage.
- Calculating an incorrect ETA, affecting rendezvous or port arrival times.
By using a tidal atlas, you can accurately factor the tidal stream into your navigation planning, ensuring a safer and more predictable journey.
