Reading the depth on a nautical chart can be challenging for a beginner boater. However, with a step-by-step guide, it can be easy to get to grips with.
How to read depth on a nautical chart?
Here's how to read the depth on a paper nautical chart:
Find the depth soundings
The first step is to locate the depth soundings nearest to your location or your intended location, these are marked by small numbers on the chart.
Look for the water depth
The depth of the water is measured by soundings usually acquired by marine hydrographic surveys. These depths are usually measured in either feet or fathoms (the chart will indicate which). A fathom is a nautical unit of measurement equal to six feet (about 1.83 meters) and is used especially for measuring the depth of water).
Identify the contour lines
Water depths may be connected with a line known as a depth contour as they connect areas of the same depth together. Depth contours present a picture of the sea bottom for boaters. The closer the contour lines are to each other, the steeper the slope of the seabed. The further apart the contour lines, the shallower the slope.
Determine your safety depth
The safety depth is the minimum depth required to navigate safely (to ensure you have sufficient clearance under your hull/keel at any given time, including during tidal fluctuations). It is determined by adding a safety margin to the charted depth. A safety margin of 10% to 20% is usually recommended.
Take into account the tide
The water depth on the chart is usually referenced to a specific tidal datum. Therefore, it's important to take into account the current tide level when navigating to ensure you have enough water below your boat.
Calculate the under-keel clearance
The under-keel clearance is the distance between the keel of the vessel and the seabed. It should be at least twice the draft of the vessel.
By following these steps, you can use nautical charts depths to assist you in navigating safely through shallow waters. It's important always to exercise caution and be aware of changing conditions, such as weather and tides and to be prepared to adapt plans as conditions change.
Use digital first navigation tools to help you explore chart data
Modern boaters are increasingly using digital first tools like Savvy Navvy to read, use and navigate with charts. Digital First charts are electronic and unlike paper charts they are updated regularly to ensure you have the most accurate information possible.
Apps like Savvy Navvy allow you to set your boat settings including your minimum depth and draft. This means you can plan routes using the boating app whilst not only using the digital charts but also taking into account wind, weather and tide for optimal safety and hassle free planning.
To see that charted depth you can tap on the electronic chart to get a depth reading at any given location. The app also displays contour lines as a visual overview on the chart. Some areas such as the UK and Australia also have relief shading to provide further detail for planning a trip at sea.
Click here to view digital first Savvy Charts™.
Reading the depth on a nautical chart - the summary
Are you wondering how to read latitude and longitude on a nautical chart? Check out Savvy Navvy blog for more details!
