how to read depth on a nautical chart

How to Read A Nautical Chart Depth? A Step-by-Step Guide

Reading the depth on a nautical chart depth 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).

depth contour on nautical chart

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.

keel depth and chart calculation for depth

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. 

Consider electronic options as a cross-check

It may be an idea to also double-check your paper charted depth with electronic charts. The savvy navvy app gives you access to global digital chart coverage (where they have courage).

digital chart depths calculation\

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.

Click here to view electronic 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! 

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