3 Methods How To Read A Nautical Chart (Complete With Pictures)

How To Read A Nautical Chart - Nautical charts are an important navigational tool, even if you are familiar with a waterway. A nautical chart will help you figure out which direction to go, how deep the water is, and where ports are located. You'll also be aware of underwater obstacles that may not be visible, as well as bridges and power lines that might get in your way. Even if you have a navigation system onboard, chart reading is a skill you must master before hitting the water.

• Measure distance and depth

1. Use the dial indicator to track distance on a map. Scales are expressed as ratios and differ from map to map. The scale, printed in the top right corner of the map, looks something like 1:100,000. A ratio of 1:10,000 means that every 2.5 cm (1 inch) of the map is 25,000 cm (10,000 inches) in the real world. Smaller scale charts provide a close-up view, showing more area detail and markers.

If you're familiar with maps, scale works the same there.

A port map is an example of a small-scale map. Navigators need a close look at the harbor with all the obstacles and other details. Larger maps show land and waterways but little additional detail.

2. Find the unit of measure used to indicate water depth. Look in the top right corner of the map near the ruler. You will see the unit of measure in large letters. You need to know this because charts use all sorts of different measurements. Your chart can contain feet, fathoms, or meters.

In the US, charts generally use feet or fathoms. However, the National Oceanic and Atmospheric Administration (NOAA) has begun converting its charts to meters for international consistency.

3. Read the chart numbers to find the lowest water depth. The black numbers on the map indicate the water depth. Each number indicates the "mean lower low water" (MLLW) in an area. This is the average water depth at low tide, so most of the time the water is deeper than shown on a chart.

If you see a negative number, it means that the water depth in the area is typically shallower than shown on the chart.

4. Multiply fathoms by 6 to measure depth in feet. Numbers written in fathoms have a unique look that makes them easy to spot. A fathom listing often consists of a pair of numbers. The first number indicates the number of threads. The second number, which will be a minor index, is the number of extra feet.

For example, listing 0 and 3 means a depth of 0 fathoms, 3 feet. A listing of 3 and 2 means a total of 3 fathoms, 2 feet, or 20 feet.

5. Multiply meters by 3 to convert to feet. For consistency and simplicity, many modern charts use gauges. This can be difficult to understand if you are used to measurements in feet or fathoms. Multiplying by 3 is a quick way to get a reasonably accurate estimate of water depth.

For example, multiply a listing of 6 meters by 3 to estimate that it is at least 18 feet deep. Actual depth is 6 meters (20 feet) so not entirely accurate but useful to avoid shallow areas.

If the depths are in feet, you can also divide by 3 to quickly convert to meters.

• Using colored contour lines

1. Follow the dotted contour lines to monitor similar water depths. Contour lines are intended to give you an idea of ​​what is happening below the water surface. Contour lines, often red, connect similar water depth indications. This has the effect of dividing areas into shallower and deeper waters. Use the contour lines as a reference for the total water depth in an area.

For example, contour lines can connect points listed as 20 feet (240 inches) deep. All points within these lines are even flatter.

2. Look for light blue areas to identify shallow water. On nautical charts, the white area is usually water, making the blue areas more prominent. Shoals and other shallow areas are blue. These spots are typically no deeper than 5.5 m (18 ft), although this varies from map to map. Flat areas are often marked by contour lines.

The exact depth of the shallow areas depends on the map. Some maps stain in areas under 3 fathoms, while others mark areas less than 1 fathom deep.

3. Find areas of yellow or brown to identify land and sandbars. Dry land is usually within a contour line, but not always. Pay special attention to brown spots in the middle of water channels. Sometimes these spots are underwater and can damage your boat, especially in shallow water.

Land is always light in color to distinguish it from water and other navigational aids.

• Identify common chart symbols

1. Use the compass rose to determine the direction you are traveling. The compass rose is a series of 3 circles surrounded by numbers. The outer circle is designed to help you find true north, represented by a star or a 0. The inner circle points to magnetic north at the time the map was printed. You can use a magnetic compass to navigate using the compass rose as a guide.

Keep in mind that the magnetic field over the earth changes over time, so you'll need a magnetic compass or an updated map to use the compass rose.

2. Follow the red and green buoys for navigational aids. These buoys mark water channels. The colored circles you see on a map correspond to buoys you'll see floating along the canals during your voyage. Travel markers are numbered, making it easier to follow your route.

Red markers show the right side of a channel, while green markers show the left side. The different colors give you orientation, no matter in which direction you travel.

3. Look for anchors to find anchor points for boats. The anchor is a familiar symbol on charts showing waterways near land. An anchor means you can safely drop anchor your boat in that area. Anchor points may be numbered or labeled to describe what type of boats are allowed to stay there.

For example, an anchor point marked “DW” means deep water. Only deep sea vessels will anchor there.

A number like 24 represents 24 hours, so you can only leave your boat in the area one day at a time.

4. Note rocks and other hazards circled on the map. Dangerous obstacles like rocks and shipwrecks are all circled. Some circles are dotted, which means the obstacles are under water. When the lines are taut, the obstacles are at least partially above the water. Many modern maps now mark these points in purple.

Obstacles are generally represented by circles with symbols such as asterisks and plus signs. There are many different symbols and they often vary between charts.

Some cards use pictograms, e.g. B. a boat to represent a partially sunken shipwreck.

5. Follow the arrows for the speed and direction of currents and tides. Currents are indicated by arrows with numbers above them. The direction of the arrow indicates in which direction the current is moving. The number tells you the water speed in knots.

Currents and tides are constantly changing. Take the arrows as a prediction of the way water normally flows. Be on the lookout for the extraordinary on your journey.

6. Read abbreviations to identify important features in the area. Because charts have limited space, mapmakers must rely on shortcuts to denote markers. Some letters indicate the type of seabed. Others label landmarks such as towers, shipyards, and government stations.

Some common abbreviations for seabed are S for sand, M for mud, and C for coral. Rky stands for a rocky ground.

Tr indicates a nearby tower, while Whf stands for wharf and Bn means beacon. CG can point to a Coast Guard station on a US map.

7. Download a separate table with a list of less common symbols. The full title of the table is NOS Table #1: Symbols, Abbreviations and Terms. It's not actually a nautical chart, but it does contain a list of all chart symbols you may encounter. Download it from NOAA's website or purchase a copy from a commercial map supplier.

The chart covers all US charts. Download a copy at https://nauticalcharts.noaa.gov/publications/us-chart-1.html.

Maps from other countries may have other symbols. Check with your government's Maritime Administration for a reference.