Understanding a Nautical Chart
A nautical chart is a reduced, conventional, geometrically accurate, and flat image of a portion of the Earth’s surface. It is a representation of part of a landscape seen from above. There is no direct depiction of relief; instead, contour lines and soundings are used to interpret it.
On a chart, by international convention, the northern part of the landscape is always shown at the top, the south at the bottom, the east on the right, and the west on the left.
The legend of the chart allows symbols and colors to be interpreted, but it is incomplete on the nautical chart itself. The full legend is found in documents that must be kept on board a vessel (merchant marine regulations).
The scale corresponds to the reduction factor compared to reality on the ground. There are different scales.
When the reduction is significant, details disappear; this is the case of a small-scale chart (e.g., 1:1,000,000). Conversely, the larger the scale, the less the reduction and the more detailed the chart (e.g., 1:25,000).
The scale may be written in two different ways: 1:25,000 or 1/25,000, but it is read the same way: “one twenty-five thousandth.”
The scale makes it possible to calculate, from a chart, the real distance on the ground.
A scale of 1:25,000 means that 1 centimeter on the chart corresponds to 25,000 centimeters on the ground. For easier calculation, centimeters are converted into meters (250 meters) or into kilometers (0.25 km).
To know the distance between two points on a nautical chart, one measures the distance on the chart (in centimeters) and multiplies it by 250 meters. This gives the real distance in meters.
Land relief is represented by contour lines (highlighted in red on the chart) linking points of equal altitude.
Submarine relief is represented by isobaths (highlighted in green on the chart), also called soundings lines, linking points of equal depth. These curves are all drawn in black on the chart.
For each sounding line, depth is indicated, and additional soundings specify depths in between.
The closer the lines, the steeper the slope. Slopes represent the inclination of the terrain relative to the horizontal (the ratio between the elevation difference of two points and their horizontal distance).
Slope is expressed as a percentage. A 10% slope means a rise (or drop) of 10 meters for a horizontal distance of 100 meters.
A nautical chart is identified by its title, which specifies the geographic coverage, and a reference number from the publisher (Shom, Navicarte, Admiralty, Imray, NGA, NOAA, etc.). Different chart scales exist.
Three main background colors are used on the chart: sepia, blue, and white.
Black, pink, and green are also used.
-
Sepia represents land. Black is used for cities, roads, contour lines, and prominent landmarks (seamarks). Seamarks are very important for positioning.
-
Blue represents the sea in areas between 0 and 10 meters deep. Black is used to show ports, buoys, and isobaths (equal-depth lines). In this shallow area, precise indications are crucial: the top number shows meters, and the smaller number below shows decimeters.
-
White represents the sea at depths greater than 10 meters. Black is used for isobaths. The 20-meter isobath is highlighted in blue.
-
Pink is used to highlight areas within the blue and white zones, indicating lighthouse positions at port entrances, submarine cables, and regulated areas (marine reserves, anchorage zones).
-
Green indicates rocks that may emerge at low tide.
The nature of the seabed is symbolized with abbreviations (from English terms):
-
Uppercase letters = type of seabed:
-
S = sand
-
M = mud
-
St = stones
-
G = gravel
-
P = pebbles
-
R = rock
-
Wd = weed (seaweed, grass)
-
Sh = shells (shell debris)
-
-
Lowercase letters = seabed quality:
-
bk = broken
-
so = soft
-
h = hard
-
Compasses, drawn in pink, are used to calculate and plot a course on the chart. When plotting a course (heading), it is very important to take magnetic variation into account (the difference between true north and magnetic north).
Insets on the edges of the chart (at a different scale) show enlarged plans of different ports.
Lighthouses and buoys are marked on nautical charts with information about the type of light, its period, and its range.
For example, the La Garoupe lighthouse (on the hill at Cap d’Antibes) is described as: FI(2)10s104m31M. Referring to the legend, this means: La Garoupe is a flashing light (short light period, longer dark period). It produces 2 flashes every 10 seconds (0.1s on – 2.4s off – 0.1s on – 7.4s off). Its altitude is 104 meters, and its range is 31 nautical miles (1 NM = 1,852 meters).
Charts specify whether lights are flashing or occulting. In an occulting light, the illuminated period is longer than the dark period.
Seamarks are distinctive points in the landscape (buildings, capes, lighthouses, buoys, mountains) used for position fixing at sea.
Position Fixing
The geographical position of a vessel is defined by its longitude and latitude.
-
Longitude (G): represented by vertical meridians. The reference meridian (0° longitude) is the Greenwich meridian (England). Points east of Greenwich are noted G = X°E; points west are noted G = X°W. By convention, “W” (West) is used to avoid confusion with the digit 0.
-
Latitude (L): represented by horizontal parallels. The reference parallel (0° latitude) is the Equator. Points north are noted L = X°N; points south are noted L = X°S.
Example: a point at the intersection of meridian 7°6’E and parallel 43°34’N has the coordinates L = 43°34’N and G = 7°6’E.
To determine the vessel’s position, plot a course, and calculate distances on the chart, a navigation ruler is used.
The Cras ruler is one such tool. It has:
-
A scale in centimeters,
-
A scale in nautical miles (at 1:100,000 and 1:50,000),
-
A black arrow “000 to 180,”
-
A red arrow “180 to 360,”
-
The center of the protractor,
-
A graduated protractor in degrees.
To determine the boat’s position, one uses a bearing compass and the Cras ruler.
-
The bearing compass gives the azimuth (degrees from magnetic north) of a fixed point.
-
The Cras ruler is used to plot lines from these bearings.
The vessel’s position is obtained by triangulation, ideally using three seamarks separated by at least 45°. The boat is located at the intersection of the three lines, or within the triangle they form. This is called fixing a position.
⚠️ The course plotted on the chart is the ground track. When currents, compass deviation, magnetic variation, or wind are considered, the corrected course is called the surface track.
Measuring Distances
There are two main methods:
- Use a ruler (double-decimeter) to measure the chart distance in centimeters, then convert using the chart scale.
- Preferably, use a divider (compass) in navigation.
A nautical mile is defined as the length of an arc subtended by an angle of one minute of latitude (1/60 of a degree).
To measure: place each leg of the divider on the two points, then transfer the spacing to the latitude scale (right or left edge of the chart). The real distance is read directly.
Knowing both the vessel’s average speed and the distance to cover, the duration of navigation can be estimated:
- Time = Distance ÷ Speed.
Be careful with units: - If speed is in km/h, distance must be in kilometers.
- If speed is in knots (nautical miles per hour), distance must be in nautical miles (1 NM = 1.852 km).
Navigation courses can be found in manuals used to prepare for boating licenses.