1. What Does a Depth Number Like "3" on a Nautical Chart Mean?
Imagine entering a harbor in a small boat. The chart shows the number "3" at the entrance. Whether the bottom of your boat touches the seabed depends on what this number represents. According to the Japan Hydrographic Association, for the sake of navigation safety, charted depths are measured from a level where the sea surface almost never falls below [2]. In Japan, this reference is called the "lowest water level."
If the chart says depth 3, it means there are 3 meters from this datum to the seabed. For most of the time, the actual sea surface is higher than this level. Therefore, the real depth is often the same as or deeper than the number written on the chart. The chart intentionally uses numbers that lean toward the side where ships are less likely to run aground. The Japan Coast Guard creates and publishes these charts [1].
Because this reference level is consistent, you can compare shallow and deep spots between locations even without knowing the tide height at a specific time. But how do you picture the shape of the seabed from just a list of numbers?
Safety Note: The Japan Hydrographic Association explains that charted depths are values from a level where the sea surface rarely falls, called the lowest water level in Japan [2]. Nautical charts are maps that shift numbers to one side for safety.
2. How Do Contour Lines on a Chart Show Equal Depth and Slope?
Trying to find deep spots by just looking at scattered numbers is difficult. So, nautical charts draw lines connecting points of the same depth. These are called contour lines, and the concept is very similar to contour lines on land maps [5].
In Japanese charts, lines are usually drawn at 2 meters, 5 meters, 10 meters, 20 meters, and 200 meters [2]. Shallow areas are divided finely, while deep areas have larger intervals. It is thought that this is because differences in depth are more critical for ships in shallow places.
By looking at the spacing between the lines, you can imagine the seabed. If the lines are close together, the slope is steep and the depth increases quickly. If they are far apart, the seabed is gentle. Using the same reading method as for mountains on land, the underwater terrain comes to mind.
3. What Else Do Nautical Charts Show Besides Depth?
Reading the explanation from the Japan Hydrographic Association reveals that charts contain much more than just depth [2]. First, there are aids to navigation like lighthouses and buoys. These show not only position but also properties like light color and flashing cycles, which help confirm your location on the sea at night. Lighthouses, in particular, are considered the most important markers for finding your way when approaching from far out at sea.
Next, there is bottom material. Whether the seabed is sand, rock, or mud is written in specific abbreviations, providing clues for choosing where to drop an anchor. Charts also list sunken ships, artificial reefs, submarine cables and pipes, and other underwater obstructions.
On the edges of the chart are scales for latitude and longitude. Degrees and minutes are written for each section, serving as a ruler to read the ship's position. The meanings of symbols and abbreviations can be checked in official symbol collections. In the United States, there is a symbol collection called "U.S. Chart No. 1" [7].
Safety Note: Matters concerning navigation safety are overlaid on the chart using symbols and abbreviations. Underwater hazards listed include sand waves, sunken ships, artificial reefs, submarine cables, submarine pipelines, and underwater obstructions.
4. Why Check a Chart's Title for Units, Scale, and Survey Year?
Before reading the numbers on a chart, look at the chart's title block first. According to the Japan Hydrographic Association, the title includes the scale, units for depth and height, the year of the survey, and notes on tides [2]. The survey year tells you how recent the survey data for this chart is.
In Japanese charts, both water depth and the height of mountains or towers on land are written in meters [3]. On the other hand, NOAA explains that water depths on American charts are shown in feet or fathoms [5]. One fathom is 6 feet. Calculating with 1 foot as 0.3048 meters, 1 fathom is about 1.83 meters. So, if "5" is in fathoms, it is about 9 meters. If "5" is in meters, it is 5 meters. Misreading this can cause the depth to differ by nearly a factor of two.
In charts for foreign areas, the reference for latitude and longitude (the geodetic datum) may differ from Japanese charts [3]. The scale also changes depending on the purpose. According to the Japan Coast Guard, coastal charts have scales of 1:50,000 to 1:300,000 and show detailed coastal depths and targets, while harbor charts have scales larger than 1:50,000 and show the inside of ports in even more detail [1].
5. How Are Nautical Charts Kept Up to Date with Weekly Notices?
Because the sea changes after a chart is made, charts are continuously updated. The Japan Coast Guard informs users of the latest information through weekly Notices to Mariners [1]. These notices are issued every Friday and consist of minor corrections (including correction sheets) to update charts, and temporary information like navigation warnings [4].
Interestingly, users perform part of the updating manually. They correct their own charts by hand, and if the coastline or depths have changed significantly, they paste correction sheets found at the end of the Notices to Mariners to keep the chart current [4]. To keep a chart up to date over many years, you need to update it yourself by checking the notices. The Japan Coast Guard also periodically revises entire charts in new editions [1].
Japan's first chart, "Chart of Kamaishi Port in Riku Province," was issued from the Naval Ministry's Hydrographic Bureau in 1872 (Meiji 5) [8]. Today, there are also Electronic Navigational Charts (ENCs), which are digital versions of paper chart information. According to NOAA, navigation systems using ENCs can issue warnings and alarms when a ship is heading toward shallow waters or dangers [6].
Safety Note: Currently, Electronic Navigational Charts (ENCs) exist as numerical data of paper chart information. According to NOAA explanations, navigation systems using ENCs can issue warnings and alarms when a ship is heading toward shallow waters or navigation hazards [6].
6. How Can an Atlas's Ocean Colors Help You Understand Contour Lines?
If you have an atlas at home, look at the ocean parts. In many atlases, shallow seas are colored light blue and deep seas are dark blue. The legend should list the depth ranges for each color.
Find the darkest blue area around Japan and trace how the colors change from the coast out to sea. At points where depth is divided into ranges, the color boundaries are similar to the concept of contour lines. Where the boundary is close to the shore, the seabed may drop off steeply.
If you want to learn more, you can read about chart contents and chart specifications on the pages of the Japan Coast Guard's Hydrographic Department and the Japan Hydrographic Association [2][3]. If you can read English, you can see a list of symbols in NOAA's "U.S. Chart No. 1" [7].