If you’re familiar with land maps, the scale works the same way there. A harbor chart is an example of a small scale chart. Navigators need a close view of the harbor with all the obstructions and other details. Larger charts show land and waterways but few additional details.

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

If you see a negative number, that means the water depth in the area is typically less than what is listed on the chart. [4] X Research source

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

For example, multiply a listing of 6 meters by 3 to estimate it as being at least 18 feet deep. The actual depth is 6 metres (20 ft), so it isn’t completely accurate, but it is helpful for avoiding shallow areas. If the depths are listed in feet, you can also divide them by 3 to quickly convert them to meters.

For example, contour lines may connect points listed as 20 ft (240 in) deep. All of the points inside those lines are even shallower than that.

The exact depth of shallow areas depends on the chart. Some charts color in areas under 3 fathoms, while others mark spots less than 1 fathom deep.

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

Keep in mind that the magnetic field over the Earth changes over time, so you will need a magnetic compass or an updated map to make use of the compass rose.

Red markers show the right side of a channel, while green markers show the left side. The different colors keep you oriented no matter which way you travel.

For example, an anchor point marked “DW” means deep water. Only deepwater vessels will anchor there. A number like 24 stands for 24 hours, so you can only keep your boat in the area for a day at a time.

Obstacles are generally represented by circles with symbols like asterisks and plus signs. There are many different symbols and they often vary between charts. Some charts use picture symbols, such as a boat to represent a partially-sunken shipwreck.

Currents and tides change all the time. Take the arrows as a prediction for the way water normally flows. Keep watch as you travel for anything out of the ordinary.

Some common seafloor abbreviations include S for sand, M for mud, and C for coral. Rky stands for a rocky floor. Tr indicates a nearby tower, while Whf stands for wharf and Bn means beacon. CG may indicate a Coast Guard station on a U. S. map.

The chart covers all U. S. charts. Download a copy at https://nauticalcharts. noaa. gov/publications/us-chart-1. html. Charts made in other countries may have different symbols. Check with your government’s maritime office for a reference.