How to navigate a boat

Learning how to navigate a boat means being continuously aware of your location. This is particularly important near shore where many obstacles exist above and below the water. Knowing your location on a chart allows you to assess how close these obstacles are to your current position and intended course. You also use landmarks and navigation aids to locate your position on the water and avoid obstacles.

Your GPS unit (Learn how to connect your GPS to your VHF radio) tells you where you are, but to navigate where you want to go, you need to use a chart (paper or electronic) to plot your current position and the intended path to your destination.

While reliable, your GPS isn’t infallible, which is why it’s important to compare what you see around you with what’s on your chart. Good navigators always cross-check their instruments when learning how to navigate a boat.

How to use your GPS

Your GPS provides positions as a string of numbers, called coordinates, which shows your current location when plotted on the chart. Once you know where you are on the chart, you can determine what’s around you, both above and below the surface.

If you want to go to a specific location, you need to find a safe path to reach that destination. Because of potential obstacles, your path likely won’t be a straight line but rather a sequence of straight lines called legs. Legs are plotted to avoid land, shallow water, and other underwater obstacles. The end point of each leg is marked by a waypoint.

Your GPS can help you navigate along these legs. Starting at one end of a leg, you enter the waypoint coordinates for the other end of the leg into your GPS and then activate the waypoint. The GPS provides the appropriate direction to steer and shows the distance to the waypoint. Your GPS also shows your progress along the leg and indicates if you have deviated from the leg’s straight-line path while underway. This process is called waypoint navigation.

Chart basics

Unlike roadmaps, charts lack clearly defined highways. You must plot your path using information found on your charts. To do that, you need to understand some fundamental information about what charts show.

Scale

Charts provide an accurately scaled depiction of the land and water area covered. The chart scale represents a ratio. For example, a 1:40,000 scale indicates that one unit of measure on the paper chart is equal to 40,000 of the same units in the real world. Thus, one inch on the chart covers the same distance as 40,000 inches (0.6 nautical mile) on Earth. A chart with a small value of n, such as 1:20,000 is called a large-scale chart because it shows greater detail. On a 1:20,000 chart, one inch represents about a quarter of a mile. This means every feature on a 1:20,000 chart is twice as large as on a 1:40,000 scale chart. However, the area covered by the same size chart is only one-quarter as much. Small-scale charts, such as 1:1,200,000, show a much larger area with much less detail.

Distance

Charts also provide distance scales. Offshore and coastal charts usually show distance in nautical miles. Nautical miles are recognized internationally and relate directly to the coordinate system. Most charts show more than one distance scale and often include statute miles and kilometers as well.

Depth

While nautical miles represent an international standard for distance on nautical charts, vertical measurements of water depth and object heights differ. In the U.S., we measure depths and heights in feet or fathoms (6 feet); you will find most depth contour lines plotted at 6-foot increments, harking back to when the fathom was a more popular measure. In countries using the metric system, depths and heights are plotted in meters.

Soundings and depth contours

The features charts present on and under the surface of the water make them different from maps. A marine chart indicates depth with spot soundings showing a number for the depth of the water at a specific location using a particular stage of tide. It also shows corresponding depth contours, which are lines joining areas of equal depth. These contours are the underwater equivalent of topographic contours found on many land maps.

Obstacles

Marine charts provide information about the bottom and obstacles that may impede your boating. Generally marine charts show little information about the land other than the shoreline and objects that may be seen by a boater on the water.

Grid lines

Lastly, charts have grid lines. The grid is laid out with vertical lines intersecting the Earth’s poles, called meridians, which run true north and south and horizontal lines running east and west called parallels, since they are parallel to the equator. You use these grid lines and their intermediate scales to compare your GPS coordinates with the chart.