Prudent boaters rely on more than one device or technique for navigation. Safe navigation depends on maintaining a sense of your environment. In coastal waters with good visibility, this means comparing your visual observations with your charts or GPS.
Before leaving home, store charted landmarks as waypoints in your GPS for quick recognition on the water. Annotate their names on your charts.
Relative bearings using the boat
You can take relative bearings using your boat, but you must convert them to magnetic or true before comparing them to your GPS or a chart.
A quick visual bearing taken over your bow corresponds to your heading. Like bow bearings, beam bearings (taken 90 degrees off the bow) are easy to sight and reasonably accurate
To convert a relative bearing, add your boat’s heading (usually magnetic) to the relative bearing as measured clockwise from the bow. For example, if your boat’s heading is 070 magnetic, and your sighted object is directly off the starboard beam (90 degrees relative), your plottable magnetic bearing would be 160 magnetic. If the resulting number is greater than 360 degrees, simply subtract 360 degrees to get a plottable magnetic reading.
You can also take a quick visual sight over a protractor or plotting tool aligned with the boat’s centerline to estimate a relative bearing. Many boaters note the angles sighted across the various features of their boat, such as a stanchion or window edge. Others place marks on rails at intermediate relative angles for quick visual reference.
Absolute bearings using the ship’s compass
More precise and easier to compare with GPS readings, compass bearings are sighted forward of the helm using your steering compass as a guide.
Lay a plastic (not metal) plotting tool across the top of the compass and use it to sight a charted object for an absolute magnetic bearing you can plot without conversion.
Quick GPS comparisons
Comparing visual bearings with their GPS bearings isn’t precise, but it’s quick and easy and gives you confidence in your GPS.
On your GPS map screen, zoom out to display the nearby landmarks you programmed as waypoints before leaving home. Compare a quick relative visual bearing with the landmark on the GPS screen. If the two agree, your GPS is probably fine. If not, take more precise bearings.
If any of your quick visual observations don’t match, compare your bearings with a chart, which is always a good idea when navigating by GPS. For more information on this and other navigation topics, take the USPS Piloting course at a squadron near you.