Navigation is the skill of knowing where you are, and how to get to where you want to be.

Position Fix

By taking a fix, or “fixing” your position you determine where you are. Fixing your position is the primary job of your GPS receiver, although it also does the math for a whole host of other naviagational duties. Another way to take a fix is with a compass, by measuring and plotting the bearings to two or more landmarks and observing where the lines of position intersect. This is the technique behind celestial navigation, but instead of using a magnetic compass the lines of position are determined from knowledge of the position of celestial bodies and an almanac. Now that you know where you are, you need to establish how to get where you want to go.

If your fix gives you coordinates and you know those of your destinatation then you can use math to calculate the direction (bearing) and distance to your destinataion. Fortunately your GPS does all this math for you very quickly and updates everything in realtime.

Dead Reckoning

If you know where you are, and you plan to travel in a given direction at a known speed for a certain amount of time, then you can calculate your final position. This is called dead reckoning, or DR for short. A DR position may be figured mathematically (which is what your GPS does) or graphically on a chart or map. While the calculations may be accurate, the DR position may be in error because of effects not accounted for when making the calculation, such as current drift while on the ocean. Errors following the course will lead to differences between the DR position and the actual position.

Rhumb Line & Great Circle

If you know where you are and where you want to be then you can calculate or plot the course and distance to your destination. The basic form of this navigation is the rhumb line, which is represented by a straight line on a mercator projection chart. However, on the earth, the shortest route may not be a straight line. For widely separated points the shortest path lies on what’s called the great circle. The great circle is the intersection of a plane and the sphere such that both points are on the plane and the plane passes through the center of the sphere. A great circle cannot be followed directly because as you travel the bearing to your destination is constantly changing. In practice, the great circle is approximated by a series of rhumb lines.

Routing

It’s not always possible to follow a direct course from your starting point to destination. If you’re on foot there may be lakes, impassable cliffs, or private property that you must remain clear of. While driving, you’re restricted to roads (for the most part). On the water you may have to avoid dangerous shoalwater, submerged rocks, or the shore in coastal and inland marine navigation. Ocean navigators must plan their voyages to avoid storms and adverse sea conditions. In all cases, the navigator’s job is to select a series of waypoints that, when followed, get you to your destination while avoiding obstacles and hazards. Virtually all GPS receivers have the ability to set routes and manage waypoints. Navigating without a GPSr involves the same tecnhniques; choosing waypoints, following the course from point to point, and identifying the waypoints when you’ve reached them. Doing this without a GPS actually requires some knowledge and skill.