# What is the minimum knowledge to navigate with only a compass?

Assuming I have memorized the major Latitude and Longitudes as they relate to the world, as shown in the image below and that I know how to identify latitude & longitude with a compass. What else do I need to know?

I am not talking about everything I need to know to find my way to a specific small island in the middle of an ocean, but more for general preparedness.

Image Source Wikicommons

• See my answer to your linked question. I do not think this is possible, though kudos for wanting to try. Commented Feb 25, 2017 at 13:47
• I've answered assuming you are attempting global (ship or aircraft) navigation via compass. The topic of local (hiking/paddling/skiing/etc) navigation may have been covered by other questions. Commented Feb 25, 2017 at 21:12

As you can see from an answer to your other question, you cannot determine your global coordinates via a magnetic compass. Navigation, however, where you wish to use a compass to help you travel from one known global location to another, has been achieved as follows:

1. Using a Gnomonic map, plot a straight line connecting your "from" and "to" points. The Gnomic projection has the unique property that all great circle routes (i.e., shortest courses on a globe) project as straight lines, no matter what direction they have.

2. Carefully transfer the route from the Gnomonic map onto a Mercator map. Generally stated, your shortest route will plot as a curve. The Mercator projection has the unique property that all loxodromes – lines of constant azimuth (or bearing) – project as straight lines, no matter their initial directions. It also has the property that angles measured between lines on the map are correct.

3. Segmentize the curved route on the Mercator map. That is, approximate the curve via a sequence of straight lines. Each segment will have constant direction throughout its length.

4. For each segment, set a course for that segment and travel for the distance indicated by that segment. Note that each True course must be "corrected" for magnetic declination to get the Magnetic (compass) course. Also note that each measured map distance will need to be corrected for scale distortion and this is a mathematical function of latitude. While your overall line of travel will not be exactly the shortest route, it will always be following a set course.

The following example is from GeoGarage.com: First, the shortest route from Seattle to London on a Gnomonic map. Notice its direction changes throughout.

Now that same great circle route plotted as a curve on a Mercator map and, in this case, the curve has been "approximated" by only a single straight segment, representing a line of constant bearing.

Such has been the method of global map and compass navigation prior to the age of electronic instrumentation.

• Since the OP didn't say they needed the shortest route, a much simpler approach would be to determine the bearing of the destination and then proceed in that direction. You can use a Mercator projection for that. It's not the shortest route but you get there (with some pathological exceptions near the poles.) Commented Feb 25, 2017 at 20:30
• Yes, @DJClayworth, but you could easily travel twice as far as needed. Commented Feb 25, 2017 at 21:29

You need know the magnetic variation for the area you are in. This gives you 2 positions to triangle your position & were the sun rises at in the area you think you are in at that time of year. You now have 3 points to go on. Look up magnetic variation on line. Now you are in the area. Next you need know how to float a stick. This will tell you current variation. Even small islands change the course of currents 200 miles out. With that you head for land & hope you are on the right island or close to the one you want. But I cheat. take a small transistor radio. Pull out the antenna. Know what radio towers are were & station they are. Note on your chart. Buy knowing were they are by the way a cheap radio fades in & out. You know what way to go to that tower. 2 towers are better for exact location to land at.

• What is "float a stick"? Commented Feb 26, 2017 at 10:37
• It is a Y shaped stick. You put a round stick into the Y or a hole at the Y . The way it moves tell you current change. Currents change as they sweep around islands. Or land. Also good to find reefs for fishing. Very primitive way to find land or sea bottom change. By S, Pacific Islanders. The fisher folk are the best to learn from in how to do this. They have been doing it for over 1,000 years. Many outlanders can not manage to ever do this. Here floating a stick. Knowing what to look for in current change will get you to land. Mostly used on native outriggers small fishing boats. Commented Feb 26, 2017 at 11:08