Position lines
Every fix is built from position lines: lines on the chart you know you're somewhere on. One position line isn't a fix. Two that cross give you a position; three that cross give you a position and a check on it.
- A compass bearing of a charted object, such as a headland, lighthouse or church spire.
- A transit: two charted objects in line. It needs no compass, so it has no compass error, and it's the most accurate position line you can get.
- A depth contour: when the echo sounder, corrected for the height of tide, matches a charted contour.
- A radar range: your distance off a charted feature that shows clearly on radar.
The three-point fix, step by step
- Choose three charted objects you can identify for certain, ideally about 60 degrees apart so the lines cross at good angles. Avoid objects nearly in line with each other or nearly opposite.
- Take the bearings quickly, one after the other, and note the time and log reading. Take the bearing that's changing fastest, the one nearest the beam, last.
- Convert to true: apply deviation and variation to each compass bearing. See variation and deviation.
- Plot the lines on the chart from each object, and mark the fix with a circle and the time.
Reading the cocked hat
Three bearings rarely meet at a single point. They form a small triangle called a cocked hat.
- A small cocked hat means a good fix. Take your position as being in the triangle.
- A large cocked hat means something is wrong: a misidentified object, a mistake in deviation or variation, a misread compass, or a plotting error. Check each step, or retake the bearings.
- Near danger, assume you're at the corner of the triangle closest to it, and navigate from there.
When you can only see one object: the running fix
Take a bearing of the object, note the time and log reading, then carry on until the bearing has changed by a good amount and take a second bearing. Work out where your boat has gone between the two (course, distance and tidal stream), move the first position line along that track, and where it crosses the second bearing is your position. It's only as good as your estimate of the ground you've covered between the bearings.
Cross-checking GPS
On passage, plot a GPS position on the chart regularly and check it against something independent: a bearing, a transit, the depth, or your dead reckoning. A position from a single source isn't a fix you can trust in close water. For working out where you'll be rather than where you are, see the course to steer guide.