Why transits still matter

There is a particular satisfaction in fixing your position without touching a screen. When the plotter flickers, the handheld GPS batteries die, or you simply want a second opinion, a transit is one of the oldest and most dependable tools in coastal sailing. A transit is two charted objects lined up ahead or astern. When they appear as one, you know you are somewhere on the line that runs through them. That line of position is not a guess; it is a fact you can see with your own eyes.

In UK coastal waters, where tides run hard, headlands overlap, and buoys sometimes shift after winter gales, transits are especially valuable. They work in daylight and, with leading lights, at night. They need no power, no signal and no subscription.

What makes a good transit

Not every pair of objects will do. A useful transit has three qualities.

• Charted and identifiable. Both objects must appear on your chart and be unmistakable from seaward. A church spire, a white daymark, a harbour wall, an isolated chimney or a conspicuous headland all work well.
• Well separated. The further apart the objects are, the more precise the line. If they sit close together, a small change in your position makes them swing apart quickly, which is sensitive but hard to read in a swell.
• Stable and sharp. Avoid anything that moves, such as moored boats, or anything vague, such as a low stretch of dark shore. Vertical edges and clear shapes are best.

Leading lights are transits at night: keep the upper light directly above the lower. The chart will tell you the sector and colours, so brief your crew before dark.

Two transits give a fix

One transit gives a line, not a position. You know you are somewhere along that line. To get a fix, use two transits that cross at a good angle. The closer to 90 degrees, the better. Anything less than about 30 degrees and the crossing becomes a long, thin triangle that hides error. Plot each line through the two charted objects on your paper chart or plotter. Where they cross is your position.

If you only have one transit, combine it with something else: a compass bearing on a third object, a depth sounding that matches the chart, or a visible distance off a headland. The transit sharpens the fix because it is independent of compass error and heading.

To plot a transit, you do not need to take a bearing. Simply lay your straight edge along the two objects and draw the line. If you are using a hand-bearing compass, you can check the transit bearing, but the alignment itself is the truth.

Transits ahead and astern

An ahead transit is the classic pilotage tool. Keep the near object lined up with the far one and you will follow the transit over the ground, even if tide is pushing you sideways. Your heading may be a few degrees off, but your track is true. If the near object appears to the left of the far one, you are to the right of the line; if it appears to the right, you are to the left. That simple rule lets you steer back on.

An astern transit works the same way in reverse. Looking back, if two objects remain aligned, you are on the line that runs through them. If they open out, you have been set to one side. An astern transit is excellent for checking whether you are being carried by a cross-tide on a long leg. Pair it with an ahead transit and you have a continuous check on both cross-track error and steering.

Pilotage, clearing lines and tide

Transits earn their keep in pilotage. A clearing line is a transit used as a boundary: keep one object open of another and you stay in safe water. For example, “keep the white cottage open east of the red buoy” means that as long as the cottage appears to the east of the buoy, you are clear of the shoal. If the cottage closes behind the buoy, you are inside the danger line. These lines are often printed on charts or in pilot books, but you can create your own for a familiar harbour.

Transits also reveal tide. Steering to keep a transit aligned tells you your course over ground, not just your heading. If the transit is ahead and you need to crab to hold it, that is the set and drift at work. Over a known distance, you can estimate the rate. In strong UK tides, such as around headlands or in estuary mouths, this is more reliable than guessing from a tidal atlas alone.

Write your transits into the passage plan. Note the objects, the bearing if useful, the day and night appearance, and what to do if the transit opens. A short list on a waterproof card is worth more than a screen full of waypoints when the wind is up.

Limitations and common mistakes

Transits are not magic. They need visibility, and they fail in fog or heavy rain. They need objects that are charted and still there; a new building or a fallen tree can ruin a transit that worked last season. Check your chart for the exact symbols and any note about the transit.

• Do not use one transit as a fix. It is a line of position. Add another line or a depth.
• Do not trust a transit you have not identified. Confirm both objects on the chart before you rely on them.
• Do not ignore height of eye or a swell. In a rough sea, your view of a low mark may disappear in the troughs. Choose higher objects when possible.
• Do not forget the night case. Leading lights may be obscured by another vessel or shore lights. Have a backup bearing ready.

Use transits with your other tools, not instead of them. A quick check against GPS, a depth sounder and a compass bearing will catch a blunder. But when the electronics go quiet, a well-chosen transit is a quiet, honest answer. It connects you to the coast, the chart and the long tradition of sailing these islands. Keep binoculars handy, look ahead and astern, and let the land tell you where you are.