NAVIGATION · Lesson 1 of 4
Ground Track vs Water Track
25 min · Beginner friendly
You’ll learn toDefine heading, water track, ground track and COG/SOG · Explain how tidal stream displaces a vessel from its heading · Recognise when a course to steer calculation is needed
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About 9 min
The single most important idea in coastal navigation: the direction your boat points is not the direction it travels. The sea itself is moving — around most coasts the tide drags the whole body of water along at up to several knots, and your boat rides on top of it. Point the bow east all day; if the water beneath you is sliding south, you end up somewhere south-east.
Take a guess
Decide before you read on. Mid-crossing, your steering compass reads a steady 090° while the GPS shows a COG of 110°. Which instrument is broken?
Commit to an answer before reading on — a wrong guess here helps the right idea stick.
Four Tracks, Four Names
- Heading
- The direction the boat is pointing at this moment — what the steering compass reads. It says nothing about where you are actually going.
- Water track
- The path the boat follows through the water. With no wind effects, equals . When the wind shoves the boat sideways (), the water track lies a few degrees downwind of the heading.
- Ground track
- The path the boat follows over the seabed — the line you would draw on the chart. It is the water track plus the effect of the .
- Course over the ground (COG)
- The direction of the , as a GPS reports it. is measured from where you were to where you are — it looks backwards, not forwards.
- Speed over the ground (SOG)
- How fast you are moving over the seabed. With a (favourable) tide, is higher than boat speed; against a foul tide, it is lower.
- Set
- The direction the tidal stream is flowing towards, given in degrees true. A stream setting 180°T carries you south.
- Drift
- The distance the tidal stream carries you in a given time, in nautical miles. A 2- stream produces 2 miles of in one hour.
Notice the chain: heading, adjusted for leeway, gives the water track; the water track, combined with the tidal stream, gives the ground track. You steer the first, but you care about the last, because rocks, sandbanks and harbour entrances are all fixed to the ground.
Knowledge check
A yacht steers a steady 090°T at 5 knots through the water. The tidal stream is setting 180°T at 2 knots. Which statement is true?
Set and Drift: Naming the Stream
Knowledge check
A tidal diamond gives the stream as 045°T at 1.8 knots. What does this mean for your boat?
Stream predictions come from tidal stream atlases and the tidal diamonds printed on charts: for each hour before and after at a reference , a in degrees true and a in knots. Multiply the rate by the time it acts and you have the drift in miles — the length of the tide arrow you will soon be drawing.
When You Need a Course to Steer
A course to steer (CTS) answers the forward-looking question: given the tide, what heading makes my ground track run along the line I want? Its mirror image is the (EP), which works forwards from what you did — same triangle, opposite question. You need a CTS when the stream is strong enough, and across your course enough, to push you meaningfully off the line:
- Cross-tide passages — any leg where the stream runs roughly across your course, such as a channel or estuary crossing.
- Long legs in open water — small angular errors grow with distance; over 10 miles, 5 degrees of unnoticed set is nearly a mile of .
- Slow boat, strong stream — whenever the stream is more than about a tenth of your boat speed, the offset is worth calculating.
- Approaches with dangers either side — when arriving even slightly downtide of the entrance would put you into shallow or foul ground.
When the stream runs directly with or against your course, no steering correction is needed — the tide only changes your SOG, and therefore your timing.
Work it out
You plan a 12-mile crossing from Westhaven to Salt Island at a boat speed of 6 knots. The tidal stream runs across your course at 2 knots throughout. How long does the crossing take through the water, and roughly how far would the tide displace you if you simply steered straight at Salt Island the whole way? Then decide: does this leg need a course to steer?
Within 10 min is fine.
Within 0.5 nm is fine.
Key points
- Heading is where the bow points; water track is your path through the water; ground track is your path over the seabed.
- The tidal stream displaces the whole boat bodily: ground track = water track + tide.
- GPS reports COG and SOG (ground track, looking backwards); the compass reports heading (looking forwards).
- Set is the direction the stream flows towards; drift is the distance it carries you (rate x time).
- A course to steer is needed whenever a cross stream is significant relative to boat speed - especially on long legs and slow boats.
- Stream directly with or against your course changes SOG and timing, not the course you steer.
Knowledge check
On which of these legs is a course to steer calculation LEAST necessary?
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