Comparing Multiple Sailing GPS Tracks: How Course Analysis Works

There is a moment in almost every post-race debrief when a sailor or coach pulls up a GPS track and says "I went right on that upwind and it paid off" — or lost, as the case may be — and the conversation turns to whether the course had something to do with it, or whether it was boat speed, or the timing at the start. That conversation is useful but limited when you are looking at a single track in isolation. You are comparing what happened against what you remember, and memory is unreliable even at the best of times. The really valuable question is not "what did I do?" but "what did everyone else do at the same moment on the same leg, and how did the numbers compare?" That question requires looking at multiple tracks together, on the same map, cut up by the same course elements, and measured against the same distances and times. That is exactly what the Course Analysis tool in SailingMetrics is designed to do.

Why Reviewing a Single Track Is Never Enough

Single-track analysis has come a long way. You can load a GPS activity, watch a replay, read off your average speed on each tack, and see where you gained and lost through the fleet. All of that is genuinely useful for identifying broad patterns in your sailing. But it misses the comparative dimension that makes coaching effective and that helps sailors internalise what actually matters.

When a coach reviews a session with a group of six sailors, the interesting things are almost never absolute. Whether someone averaged 5.4 knots on a particular upwind leg matters much less than whether the sailors who went left averaged 5.4 knots while the sailors who went right averaged 5.1 knots on the same leg in the same breeze. The direction call, the sailing mode, the boat settings — all of these are evaluated relative to each other, not against some notional ideal. To do that analysis properly you need to cut every track with the same knife: the same gate crossings, the same leg definitions, the same time zero.

There is also the question of what "the same leg" actually means. If you compare average speeds over an arbitrary time window, a sailor who rounded the top mark two minutes before another sailor will have sailed in different pressure and different shifts. Leg-based comparison, where the segment starts when each individual sailor crosses the start gate and ends when they cross the next gate, accounts for this naturally. Each sailor's leg is measured on their own time, so the statistics reflect actual performance over the water they sailed rather than an artificial window imposed from outside.

None of this is new as a concept in sailing coaching. Coaches have been cutting video footage and comparing split times for decades. What has changed is that GPS data makes it possible to do this analysis with objective numbers rather than estimates, and modern software can automate the segmentation so that what would once have taken hours of manual work can be done in minutes. The prerequisite is simply that each sailor records their session with a GPS device — something the SailingMetrics app handles automatically in the background while they sail, logging position, speed, and heading throughout the session without any manual interaction required.

Loading Multiple Tracks for Comparison

The Course Analysis page opens with a clean workspace. On the left is a panel for managing the tracks you want to compare and for defining the course elements. On the right is an interactive map. To get started you click the "Add track for analysis" button, which opens a dialog showing your uploaded GPS activities — paginated, with date, sailor name, and boat name — and you select the first one. You can add as many tracks as you like. Each appears in the list on the left, labelled with the sailor's name, the date of the session, and the boat, and is assigned a distinct colour so you can identify it on the map at a glance.

The tracks can come from any GPS activity in the system, which means you are not limited to sessions that were recorded as part of the same race. If a group of sailors all went out on the same day and recorded their own sessions independently, you can pull those sessions together into a single comparison. If you want to compare this Tuesday's training against last Tuesday's to see whether the fleet is improving on a particular leg, that works too. The tool does not require that the activities share a race or event — it only needs the GPS coordinates and enough overlap in the course sailed to make the gate crossings meaningful.

You can also open the tool pre-loaded from an existing activity. If you are viewing a GPS activity in your account and want to compare it against other sessions, a link takes you directly to the analysis page with that track already loaded and the course marks already populated from the associated race or training session. This saves the extra step of hunting for the activity in the selector when you already have it in front of you.

Watching the Entire Fleet Sail at Once

Once you have loaded two or more tracks, the map shows all of them simultaneously. Each track is drawn in its assigned colour, so you can see immediately where sailors diverged, how wide the fleet spread was, and how the boats converged or separated through different parts of the course. Below the map, a metrics panel shows live figures for every loaded activity: the current time in the session, the speed over ground in knots, and the compass heading for each sailor at that exact point in the replay.

The replay is controlled with a slider and a play button. A speed selector lets you watch at one, two, four, or eight times real speed. At 8x you can get through a forty-minute race in five minutes, which is usually enough to form a clear picture of where the fleet split and where time was made or lost. At 1x or 2x you can pause and examine a particular phase in more detail — the final seconds of a starting sequence, a key mark rounding, or a windshift that separated the fleet.

Each boat is shown as a moving icon on the map as playback runs, and the icon rotates to reflect the boat's actual heading at that moment. This means you can see not just where each boat was at any given instant but also which way it was pointed, which is useful for identifying moments when two sailors were in the same patch of water but sailing at meaningfully different angles. The metrics panel updates in real time as the slider moves, so if you pause at any point you can read off exactly what each sailor's GPS was recording.

The combination of the spatial replay and the live metrics is more informative than either alone. The map gives you the picture of what happened in space, the metrics give you the numbers, and because they are fully synchronised you never lose track of which moment you are looking at. A coach watching alongside a sailor can point to a position on the map and have both the visual and the data available for discussion simultaneously.

Defining the Course: Gates and Marks

Watching a replay is illuminating, but the real analytical power comes from defining the course and letting the system cut each track into legs. The drawing toolbar underneath the map gives you three tools. A gate tool draws start lines, finish lines, and rounding gates. An upwind mark tool places buoys that the boats round at the top of the course. A downwind mark tool places marks at the bottom. Together these three elements can describe any standard racing course — windward-leeward, triangle, trapezoid, or combination.

To draw a gate you select the gate tool and click on the map to define the line, then double-click to finish. The line needs to cross the GPS tracks where the boats actually sailed through it; if the gate line does not intersect the track the segmentation will not register a crossing for that boat. The first gate you draw defaults to a start gate type, displayed in green. Subsequent lines default to generic gates, displayed in blue, but you can change any gate to start, finish, or generic using the selector in the course elements list. Marks are displayed as pins, deep orange for upwind and purple for downwind.

The course elements list on the left shows every drawn element in order. You can drag and drop items to reorder them, and the sequence you end up with is the sequence the system uses when segmenting the tracks. The leg from element one to element two is always the first segment, the leg from element two to element three is the second, and so on. A standard windward-leeward race with a start line, top mark, and finish line produces two legs: start to top, and top to finish. Adding a bottom gate between the top mark and finish gives you three legs. You can represent any course architecture by placing and ordering the elements to match what was actually sailed.

Marks that sit between two gate lines in the sequence contribute to the leg label in the statistics table. Rather than seeing "Leg 1, Leg 2, Leg 3" you see "Start gate to Upwind mark", "Upwind mark to Downwind mark", and so on, which makes the results much easier to read when you are flicking back and forth between the statistics and the map.

When the Course Is Already Known

Drawing the course manually works well when you are setting up a comparison session from scratch, but if the activities you are loading were recorded as part of a structured race in SailingMetrics, the course is often already defined. When the system recognises the course marks associated with an activity, it populates the course elements list automatically as soon as you add the first track. You will see the start gate, finish gate, and any intermediate marks appear without having to draw anything at all.

This auto-population is particularly useful when reviewing races where the course was set in advance and stored in the competition management system. Race officers who define their courses digitally through SailingMetrics can share those course definitions with the whole fleet, and anyone analysing a track from that race will have the correct course elements loaded immediately. The marks reflect where the actual buoys were, which is more accurate than placing them from memory on a satellite image after the fact.

You can always adjust auto-populated elements. If the actual start line was slightly different from the recorded position, or if a mark was moved during racing, you can delete the incorrect element, redraw it in the right place, and reorder as needed. The auto-population is a starting point and a time saver, not a constraint.

How the System Cuts Each Track into Legs

Once you have at least one activity loaded and at least two gate lines defined, the system automatically sends the track data and course element definitions to the backend and calculates leg statistics. This happens reactively: as soon as you add a gate, remove one, or change the order of the course elements, the statistics update without you needing to trigger a calculation manually. The loading indicator appears briefly and the new results replace the old ones.

The segmentation works by finding the moment each GPS track crosses each gate line. The system looks for the intersection between the sequence of GPS positions and the gate line, computes the crossing time, and uses that as the precise boundary between legs. If the GPS positions are dense enough — which they typically are at modern recording rates — the crossing time is accurate to within a second or two.

If a boat fails to cross a gate within the recorded track — because the track ended before the boat finished the course, because the sailor retired, or because the gate line does not intersect the actual path sailed — that particular leg will simply not appear in the statistics for that sailor. Other sailors' legs are unaffected. This means partial results for one sailor do not corrupt the analysis for the rest of the group.

Reading the Leg Statistics Table

The statistics appear below the map in a separate table for each leg. Each leg shows one row per sailor, with columns for sailor name, boat, the start and end time of that leg, the duration, the distance sailed, the average speed over ground in knots, and the VMC in knots.

Duration is probably the most immediately useful column for comparing performance. Two sailors on the same leg who both rounded the marks cleanly will have sailed similar distances, but the one who finished in less time had a genuine speed advantage over that stretch of water. Reading down the duration column for a single leg gives you an instant ranking that accounts for different individual start times — unlike raw elapsed time from a gun, which is confounded by how quickly each sailor crossed the start line.

A leg distance figure sits above each table as a subtitle. This is the straight-line distance between the two gate points, giving you a reference for that leg. If a sailor's distance sailed is significantly more than the leg distance on an upwind, they covered extra ground through wider angles or more tacks. If it is close to the leg distance they sailed an efficient angle or benefited from a very direct shift. Distance sailed alone does not tell you which it was, but combined with the duration it points you in the right direction before you go back to the replay to look more closely.

Average speed over ground gives you the raw speed picture. On a reaching leg where everyone is sailing a similar angle, a consistent speed advantage shows up clearly here. On an upwind leg, speed over ground is less meaningful in isolation — a faster number could reflect a faster mode, a freer angle, or better pressure — but it is still a useful first filter when you are trying to decide which legs and which sailors deserve closer attention.

VMC: The Metric That Shows Real Leg Performance

The VMC column is the one that most directly answers the question coaches and sailors care about most: how effectively did this sailor convert their sailing into progress toward the next mark? VMC stands for Velocity Made on Course, and it is calculated as the component of the boat's velocity in the direction of the next gate. A boat sailing directly at the mark at 6 knots has a VMC of 6 knots. A boat sailing at 6 knots but at 60 degrees off the direct course to the mark has a much lower VMC even though the raw speed is the same.

This makes VMC a fairer comparison metric than average speed on legs where boats are sailing at different angles to the course. Two sailors who both averaged 5.5 knots on an upwind leg but where one had a VMC of 4.8 knots and the other had 4.2 knots made genuinely different amounts of progress toward the mark per minute sailed. The one with the higher VMC arrived at the mark faster even if their raw speed was identical, because they were sailing a better angle for the conditions at the time.

On a downwind leg, VMC tells you how well a sailor balanced depth against speed. Sailing very deep and slow produces a lower VMC than gybing at a faster but freer angle, even if the deep boat eventually covers the direct distance. VMC captures both the angle and the speed in a single number, which is why it tends to be the headline metric for leg analysis rather than speed alone. It is the closest single number to an answer for the question "how well did this sailor sail this leg?".

How Coaches Are Using This in Practice

Coaches who have integrated the Course Analysis tool into their training programmes have found a few workflows that keep the debrief efficient and focused. The most common is to load the tracks immediately after coming off the water, let the course marks auto-populate from the session definition, and allow the statistics to generate while sailors are rinsing gear and changing. By the time everyone is gathered for the debrief, the leg tables are already on screen and the data is ready.

Opening the debrief with the 8x replay works well as a shared orientation. Running through the full session in a few minutes gives everyone a common spatial memory of what happened, which makes the discussion of the statistics more grounded. Sailors often notice things in the replay that they did not register while racing because they were focused on their own boat and not watching the fleet as a whole.

The leg statistics table is most useful for directing attention rather than providing answers. If the VMC column shows one sailor consistently higher than the others on the first upwind leg, that is worth spending time on regardless of what the coach thought they observed from the chase boat. The data removes the subjectivity from the initial question of where to focus. The discussion about why something happened still requires human judgement, but the data tells you where to point that judgement.

For sailors doing self-coaching without access to a coach, the same principle applies in reverse. The data tells you which legs are underperforming relative to your own previous sessions or relative to other sailors in the same group, and the replay gives you the visual context to form a hypothesis about why. The combination of the map and the statistics makes it possible to do meaningful post-session analysis without needing someone else to watch you sail and offer their impressions.

Getting Started with Course Analysis

The Course Analysis tool is available to all SailingMetrics users who have recorded GPS activities in their account. You can find it under the Sails section in the main navigation. If you already have an activity you want to analyse alongside others, opening it from your account and following the link to the analysis view will take you there with that track pre-loaded and the course marks already placed. If you do not have an account yet, signing up is free and takes only a minute.

If you are starting from scratch, navigate directly to the Course Analysis page, add the activities you want to compare using the track selector, and draw the course elements on the map. Once you have placed two gate lines and loaded at least one activity, the leg statistics will appear below the map automatically. The main workflow from there is adding and removing tracks, adjusting the course elements to match the actual gates sailed, reordering the elements if needed, and reading the statistics to identify which legs and which sailors deserve closer attention.

The amount of analytical value you get from the tool scales with the number of sessions you have recorded and the consistency with which the course is defined. A group of sailors recording every training session through SailingMetrics will build up a body of leg-by-leg data that makes it possible to track trends over weeks and months rather than just session by session. Whether you are a coach working with a junior squad, a club sailor trying to understand where you are losing time to the sailors around you, or a competitive sailor preparing for a major event, the ability to cut every GPS track with the same course geometry and read the numbers side by side is a meaningful step forward in how you can use data to inform your sailing.

For a detailed walkthrough of the interface, including step-by-step instructions for drawing course elements, adjusting gate types, and interpreting each metric in the statistics table, see the Course Analysis documentation.