Measuring Non-Straight-Line Swim Routes

In the previous article, I discussed the difference between a swim route and a swim track, and how the measured distance of a marathon swim is always the length of the route, not the length of the GPS track. To demonstrate this principle I contrasted the standard English Channel route (a 20.5 statute mile line between Dover and Cap Gris Nez) and a typical English Channel track (a swooping S-curve as the swimmer is pushed back and forth by the tides).

Aha!, you might say: Not every swim route is a straight line! What about an island circumnavigation? Or a swim down a curving river?

Reviewing the definition of “swim route” from the previous article (emphasis added):

A swim route is a predetermined, abstract path between the start and finish, composed of either a straight line or (if the straight-line path is interrupted by another land-mass) a series of connected straight-line segments.

So, by this definition, even a non-straight-line route can be understood as a “series of connected straight-line segments.” The key in measuring a non-straight-line route is knowing how to select the intermediate waypoints - the “nodes” connecting each line segment.

The underlying principle is to create the shortest swimmable route between the start and finish, using fixed land features to define the waypoints.

Waypoint-Selection Algorithm for Non-Straight-Line Swim Routes

1. From the start or any intermediate waypoint, move in the general direction of the finish.
2. Find the location where your route line encounters land and creates the longest possible line segment from the previous waypoint. Create a waypoint at that location.
3. If the route line does not encounter any land and heads off “toward infinite sea” (this will happen at least twice, usually more, on an island circumnavigation) then create a waypoint at a location where the next point of land is “visible” via straight-line path.
4. Repeat steps 1-3 until you reach the finish.

Let’s work through some examples.

Case Study: Santa Rosa Island circumnavigation

How would one measure the distance of a circumnavigation of Santa Rosa Island (one of the eight Channel Islands of California)? This is an actual problem I was tasked with a few years ago, when I produced official measurements for the 54 swim routes governed by the Santa Barbara Channel Swimming Association.

Applying the waypoint-selection algorithm, we find a route distance of 41.1 statute miles.

For island circumnavigations, the shortest route will typically follow close to shore. Even if you opt to swim further out, you can’t ”take credit” for a longer route!

Note, though, for islands with sharply defined points and coves, the shortest circumnavigation path will “cut across” the coves rather than follow the shoreline. To understand why, think about the triangle inequality theorem. For the northeast portion of the above route (between Carrington Point and Skunk Point) the route-line is nearly 2.5 miles offshore!

Case Study: Angel Island “Round Trip”

A “Round-Trip Angel Island” (RTAI) swim in San Francisco Bay starts at the South End/Dolphin Club beach, heads straight out past Alcatraz to Angel Island, then around Angel Island, then back past Alcatraz again and returning to the start. It is a challenging cross-current swim that can only be attempted on certain tides. In 2013, I observed Cathy Delneo become the first woman to complete an RTAI.

Using the waypoint-selection algorithm, I measure the shortest RTAI route at 9.99 statute miles - let’s call it 10. And for sure, it’s an _honest _10 miles - Cathy finished in 6 hours, 10 minutes.

Note that from the Aquatic Park “Opening,” it’s an uninterrupted straight shot to either the east or west end of Angel Island. Here’s a zoom on the start/finish at Aquatic Park:

Then, going around Angel Island the route follows a typical circumnavigation course, but with the “bottom” cut out.

Case Study: 8 Bridges Hudson River Swim, Stage 4 - Newburgh-Beacon to Bear Mountain

What about river swims? I chose a particularly curvy stretch of the

And… I get 15.0 miles exactly. Nice!

It’s worth noting again: The route depicted above doesn’t necessarily reflect the actual path swimmers would take (i.e., the track). In a river, the fastest currents are often found in the middle. But we measure the official distance of a swim with the route, not the track, in order to maintain consistency and repeatability across different attempts of the same swim.