# MIMS GPS analysis: Three rivers, Three races

In the “GPS snapshots” I showed in the narrative report, you can see how far apart the top swimmers are in terms of distance. Four hours into the race, for example, Erica Rose was 455m ahead of Ollie Wilkinson, who was in turn 135m ahead of John Van Wisse.

Another way to model the race is to look at when each swimmer passes a given landmark. This shows how far apart each swimmer is on a different dimension - time. Using the GPS tracks provided by NYC Swim, we can actually calculate “split times” for each swimmer between any landmark we choose. And, using those split times, we can calculate each swimmer’s speed (including current) for each segment.

For the purposes of this study, I chose 11 landmarks - three in the East River (Pier 11, Queensboro Bridge, and the Randall Island footbridge), two in the Harlem (Macombs Dam Bridge and Spuyten Duyvil), and six in the Hudson (GW Bridge, Riverbank Park, 79th St, 34th St, Pier 40, and the finish at South Cove). Hopefully they’re all fairly obvious reference points. Here they are on a map:

### East River

As you may recall, Erica Rose was about 85m ahead of Ollie Wilkinson and me at Pier 11. However, the GPS tracks were taken at 1-minute intervals - so I was only able to calculate splits to about half a minute of precision. That’s why Erica, Ollie, and I have the same split for the Start-to-Pier 11 segment.

In any case, everything here pretty much corresponds to what we’d expect. From the start to Pier 11 we were swimming through mostly slack water - thus slower overall speed (probably 1 mph net current). The first part of the East River was fast (up to 6 mph average speed). In the latter part of the East River (along Roosevelt Island), the current slowed by about a knot.

Erica pretty much dominated the East River.

### Harlem River

Some thoughts:

• According to these splits, the field was pretty evenly matched in the Harlem. Erica gained a minute on John (and 2 minutes on Ollie) between the Footbridge and Yankee Stadium, but then gave back 30 seconds to John (and 90 seconds to Ollie) in the upper Harlem.
• This isn’t necessarily because Erica was swimming less well. It’s possible that the trailing swimmers gained some relative advantage by swimming in the Harlem during a faster part of the tide cycle. Erica entered the Harlem River at 11:32am -  5 minutes ahead of 2nd place Ollie and 7.5 minutes ahead of 6th place Miguel S. Those may have been crucial minutes. Based on the slow speeds of all the swimmers in the lower Harlem (just under 30 minutes per mile), we were probably swimming into a head current at first.
• The current really picked up in the upper Harlem. Again, the trailing swimmers were relatively advantaged by this, as they spent more time in the faster current. This may explain Erica’s surprisingly slow split in the upper Harlem.
• I had a terrible 1st half of the Harlem; but, I already knew that.

Here’s the order of the top 6 swimmers as they passed under the Spuyten Duyvil railroad bridge and into the mighty Hudson:

1. Erica R. - 2:08pm
2. Oliver W. - 2:13:30
3. John V.W. - 2:14
4. Miguel S. - 2:15
5. Miguel A. - 2:16
6. Evan M. - 2:17:30

### Hudson River

Erica extends her lead to half a mile. But I gain on everyone else.

Erica is still pulling away, despite moving in close to shore. Usually, the faster currents in a gathering ebb tide would be found in the middle of the river. Possibly, her pilot was trying to evade the chop (which would also be greatest in the middle of the river). Or perhaps they saw something the rest of us didn’t. Either way, it didn’t seem to hurt her: She ties me for the fastest split of the segment.

John is pulling away from Ollie.

### Overall River Split Times

Readers may be curious to see split times for the full rivers (East, Harlem, & Hudson) - not just the “segments” shown above. So here they are.

You’ll notice a new swimmer in the mix here: Sarah Thomas from Colorado. Originally seeded 8th, Sarah managed to slip into 5th place overall with a roaring swim down the Hudson. Well done, Sarah!

Posted 17 July 2011 in: analysis , swim reports Tags: MIMS