coxmate We recently received a great question from a sport scientist and rowing coach in Australia questioning the accuracy of GPS in rowing.
Question:
“I’m curious about the accuracy of GPS as a valid and reliable way of measuring speed and distance in rowing. I use GPS a lot with running, triathlon and cycling athletes that I coach but I’m interested in how it can be applied to rowing in an effective way.
“In what way does CoxMate counteract the discrepancies that the current in bodies of water would create with regards to speed over ground as measured using the GPS satellites versus actual boat speed through the water?
“I would be open to considering a change to CoxMate for my crew in place of the traditional impeller but am sceptical about what I can see as potential issues with accuracy.”
Answer:
You are right, GPS will only provide land speed. However, the majority of impellers on school boats are rarely accurately calibrated and often physically broken. GPS has the advantage that it requires no installation or calibration to work. The Coxmate SX does offer impeller speed sensing, so if water speed is needed, it can be plugged in—impellers are an option for $100. The GPS can then be used to calibrate impeller. Just row 250m, turn, row 250m in opposite direction and the SX will calibrate the impeller. This provides the accuracy of a GPS system with the functionality of an impeller. When SX is turned on, you can select impeller or GPS for speed sensing. Impellers have the advantage of being able to provide details of stroke velocity profile, i.e. how boat speed changes through stroke. So it’s important to have the option of both.
When it comes to measuring distance by GPS over a marked course, we have never been able to determine any error. The following observation by a Tideway Scullers' cox at Henley emphasizes this point: "The Henley Regatta is traditional and has its own unique markers and distances. From the Start the first timing marker is called 'The Barrier'. This is in fact 637.3 mtrs from the start - according to a brass plaque fixed to the barrier post. We did a piece to the barrier yesterdayand the Coxmate recorded 637 mtrs."
Facts: GPS vs. Impeller
To further explain the relative benefits between these two systems, let's take a look at their differences. GPS is simple to install, produces no drag, is not prone to interference from weed or damage from impact (on and off the water), and requires NO calibration. A GPS gives land speed. It can easily be moved between boats. Impellers give a speed reading every few milliseconds, compared to around a second for GPS, and they use less power—GPS uses comparable power to the audio amplifier. The high frequency speed reading of impellers enables stroke velocity profiles to be analysed—not possible with GPS. Impellers give speed through the water which is a better indication of rowing performance if the water is flowing. Impellers are not affected by overhead line of sight obstructions - bridges and buildings can affect performance of GPS.
Given that not many coaches use the stroke profile analysis, and that few impellers are maintained in a working and calibrated state, then if you row on still water, the GPS is an obvious choice. However, Coxmate offers both options with the Coxmate SCT, and you can choose, or if funds permit, have both, and choose the one most suited to rowing at the given time, i.e. if training on a tidal flow, use impeller, but if racing at Nationals on 2K course, use GPS.
