Volker Nolte Study on GPS vs Impeller for NK

We seem to have hit something with this series, with today being a record day for readers - double the previous day. More importantly, we have some impassioned comments - all from GPS users, but replied to by NK! Any passionate stroke coach users out there?

As promised for today I present the study by Volker Nolte as published in the NK 2005 Catalog. I hope that they don't mind this copy as I have no way to host their catalog - I would be happy to provide a proper link to the full catalog if it becomes available.

First my comments after reading the study.

Dr. Nolte's comments are presented quite clearly. A few points though to highlight:

GPS calibration is automatic. The Speedcoach calibration requires a known distance. If you don't have one - how will you create one accurately?

The accuracy measures suggest that for a few strokes with the GPS off by 1-15 m it will be extremely inaccurate, but it is also 1-15 m off for ANY distance. The impeller though is listed as 1-2% off for any distance. For a 2000 m course then, assuming only 1% error it is off by 20 m. The GPS would seem to do a better job, and gets even better as the distances get longer. NK seems to understand this though (see yesterday's post for the hint at combining the two technologies).

It is widely reported that on a body of water with a current the GPS will be wrong - which basically means that the satellite will see a boat that is stopped, but sitting in a current, as moving. The impeller though won't move while sitting still in a current. I would think that for distances this is in favour of the GPS - rowing courses do not change their length based on a current. Nolte would seem to agree, as under GPS, with current, he lists the measurement of distance over ground as a positive.

What the reports of current vs. still water do report correctly is the registered velocity - sit still in a current and the impeller reports zero velocity - which is what you would want if you use velocities to indicate training intensities. The GPS does not accurately report what you are accomplishing as an athlete with respect to velocity.

If all you care about is average pace for training intensity - again, each unit has its strengths. In still water the GPS would seem to win - but in a current the impeller wins - though again, it needs to be calibrated.

What most reports seem to rely on to discourage the use of the GPS is the rate of reporting. If the impeller can report at around 100 Hz an athlete can see virtually instantly how their technique affects their velocity. As a feedback mechanism for technique this is vastly superior to the GPS, which reports only once every second. The GPS then is only good for AVERAGE pace or velocity, not INSTANTANEOUS.

Garmin who have a considerable share of the handheld GPS market have agreed to an interview and I hope to have them address some of these concerns. NK will also be providing more information, so stay tuned for more.

Nolte's Study follows:


With emerging technology and affordability of GPS, coaches and rowers often ask why NK's speed measurements still rely on an impeller. Below is a recent article written by Dr.Volker Nolte, an expert on biomechanics as well as internationally accomplished coach and rower, comparing GPS and Impeller based speed measurements:

Thanks to the improvements in technology, it is now possible to give rowers invaluable feedback about their performance in the boat. Electronics developed at a pace that was unthinkable a few years ago measures time and distance with minuscule sensors and computer chips. The computer then calculates related quantities like stroke rate and speed, displays it on monitors and still is small and light enough to be used in a racing shell without any measurable influence on the performance of the rower. Therefore, it is understandable that more and more rowers
utilize such equipment.

Rowers use electronic feedback equipment to direct their training towards very specific, but different goals. High performance athletes want to control their training intensity to gain the desired fitness improvements in the most effective way and look for feedback during their races. Recreational rowers need information to direct their power output for health reasons. Researchers use training and race data to analyze performances for physiological, biomechanical and strategy studies.

For each of these tasks, it is critically important that the feedback one receives from the equipment is correct. The electronics provide indicators that potentially have very serious and important consequences for the rower. In case of incorrect data feedback, athletes may choose inappropriate intensities that do not lead to improvements, or even put the athletes' health at risk. Obviously, researchers need proper data to interpret their findings correctly.

Therefore, it is very important for any user to know exactly which data the respective equipment provides. In general, two different methods are currently utilized to measure the quantities mentioned above: Impeller and GPS.

Impeller measurement is based on the principle that the water sets it in a motion that corresponds with the speed of the boat that it is attached to. The motion of the impeller is directly dependent from the flow of the water relative to the boat. This means that the movement of the boat relative to the water is measured. Sensors in the boat record the spinning of the impeller to calculate the required data.

Global Position Systems (GPS) measure the position of the receiver as a place on the earth's surface. If the receiver is connected with the boat, GPS is able to track its movement and can calculate various data from that. This means that the movement of the boat relative to the ground is measured. Below, you find a comparison of the two measurement systems.

(Data from tables presented as photos - click on image to see original size.)

In conclusion:

Despite its very simple usage, GPS systems have to be operated with care. The information generated by GPS is potentially extremely inaccurate, especially when used on a body of water with current. Used without consideration of this fact, the training feedback could harm an athlete's development or even health.

The impeller system, however, clearly shows advantages when calibrated properly. When not calibrated, the impeller system will still show accurate, corresponding changes in speed.