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Feb 28

Wakesurfing with a GPS data logger

Right? Who doesn’t want to do that? If you’re not familiar with a GPS Data Logger they are small little devices that link up to a number of satellites and record position data. Internally, the data recorder can do some calculations a determine things like speed. This is the very same technology that is used with the current state of the art speed controllers, like the Stargazer system from PerfectPass to name one.

These little handheld units have amazing versatility in that they can record any number of items that can be detected via satellite, such as longitude and latitude, but also elevation. They are also used frequently in races where a person needs to maintain a certain pace. They can record specific locations and all manner of location data and then from that infer and calculation speed and the like.

Here is a picture of one such unit.

data logger

Now the above GPS Data Logger unit isn’t great for recording altitude, it’s only accurate to within 2 meters or about 6 feet. BUT they can record speed to within 0.07 mph, which is really accurate! 

Many sport disciplines have speed races, or speed records is probably the best descriptor, with websites dedicated to that pursuit.  These devices can be used to record runs and then the data uploaded to the site as part of a series of who went the fastest recordings.  We don’t really have that in wakesurfing, but we can still use the speed data.

The data loggers record what is referred to as sentences. That is there is a data file and each line is a record, like a piece of paper in a regular paper file folder. But each record can be of a different type, so one might be for GPS fixed data and another might be for Date and time. Each sentence then has a comma delimited format that is known and defined. Here is an example of one such sentence.

$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47

The field definitions are:

GGA Global Positioning System Fix Data
123519 Fix taken at 12:35:19 UTC
4807.038,N Latitude 48 deg 07.038′ N
01131.000,E Longitude 11 deg 31.000′ E
1 Fix quality:
0 = invalid
1 = GPS fix (SPS)
2 = DGPS fix
3 = PPS fix
4 = Real Time Kinematic
5 = Float RTK
6 = estimated (dead reckoning) (2.3 feature)
7 = Manual input mode
8 = Simulation mode
08 Number of satellites being tracked
0.9 Horizontal dilution of position
545.4,M Altitude, Meters, above mean sea level
46.9,M Height of geoid (mean sea level) above WGS84
ellipsoid
(empty field) time in seconds since last DGPS update
(empty field) DGPS station ID number
*47 the checksum data, always begins with *

That format is defined by a body called the National Marine Electronics Association or NMEA for short. There are two widely recognized formats the 0183 and a 2000 which is more current. The point being that if we know the format of the data and can decipher it, we can get very accurate details on things like speed.

So we are hoping that we’ll be able to determine a riders speed when in trim, the riders speed when pumping or going up for an aerial, or maybe we’ll do a few tests and just have a rider go as fast as they can down the line. One last thing we are hoping to determine is aerial height and possibly wake height. We aren’t too confident about these last two items because this GPS Data Logger unit isn’t really all that accurate in this regard.

What we hope, is that the data will give us a consistent error. So the altitude is based upon sea level and while it may not be an accurate reading of the height above sea level, it will be consitently inaccurate with all readings and not vary by say 2 meters between readings. So lets say that someone rides to the top of the wake and we get a reading of 12.7 meters above sea level. If we always get that same recording at the top of the wake and say 12.0 meters at the bottom of the wake then we can calculate the height of the wake as 0.7 meters. It may not actually be 12.7 meters above sea level, but we don’t care. However, if we go to the top of the wake 3 times and get readings of 12.7, 11.3 and 12.1 then that data is probably not very reliable.

So like we said, the hope is that while the data may not be accurate with regard to the ACTUAL height above sea level, that the error is consistent on all altitude recordings.

Oh and yes, we’re crazy we know that.

It’ll be interesting to see if we can gather any relevant emperical data that we can use for comparison purposes. We grow so weary of the “my board/boat/device” is the best at “everything/nothing/draining your wallet” claims. At times a board may “feel” fast, but the reality is that it isn’t. So we’ll hopefully be able to get some REAL numbers on speed against the wake and possibly some indication of acceleration.

Thanks so much for following along, we appreciate it!

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