Mar 02

Wakesurfing Calculations

Do you work in Excel? Microsoft’s spreadsheet? Well here you go:


Triginometry enough to make you puke! We’ve got our GPS Data Logger and according to the documentation, the logging function will record the GLL sentence which will give us the riders position, in longitude and latitude every second. Cool! Except WTH do we do with those points? The formula above, allows us to take the two sets of points and then convert them to a distance using all manner of triginomic functions and the Radius of the earth!!!!

Now we know that the rider, in trim, will always be traveling at approximately 11 mph, what we are interested in is the INCREASE in that distance as the rider pumps or pushes forward. That can help us determine how far a rider can travel with a pump, leaning forward or whatever.

So here is what we have decided to test and hopefully will have enough accurate data to be able to do the calculations.

1) Height of the wake at the peak

2) Terminal velocity after pumping down the line

3) Acceleration

4) Altitude of a simple aerial

5) Distance traveled with a single pump

So we set up the GPS data logger, charged the battery and the like. Then last evening we tested the file creation functions and also tested the relative accuracy of the GPS itself. Now this first picture is going to seem weird and we’ll explain afterwards.

GPS Data Logger 002

So it’s showing a reading and sitting there on the coffee table! We hadn’t done anything at this point other than turn it on! It doesn’t show in the picture, but the reading fluctuated a bit and then finally settled down to about .20 MPH. Sitting on the table. It’s labled as MAX SPEED and it could be that it was reading the movements of the unit in our hand as we moved it down to the table, or when it was stationary on the table, we were concluding that it was reading the revolution of the earth! It seems that the speed readings are sort of averaged and smoothed over a period and so aren’t 100% accurate. But as we said, it settled down to a reading of 0.20. 🙂 That’s probably way more accurate than our typical, man that feels faster than a Coex type reading 🙂  Right?  We mean that, current state of the art in determining the terminal velocity of a wakesurf board is “man that feels faster than whatever other board”.  It’s JUNK! Our current state of the art speed determinations are absolutely worthless.  What’s worse is that folks are forced to make buying decisions based upon that. That’s also state of the art in terms expertise. “Well in my expert opinion, it’s faster than a coex.”  Whatever!  What we should be saying is we have no idea how fast the board really is within the working confines of a wakesurf wake. So there it is, state of the art determination of speed of a wakesurf board is: “I dunno”. 

Ok, now that we’ve finished THAT rant, the next thing that we tested was the the GPS units ability to write the data file we’d be working with. As we mentioned in one of the previous posts, the records are from various sources and so each one is most likely different. They are referred to as sentences, rather than records for this reason. Each sentence has an identifier as the first field in the record and based upon THAT identified we can tell what data is contained in it because THOSE are defined by the NMEA and are static. So here is a short snippet of what the file looked like:


We won’t spend a lot of time on this, but you can see that each line/record/sentence starts with $GP something or other followed by 3 characters. Those characters identify what TYPE of record/sentence it is. So the first line/record/sentence shows: $GPGGA and that is a GGA record. Which has a specific definition. The commas that you see in each line/record/sentence delimit the fields.

Now if you’ve done any work with spreadsheets you know that are remarkably powerful and they also have the ability to worth with flat files that are fixed length or delimited with commas or any number of characters that define the end of a field. The NMEA specification is a comma delimited type, as you can see above.

What Excel does, as you load the file, it parses each line/record/sentence into the different fields and places them in a separate column and the each line or record goes on a separate row! Viola! Our data is now in the form of a spreadsheet and we didn’t have to key in all the data with it’s attendant potential for error and inaccuracy.

Here is one spread sheet we created to test the validity of the speed recordings. Now sadly, NMEA definition saves the speed data directly, but as Knots and Kilometers per hour. We converted the Knots data into MPHS by multiplying it by a factor. If you look at the spreadsheet in the picture below you can see the two columns of speed data and then a third column which we calculated.

GPS Data Logger 004

You’ll see there are a LOT of readings and several different speeds recorded. We walked around the block with the GPS in hand, running at times, walking fast at times and just walking normally. We’ve intentionally sorted the records, first to segregate all of the VTG records/sentences and then again on the calculated speed column for MPH. You can see that the fastest speed we attained in our walk was the 8.58 MPH. It was interesting, as we looked at the original raw data, we could identify the places where we ran and walked fast and they looked appropriate in terms of their location wihin the file.

Ok, one last picture. This is of a preliminary spread sheet where we have parsed the altitude data.

altitude 002

We’ll reiterate that this spread sheet isn’t finished yet, but you can see a few things. One is the columns for the Latitude and Longitude that we talked about yesterday, so we have the basis to determine the distance traveled at any particular time. Also, we have a column that is directly recorded for altitude. What we did to record that data was Hold the GPS unit up above our heads, standing on top of things and in general moving it around. We have no idea of the resultant number above sea level is accurate but we can see that we have some different altitude readings! We’re hopeful that we have enough there to be able to accurately estimate the height of an aerial and also the height of our wake.

It should be noted that we don’t plan to affix the GPS to our boards. That would give us the most constant readings, but that isn’t in the cards just yet. So that means the rider will be carrying it n the hand. That won’t yield the most accurate readings, but hopefully it will give us something to work with. We’ll need to make some allowances and correct for those things. For example the likelihood that the GPS will make it’s 1 second recording at the height of an aerial is slim to none, right? But we should be able to get an idea of the pattern and possibly can extrapolate from that. Also, if the rider has the unit in his hand, where is that likely to be at the top of an ariel? Above his head, huh? It’d be nice to have 8 foot tall airs, but not likely, so we’ll have to get a reading at the top of the wake with the riders hands above his head. 🙂

So not 100% accurate, but it’s definately heading in the right direction.  AND certainly better than either “I dunno” or the whole in my expert opinion it’s faster than a coex, nonsense. 🙂  Hopefully we’ll be able to gather some useful objetive gps data while wakesurfing that can be converted into something meaningful.

Thanks so much for following along! We appreciate the patience and understanding as we undertake this experiment.

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