Histograph of board speed using GPS

General Discussion
1

Here is my first attempt at measuring my speed using a GPS device. I borrowed my wifes Garmin Forerunner 305. This is a pretty cool little unit. You can change the settings so that it records data every second. It has enough memory to record at this rate for 3.5 hours.

This first time out was mainly a learning experience for me. The waves were small, blown out beach breakers. I didn’t have an expecially fast board either (9’ long board). The GPS device is not water proof, so I put it in a water proof camera bag, and tied it to my wet suit zipper tab. I just let it dangle outside of my wet suit, which was a mistake. As it flopped around, about ten minute into the session, the start/stop button was hit somehow. It only recorded data for one wave even though I was out for about an hour. At least I got one wave.

When I returned home I downloaded the data file, and put it into Excel. Here is the graph that I made. Check it out.

Hopefully I’ll have something more exciting to show you next time I go out.

2

Well done, looks like accurate information, keep it coming please.

Spuuut

3

Nice one, that’s fantastically slow ! :wink:

I’ll see if I can get yesterday’s track out of the Gecko. . . . it’s not as user friendly as your one, we have to use a special cable to convert the gps data to USB because our laptops don’t have those computer sockets with lots of prongs in them, forget what they are called.

Cheers,

Roy

4

you can get software that will overlay the data onto google earth, etc.

from a thread on another site where the guy did it. pretty cool.

The GPS I use is a Garmin eTrex Venture. I bought a combined data/12v adapter cable off eBay for $25 so that I can download the data to GPS trackmaker (http://www.gpstm.com/). Once you’ve downloaded the data (very simple procedure), you just click on a link to google earth and presto, you can see where you’ve been.

http://forum.realsurf.com/forum/viewtopic.php?t=6116&highlight=gps

http://www.realsurf.com/RS_Images/20060222/20060222_1552.jpg

5

Here’s a video of a session from July last year, we downloaded the track, superimposed it on Google Earth, took a screenshot, and the made a video synchronising the speeds with the video footage (as closely as possible, within a second or so) It was a major amount of work for a lacklustre session (2 months out of water, poor waves), but just testing the concept, haven’t done it since:

http://www.olosurfer.com/17Jul06garmin.wmv

( OOPs sorry wrong one will try again)

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6

I plotted the path of my ride based on the latitude and longitude readings. Each point on the graph is one second apart. The further apart any two points are, the faster I am going with respect to the ground.

Can anyone figure out where I am surfing?

7

That is pretty cool. From the sound of it though, you’re numbers represent average speed over each of the 1 second intervals. Unfortunately your instantaneous speed - the kind given by radar detectors, for example - can change pretty drastically over a 1 second interval when surfing. So, your peak speed over any of these given intervals is higher than what your chart shows unless you maintained a constant speed during the entire interval. In some cases, like during a drop, it could be significantly higer. As an example, during a one second interval of your drop you averaged 5.4mph. At that speed you are traveling almost 8 ft per second - probably the entire distance of your drop on a small day. So that 5.4 number probably averages your speed during the most vertical part of your drop with your speed during the beginning of your much slower bottom turn. Unless your manual says that each speed reading is max speed during the interval, in which case ignore everything I just said and I’ll set down my crack pipe.

8
Quote:

That is pretty cool. From the sound of it though, you’re numbers represent average speed over each of the 1 second intervals. Unfortunately your instantaneous speed - the kind given by radar detectors, for example - can change pretty drastically over a 1 second interval when surfing.

Actually in practice the top speed shown with a 1 second interval is not much different from the doppler reading (while surfing) which upadates every 1/10th of a second. . . within 0.5 mph (well that’s what we have found anyway)

A one second interval is fairly standard and gives a useful indication of what’s going on IMO.

BTW there is no such thing as an ‘instantaneous’ speed. . . by definition an instant has no length, therefore there can be no distance travelled and no speed, only a position in time and space

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9
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Can anyone figure out where I am surfing?

You Were HERE. :smiley:

10
Quote:

Actually in practice the top speed shown with a 1 second interval is not much different from the doppler reading (while surfing) which upadates every 1/10th of a second. . . within 0.5 mph (well that’s what we have found anyway)

A one second interval is fairly standard and gives a useful indication of what’s going on IMO.

I don’t doubt this is true for the extremely heavy boards that you ride, Roy, but for typical surfboards, speed and direction can change very quickly. Consider the example that I gave and apply it to a typical weight surfboard.

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BTW there is no such thing as an ‘instantaneous’ speed. . . by definition an instant has no length, therefore there can be no distance travelled and no speed, only a position in time and space

Roy, Roy, Roy… I think you should crack open a calculus or physics book before posting such a silly statement. This is simply not true. Google it. Instantaneous velocity is defined as the time rate of change of distance as the distance becomes infinitely small. In otherwords, it is the velocity at an “instant” of time.

11

One other thing to notice about the posted chart Roy. The average speed when dropping into the wave was 5.4 and the average speed the previous second was 3.2. If you could treat these average speeds as instantaneous speeds, then the change in speed over this 1 second interval was 2.2 mph per second (this is the acceleration for the interval). Similarly, the following period has a speed of 3.9 mph for a difference of 1.5 mph. So, what if the experiment was started .5 seconds later? If you assume that the acceleration and deceleration between these velocities were constant (since we don’t know, this is the best way to go), then the time readings observed would have been 4.3 mph and 4.65. So, the highest observed speed would have been 4.65 mph. That’s about a 16% error and this is the absolute lowest possible error for the data. The speed could have increased above 5.4 mph before going back down, which would have resulted in even higher error. This clearly shows that for the data reported you cannot accurately assume average speed is equivalent to instantaneous speed. It also shows that a 1 second interval is really too big if you’re looking at peak speeds. The data shows that your speed can change 2.2mph/5.4mph or 46% between two readings - and that’s on a small wind-blown surf day.

12
Quote:

Actually in practice the top speed shown with a 1 second interval is not much different from the doppler reading (while surfing) which upadates every 1/10th of a second. . . within 0.5 mph (well that’s what we have found anyway)

A one second interval is fairly standard and gives a useful indication of what’s going on IMO.

I don’t doubt this is true for the extremely heavy boards that you ride, Roy, but for typical surfboards, speed and direction can change very quickly. Consider the example that I gave and apply it to a typical weight surfboard.

As a matter of fact heavy boards accelerate FASTER than light boards

Quote:

BTW there is no such thing as an ‘instantaneous’ speed. . . by definition an instant has no length, therefore there can be no distance travelled and no speed, only a position in time and space

Roy, Roy, Roy… I think you should crack open a calculus or physics book before posting such a silly statement. This is simply not true. Google it. Instantaneous velocity is defined as the time rate of change of distance as the distance becomes infinitely small. In otherwords, it is the velocity at an “instant” of time.

Very sorry but that is a rule of thumb which obscures, by way of definition, the meaning of ‘infinite’ and ‘instantaneous’. . . . that kind of thing is common in mathematics and physics.

I terms of pure conceptual or philosophical thought, infinity, or the infinite is EVERYTHING, and the opposite of the infinite is NOTHING

The problem is the difference between purely mathematical and purely logical thinking, and possibly there is no one correct answer.

The mathematical treatment of infinity leads to some strange contradictions, for example, if we say that infinitely small has a size which is greater than zero, then

(a) There is something smaller than infinitely small, i.e zero. . . this is a philosophical contradiction because there can be no limit to the smallness of infinitely small. . . and being bigger than something (i.e zero) is such a limit

(b) If infinitely small is greater than zero then it MUST have a size. . . if it has a size, that size can be halved . . . leading to the contradictory notion that infinitely small can be divided by two !. . . . naturally it can be objected that the size of the infintely small item cannot be defined because we haven’t ever finished making it smaller. . . . but this is a poor answer.

I realise that the mathematical treatment of the terms ‘infinitely small’ and ‘instantaneous’ is useful, and that my treatment of the terms is perhaps more in line with mystical thought or the philosophy of the Ancient Greeks. . . but the mathematical treatment is still contradictory in purely logical terms.

BTW there is no need to call me Roy Roy Roy. . . just the one will do

Cheers !

PS In practical terms if the 1/10th of a second reading is close to the 1 second interval reading it takes the stuffing out of your argument against the usefulness of the 1 second interval. . . In my opinion.

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13

I love all these graphs and stuff, good commentry too…

Top thread people!!!

14

I actually majored in physics when I was in college. I don’t use it very much anymore, but still love to talk about it. In this application I don’t think that knowing the peak velocity is too important. If you can’t sustain the speed that you are claiming for more than one second then you don’t deserve the bragging rights. I’m sure that I could record a pretty high peak velocity by flinging myself over the falls of a big wave, but does that really count?

15

"Very sorry but that is a rule of thumb which obscures, by way of definition, the meaning of ‘infinite’ and ‘instantaneous’. . . . that kind of thing is common in mathematics and physics.

I terms of pure conceptual or philosophical thought, infinity, or the infinite is EVERYTHING, and the opposite of the infinite is NOTHING

The problem is the difference between purely mathematical and purely logical thinking, and possibly there is no one correct answer.

The mathematical treatment of infinity leads to some strange contradictions, for example, if we say that infinitely small has a size which is greater than zero, then

(a) There is something smaller than infinitely small, i.e zero. . . this is a philosophical contradiction because there can be no limit to the smallness of infinitely small. . . and being bigger than something (i.e zero) is such a limit

(b) If infinitely small is greater than zero then it MUST have a size. . . if it has a size, that size can be halved . . . leading to the contradictory notion that infinitely small can be divided by two !. . . . naturally it can be objected that the size of the infintely small item cannot be defined because we haven’t ever finished making it smaller. . . . but this is a poor answer.

I realise that the mathematical treatment of the terms ‘infinitelel small’ and ‘instantaneous’ is useful, and that my treatment of the terms is perhaps more in line with mystical thought or the philosophy of the Ancient Greeks. . . but the mathematical treatment is still contradictory in purely logical terms. "

I just thought that was awesome…as a former math/physics major, I am speechless.

JSS

16

Well, as I said the numbers are interesting and if you’re most interested in average velocity, then they meet the bill. I only wish cops took average velocity into account when writing speeding tickets.

17
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I’m sure that I could record a pretty high peak velocity by flinging myself over the falls of a big wave, but does that really count?

I just did the calculation.

velocity = sqrt(2distancegravity)

[for a falling body close to the surface of the Earth, and assuming there is no air resistance]

If I were at the top of a 20 foot wave, and free fell straight down over the falls, then my final velocity hitting the water would be…

v = sqrt(22032) = 35.777 feet/second

= 35.7776060/5280

= 24.4 miles per hour

That’s not even close to what those boogie boarders in the other thread were claiming.

If I were Laird and fell from the top of a 60 foot wave then my final velocity would be 42 miles per hour. Of course, when we account for air resistance, then it would be a little less.

18

Thankyou Max.

19

It would be nice to know the algortihm the gps uses to calculate speed. The specs for the Forerunner 305 say that an accuracy of “<10 meters 50%, typical” in position, and an accuracy of “<0.05 m/s” in velocity measurements.

This leads me to believe that the unit calculates speed independently of the position measurements, as variations on the order of 1-10 meters in position would lead to skewed average speed calculations second by second, especially at low speeds (22 mph is around 10 meters/sec). If the accuracy of the speed/velocity measurement is claimed to be <0.05 m/s, my guess is that the reading given is of an instantaneous speed calculated independently. How, I don’t know.

For comparison, swied, can you post the data log of time, position and speed that your unit gave you, so we can calculate average speeds between recorded position points and compare them to what the gps calculated speed at the same points in time? I am just curious. The Excel file would be great.

JSS

20
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Well, as I said the numbers are interesting and if you’re most interested in average velocity, then they meet the bill. I only wish cops took average velocity into account when writing speeding tickets.

If the cops took the average velocities over 1 second intervals rather than the extremely short interval used by the radar (short but NOT instantaneous) it wouldn’t do you much good, the readings would be very close even if you rammed the brakes on at the beginning of the interval.

All measured velocities are averages, the only difference being the length of the interval. . . . and as mentioned previously, in surfing practice the difference in peak speed between a 1/10th of a second interval and a 1 second interval is less than 0.5 mph . . . no use saying that this only applies to heavy boards as you did previously, if you doubt this then perhaps ask Terry Hendricks and Bill Barnfield ( who ride light boards). . . they have also found that the doppler 1/10th interval readings are very close to the 1 second interval used by the time and distance based track recorder.

We have several ‘camps’ of objectors. . . those who think that a 1 second interval is too long ( you are in this camp) . . and those who think that a 1 second interval is too short. . . . then there are those sensible fellows who think that a 1 second interval is ‘just right’. . . it’s like Goldilocks and the Three Bears !

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