roy, i have suggestion.
how about you post your data sets. let all those naysayers and wizards calculate to thier hearts content.
one thing i want to clarify is that you are claiming a peak speed not an average on one wave? is this correct?
then we need someone to post a shortboard ride and see the PEAK speed they attain in a ride (i would especially like to see the speed reached by the time the bottom turn is initiated).
.
.
I am in agreement. If you can, upload the data set with time, position (in whatever coordinate system makes the most sense, e.g. lat/long/elevation above mean sea level, or another x/y/z form (I am assuming the small distances covered with respect to the radius(i) of the earth (radii if you account for ellipsoidal deformation) will allow us to forego spherical or ellipsoidal geometric considerations)), and speed data, as well as the algorithm used to calculate instantaneous speed (if available). I would be curious to see the data, and would love to be proved wrong, as long as I learn something in the process.
JSS
Next time I get a track you are most welcome to it, lately I have been using the Doppler based recorder which is more accurate but does not produce a recorded track.
.
While I agree with Roy that GPS measurement is far better than any method that isn’t objective, and “apples-to-apples”, it still leaves much to be desired. First, it measures ground speed, while in our application I think that water speed is more appropriate. Also, the error factors compromise it for distances as short as those typically for board surfing. A typical long ride where I surf is probably on the order of 100 yards (certainly less than 200). Best case GPS postition accuracy with Selective Availability turned off is 3 meters, call it 10 feet. Typical accuracy, according to Wikipedia http://en.wikipedia.org/wiki/Global_Positioning_System is more like 50 feet. I see no justification to assume offsetting errors at starting and ending positions (there may indeed be reasons to assume that, but I have not found them), So, if the assumption is, on average, a 50 foot error in measuring a 300 foot ride, that’s ~17% slop inherent in the system, without accounting for any errors introduced by variations between devices or users. To be really meaningful, we would need a log of enough GPS speed measurements from the same approximate ride (and ideally, the same GPS receiver chip set) to do standard deviation and other statistical assurance calculations. I’d much rather spend that effort on some repeatable way to measure water speed…
-Samiam
The positional accuracy of GPS units is degraded so that they can’t be used for military purposes. I’m only guessing but I don’t think they degrade the velocity measurements.
Sam , the gps measures through doppler shift to its sattilites
sort of like hearing the pitch of a train whistle and telling how fast its going.
it just measures the shift in frequency of the signal it bounces off a sattilite instead of sound
so if it gets a reading its very accurate and only needs an instant to make a reading
the problem lies in the gps trying to predicit the speed using data it is getting while it briefly loses signal
so if your walking at 2 mph then as you speed up to 4 . suddenly put the gps inside a tin can at 3.5 mph and block the signal then walk a few more seconds as you take off the can the gps may report you got up to 8 mph then suddenly drop back to your true 3-4 mph reading.
the track can be usefull to see things like this happen but it is useally not a long enough wave at a high enough sustained speed to use the track.
i would love to see a picture of Roy holding his gps unit, aquapak and surfboards so i can start to believe. i really want to believe 
Roy your boards look like they are awsome at catching the wave very early and maxing out the ride length.
looks like you are having a blast but honestly just havent seen the speed you talk about in the vids you posted.
i understand you go faster in fast sections etc. I do not feel that you need a big and gnar wave to get speed.
a steep section ahead of you is all i ever need.a great example of this is greenough riding mats in the state of the s footage
very small waves very fast rides.
maybe its just an optical illusion but greenough looks like hes going fast even on those tiny waves.
your resolute salmon 2 vid for example there is a wave that beats you out (at 2:53) it was not that fast yet you fall behind ? which video time section do you think has your fastest ride ?
most of my rides fall between 15-20 mph with a rare faster ride coming into the 20s mid high 20s tops on most days .
since you claim speeds almost 2x this fast. i would expect to really be able to see a difference between your rides and mine which are generaly as fast as every surfer i have ever seen in 30 years of wave riding and or filming. at a profesional level with photos in publications surfer ,bodyboarding and clips in several videos .
the gps unit of terrys was given to surfers to use at swammis a really fast point type reef.
not one of them was able to crack 30 mph with most speeds being in the low to mid 20s
this on the fastest rides from a very large sampeling of surfers ,many at a professional level !
your results just dont jive Roy esp when your vids appear to everyone who watches them to be relitively slow to most surfers.
im just calling it like i see it man no offense .
Soul
What you are talking about is ‘selective availability’ of satellites, this downgrading feature was turned off during the Gulf War because the US military had a shortage of military units and needed to use civilian ones. . . . it has never been turned back on. Your information is incorrect, or rather, way out of date.
.
I’ve tubed at 35mph before behind a boat, and it’s dang fast. Falling HURTS. That’s faster than slalom skiers go.
Uhm, I think that would be a fairly techical slamom skiing. Recreational alpine skiing WO the gates you can go much faster. Downhill skiing, double that speed and then some. Still, it sounds damn fast on a surfboard. Although highly irrelevant as it’s a very personal thing, anyone go eyewatering fast on a surfboard? I’ve never been close
regards,
Håvard
Samiam,
GPS does not measure ‘Ground speed’ it measures speed in three dimensional space using the ground as a reference point.
Selective availability has been turned off since the Gulf war.
In terms of accuracy in speed measuring positional accuracy is not directly relevant because the entire ride including all trackpoints is translated, thus the relationship between points remains correct.
Windsurfers have done tests on measured speed runs using GPS and have found that the accuracy is within half a knot when track recording. … . . using the doppler it is much higher.
.
Muhleder, that is true .its my understanding that there is a higher fom of gps (waas)that they let us use that is accurate to less then 3 meters on ave
they could block accses to that form of gps but in practice they dont.
from the garmin website
"Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers. "
Soul
Hi Soul,
As mentioned previously speed in videos is very deceptive.
In reality, many people at the beach comment on how fast I have been surfing.
The GPS is quite ruthless, it tells the truth without any attempt to fit in with your, or anyone else’s expectations… . . . thus it is your perceptions which must be altered, not the GPS reading, which merely reports the actual speed.
I am not sure what a picture of my GPS units is going to do for you, I can sassure you that they exist !
Although it obviously is disturbing to you that I can go faster than a bunch of pro surfing thruster riders, I am not going to slow down just to put you back in your comfort zone. . … I have been roasting the tail feathers of pro shortboarders for years in the speed stakes, and as for longboard riders, they are ancient, flapping slowboarding history.
I’ve got the goods !!
Bwahahahahaha (as our friend carve nalu would say)
The sweet part for me is that I know that I can keep on doing those speeds and more, while waiting patiently for everyone else’s minds (and boards) to catch up !
So far the progress of the herd in that direction has been minimal (no pun intended)
PS here’s a helpful hint:
The velocity measurements would probably be degraded because velocity is the measure of the instantaneous rate of change of position for a particular time (depending on the algorithm used to calculate it). If position data are flawed, velocity has no choice but to be.
Doppler readings hold their own inaccuraccies when the frequency of the wave pulses being measured are traveling at the speed of light, and the object traveling at the speed to be measured is substantially less. I have no experience with the algorithm or any ‘detuning’ done so that accuracy is not as good as what the military gets, but leaving a perfectly accurate speed function alone would be just as useful for someone with malicious purposes because position could be back-calculated using numerical integration methods.
If there is no ‘detuning’ done anymore, GPS units can be very powerful if used for non-so-nice purposes, but I would like to think that what we get is still slightly inferior to what is on a GPS-guided bomb or missile. Again, I could be wrong on all of this. I am never so confident of a position in an argument that I believe it is infallible.
Soul, I am with you, I want to believe, but I do not have faith. I would like to be proved otherwise, though. Roy, please post up data logs/tracks, or see if doppler data can be logged and saved (for multiple runs, if possible), especially if you think it is a much better measuring tool for speed. The algorithm the doppler function uses would be great, too, to calculate the inherent error in the device due to the system. Systematic error must always be accounted for, and can be much larger than one expects when calculated.
JSS
Faith - The belief in a concept in the face of, no, in the teeth of evidence.
Tom/Roy i hear what you are saying . but also relate to the fact that if you are moving 2x as fast as other people in the water it should be really easy for you to get a video of you passing a Normal surfer on a thruster type board who is pumping down the line on a steep wall like he is standing still.
so when i see the proof i will believe it .
i asked for you to post a picture because honestly i think you are just trolling and dont even own a gps with an aquapak.
but i want to believe
Soul
a GPS simply calculates for speed using Time/speed/distance between two positions. (or ‘fixes’ as they are called in navigation) So it doesn’t always pickup bursts of speed.
I’m pretty sure there is no doppler involved in gps.
Doppler is a measure of an increase or a drop in frequency.
you transmit a pulse of 300Mhz at an object. If it’s relative motion to you is zero (it’s stopped), then the pulse comes back at you reflected at 300Mhz.
If you transmit a pulse at something moving away from you, then the reflected pulse comes back at you at a slightly lower frequency - something a little less than 300Mhz. (blue shift)
if the object is coming toward you, then the reflected pulse will come back at a slightly higher frequency than 300Mhz. (redx shift)
While I agree with Roy that GPS measurement is far better than any method that isn’t objective, and “apples-to-apples”, it still leaves much to be desired. First, it measures ground speed, while in our application I think that water speed is more appropriate. Also, the error factors compromise it for distances as short as those typically for board surfing. A typical long ride where I surf is probably on the order of 100 yards (certainly less than 200). Best case GPS postition accuracy with Selective Availability turned off is 3 meters, call it 10 feet. Typical accuracy, according to Wikipedia http://en.wikipedia.org/…l_Positioning_System is more like 50 feet. I see no justification to assume offsetting errors at starting and ending positions (there may indeed be reasons to assume that, but I have not found them), So, if the assumption is, on average, a 50 foot error in measuring a 300 foot ride, that’s ~17% slop inherent in the system, without accounting for any errors introduced by variations between devices or users. To be really meaningful, we would need a log of enough GPS speed measurements from the same approximate ride (and ideally, the same GPS receiver chip set) to do standard deviation and other statistical assurance calculations. I’d much rather spend that effort on some repeatable way to measure water speed…
-Samiam
I disagree. From the standpoint of making a section, it is the speed “over the bottom” that counts. Imagine viewing a breaking wave with a progressing curl as viewed from above (e.g. by looking downward from a hovering helicopter). Imagine that a surfer catches the wave and maintains a fixed distance relative to the curl. Start a stopwatch when the surfer is at some particular location (call it point “A”). Stop it when he passes some later point (say “B”). The distance he travels from point A to point B, divided by the elapsed time, gives his speed “over the bottom”, not the “speed through the water”. If the component of his speed parallel to the crest of the wave matches the speed of progress of the curl along the crest he maintains his position relative to the curl; if it is greater, he moves away from the curl; if it is less, he gets overtaken by the curl.
On the other hand, if you had a surfboard with a lot of drag, and were trying to make the drop at Teahupoo, it’s conceivable that you could end up with the slope of the wave nearly vertical and getting nowhere (at least until you got overtaken by the curl). That’s a situation with a high speed "through the water (impellor-based speed), but near zero speed “over the bottom” (GPS speed).
On another tack…
The guys in NZ that build artificial surf breaks have made extensive measurements of the angle that the path of surfers make relative to the crest of the wave (see: The Science of Surfing and Surf Breaks"). The smaller this angle, the faster the surfer is going (relative to the rate of progression of the wave toward shore). The minimum angle that they have measured is a bit over 30 degrees, increasing to around 45 degrees for larger waves (where skin friction would be expected to make a greater contribution to the total drag). If the speed of progression of the wave toward shore is Vw and the path angle is 30 degrees, then the speed of the surfer “over the bottom” is:
Vs = Vw /sin (30 deg) = 2 * Vw
If he is staying at the same position relative to the curl of the wave, the rate of progression of the curl along the crest of the wave is:
Vc = Vw/tan(30 deg) = 1.73 * Vw
For larger waves, where the path angle is closer to 45 deg, the corresponding speeds would be.
Vs = 1.41 * Vw
Vc = Vw
For a beach with a nearly constant bottom slope, the speed of progression of a wave as it progresses toward shore is about:
Vw = square-root ( g * (0.75 H +h))
where:
g = 32.2 ft/sec^2
H = height of the wave
h = depth of the water
The ratio between the height of the wave and the depth of the water at the breaking point depends on a number of factors, but typically ranges between 0.8 and 1.2. For the purposes of illustration, I’ll assume a factor of 1.0. Let’s also assume a wave height, H, of 30 feet. Then the speed of progression toward shore (i.e. over the bottom) becomes:
Vw = square-root( 32.2 * (22.5 + 30)) = 41.1 ft /sec
For a path angle of 45 degrees, and assuming that the wave peels at a speed such that the surfer stays at the same position relative to the curl of the wave, his speed would be:
Vs = 1.414 * 41.1 ft/sec = 58.1 ft/sec = 39.6 mph
…(and the speed (over the bottom) would actually be expected to be a bit slower due to factors not considered here).
This speed is comparable to a speed claimed by Roy. So let’s do a similar calculation to see what the path angle would be to achieve this speed. Roy claimed wave sizes that ranged up to 1.5x, so let’s assume a height of 9 feet. That would give a wave speed, Vw, of 22.5 ft/sec (15 mph). The path angle associated with the speed (over the bottom) reported by Roy would then be about:
tan(path-angle) = 22.5/58.1 = 0.387
path-angle = 21 degrees
By way of comparison, as I recall, the smallest path angle that was measured in the surf break design surveys was 33 degrees, so this is a significant difference. Hence I would recommend that Roy make contact with the NZ group (e.g. K.P. Black) to make them aware of the inadequacy of their data set for the design of artificial waves. Since they are also based in NZ, and presumably “neutral” observers, perhaps this would motivate them to visit Roy during one or more surf sessions and make similar path angle and speed measurements.
With regard to time-series…
It would be nice to have the time-series of Doppler based speeds over the course of a ride on a wave. Unfortunately, the inexpensive (and most compact) GPS’s that are presently commonly used to measure surfer (and windsurfer) speeds do not record the Doppler-based observations–only the sequence of times and the spatial locations. The Velocitek S10 that that is the theme of this tread (or its predecessor) does record time-series of (apparently) Doppler based speeds, and hence would help answer some of the present uncertainties. Unfortunately it costs about 3 times as much as a Garmin Gecko 201 (and two Zip-Lock bags) or 2.3 times as much as the Gecko and an Aqua-Pak. For a variety of factors differentiating surfing from windsurfing, combined with the probable errors in position data (and “mapping” onto a finite sized grid in the GPS’s internal calculations) , I do not recommend reconstituting surfing speed estimates from the time and position time-series data that are recorded by the inexpensive GPS’s presently used for most surf speed measurements.
MTB,
Very well put, Dr Black’s research is great. Not to hijack the thread, but any news on the next/newest iteration of the HYPO?
JSS
I should have said Doppler effect, measured solving for speed and relative motion.
the doppler effect is simply the compression and decompression of a frequency - audio - radio - whatever
a car’s horn keeps getting higher as it approaches you, and gets lower sounding as it passes and moves again.
GPS has no transmit capability - it receives only.
“If position data are flawed then the velocity data must be”
it depends on how they degrade the data. If they just applied a random error to each position call then yes, but if they let the error meander slowly then you would expect to get an accurate velocity reading. I think they wouldn’t apply a random error actually, because with 10 position calls a second you could probably average out the errors to get a fairly good fix on your actual location. If the error slowly changed over time then you wouldn’t be able to do this.
My info is out of date anyway, the last time I used a gps was 1996.
If you transmit a pulse at something moving away from you, then the reflected pulse comes back at you at a slightly lower frequency - something a little less than 300Mhz. (blue shift)
if the object is coming toward you, then the reflected pulse will come back at a slightly higher frequency than 300Mhz. (redx shift)
You’ve got blue shift and red shift reversed.
Some thoughts: I keep wondering about the fact that the video clips really only show the lateral trimming component of his total speed, and I wonder how many of the critics realize this and what it represents—at least half of his total velocity GPS is giving is the wave’s beachward motion, which you see coming at you in the vids–ever try to estimate an oncoming Indy or F1 car’s speed or distance from the camera on a TV screen (with no reference points around)? You’re way off, usually. (I also note that there is a more diagonal camera position on the bodyboard vid–Roy, for sure the straight out position doesn’t give an accurate picture)
I know the wake/spray on more usually-sized boards and off bodyboarders’ feet etc gives a lot of the impression of speed, but again, you can throw spray and not go anywhere at all, and especially when you are slowing down drastically–and if Roy’s boards are very efficient and low-drag , which they look to be (most of the induced drag seems to come neatly off the pin), there wouldn’t be as much wake or spray per mile per hour as you expect.
I’ve seen several of his clips now and even with the distortion from the camera angle, sometimes he looks like he’s moving along at a decent rate especially off drops and going into trim. Kinda hard to say when you’re dealing with a board that big too…if you’ve never been trying very hard to judge speed of surfboards, you’ve never corrolated GPS readings with your visuals, and you’ve only ever seen wake/spray off more usual sizes and shapes and rails, how confident can you be calling bullshit on the guy from watching vids online of him trimming on an efficient 15 footer?
In sum, I think if you’re going off the way Roy is moving about on the board a bit slowly, you might be being deceived by the boards’ size and mass; if you’re trying to guesstimate his speed from a 2D vid clip where half his speed or more is oncoming to the camera, you are probably off; and if you’re trying to do it by his wake and spray, you’re going on unconscious data you’re importing from shorter, much different much higher drag boards built to turn a lot more. Think of rowing sculls on glassy water. Efficient shapes, no bow spray, clean wake, very fast, not so good on the turning.
It could be operator error or equipment misapplication or something; adding all the above factors and considering the boogieboard vid and Roy’s several GPS experiences, and MTB’s speeds with his feet in the water, I think all the chortling is maybe down to faulty viewer perception. But hell I dunno what’s going on, and I’m man enough to say so. I would probably defer to a GPS wholeheartedly, except for the fact that the last time I heard, the world’s military were keeping a 100 meter cushion of error in the system for anti-terror reasons. I’ll look for something about that somewhere… http://www.google.com/search?q=GPS+margin+of+error+&ie=utf-8&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a
I really want to know what spuut’s got now in his standup foil boards. Brett, get a speedometer going, will ya?
Any way you can post tracks, distances and speeds off the device, RS?