you can never measure tenths of a mph to many variables in wind, and currents. You’re not in an Olympic pool.
Exactly, wave height and adverse conditions are where the stories begin imho
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I raced offshore boats for years. Most people even though experienced would see different wave heights than me. Even 4-5’ waves were seen as 8-10. 
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You aren’t paying attention to what I write, but I don’t mind. By going over the raw data, I pull it apart and view it from a different perspective each time you challenge my belief. For casual viewers, there is nothing new here, so I urge you to end reading now, unless you share my interest in interpreting raw GPS data.
I don’t put as much unwavering value on the GPS as the “casual” reader may believe. My career involved intelligence analasis. I don’t approach either raw intelligencie or raw GPS data as absolute - both must be evaluated and cross checked for validity. It’s reasonable and prudent to question the accuracy of your car or bicycle speedometer, the actual speed of your boat in a no wake zone, or the true speed of a sail boat.
Ironically, mariners crossed oceans with less accurate instruments than a speedometer or GPS. Instead, they circumnavigate the globe relying on hand drawn charts, braving the risk of falling over the edge and know fearsome sea creatures, using a sand glass and a length of rope with a drogue on one end, which was thrown from the aft of a ship to measure relative speed. As the rope, knotted at specific calibrated intervals, slipped through the caster’s hand, the number of knots counted over a 30 second interval represented the raw speed in knots (a knot represents a fragment of minutes and seconds of a degree of longitude/latitude, so the nautical mile can convert easily to represent the imaginary meridians.
As crude as that technique may sound, not many had the stones to challenge the speed of the ship once an experience officer converted the figures, after considering known currents, drift and other factors. {Oye, you honor, and beg’in pardon cap’n sohr, but you ough not trust those calc’lation on account of them is bogus, by’n’which I means that there’s a pow’ful lot of wadder what runs on up-in-alongst this here coast h’er. You shud oughtbta take heed and compin’sate, lest we run outta salt pork and grog! See what I mean!}
They carried off extrodinary feats with crude devices, and I’ll bet a dollar against a jelly donut that few of them from the period rejected their methods as unsound. Yet I’m stymied - not upset, insulted or dishonored, mind you, nor have I felt less than adequate - instead, I feel compelled to explain when I feel someone is missing my point. It’s not an arguement, but my responsiblity for failing to make my point clear. Sometimes we labor upon the mistaken assumption that like minded people see things from the same perspective. It’s fair to point out that some experienced boaters may have a more highly developed sense of motion than myself, therefore, I accept that some are more keenly tuned than others, and they understandably may view a GPS as an unnecessary encumbrance. I for one, do not have that talent, so the GPS is a valuable crutch for me. Perhaps a ship Captain from 1800 would have considered a GPS an unnecesssry encumbrance. The true exploits of Captain William Bligh, set adrift by mutineers in an overcrowded ships boat, navigated 2,000 miles to safety using only limited navigational aids. Base on the impression I gained from reading about his character, I’d wager that he would have thrown a GPS overboard if given the opportunity.
I do not accept GPS data as absolute. It isn’t about just meauring tenths of a mph or seconds on a trip. In fact, I know my actual recorded moving times are incorrect. The speed and time data is viewed as raw data that has consistent error factored into each trip. While setting up, writing down start times, activating start times, stuffing note pad and pen in the map case, scrolling to the GPS readout screen, starting the phone app, then tucking it in the Pelican box and stowing it on the the seat stay take between 30 to 60 seconds. Then at the end of the trip, it takes time to record the finish and stop times, retrieve the phone, open the screen, take a screen shot of the stats and the trip track, making sure the info is centered. That can also take between 30 to 90 seconds. Depending on fingers and limbs numb from paddling and the ease at which windows open and scroll, especially when the touch screen is wet, while the screen is obscured by bright sun and polaroid prescription bifocal glasses obsure visibility through eyes squinting for the past hour with sunshade and perspiration causing vision issues.
Furthermore, when I stop padding, typically no more than 30 seconds (based on watching the GPS time display, which is factually accurate, based on cesium beam standard time technology, but that is only accurate to one three hundred millionth of a secind per year), drift causes one device to continue record movement (accurate or not) down to .1 mph, while the other is set at cutting off below .5 mph. That error will skew the accuracy of the overall reading (negatively, I must be point out). Consequently, I can mince hairs (phewwt), and calculate that three stops for water totals 90 seconds. Combined with a conservative estimated 90 seconds lost while both setting up and documenting trip data. That combined total is over 3 minutes, which equates to adding .1 mph to the raw data, so it isn’t actually 4.2 mph (or according to the app, 4.22 mph), but closer to 4.32 mph. So should I drop the .02 mph and claim 4.3 mph. Silly isn’t it - it’s simpler to just record the raw data displayed.
Last year I actually discussed this with you, when I was jubilant after reaching 4.84 mph, proclaiming I was within .16 mph of my goal. I suspect then that you didn’t read a jot of what I wrote and again you raise the issue.
You’re an experienced kayaker. You seem to be astute in these matters and have previously demonstrated your skill with math, so I’ll allow you to advise me. Assume that without access to my logs or GPS data, 5.0 mph accurately reflects a high speed over the same distance in 2012, when I was 61 yrs old and unencumbered by physical maladies.
How do you interpret the raw GPS data on 10 Nov 22:
Would you offer instead, a random number, round 4.84 down to 4.5, simply view it as 4.8, round up to 4.85, or to 4.9 mph. Would you throw the GPS away and guess, or plot the recorded track on a printed chart and calculate bases on the chart and travel time, while forgetting about factoring in deviations. How closely do you feel I came to my previous record? I just recorded the raw GPS data. Do you think it unreasonable for me to assume I can ever match my performance from when I was 61, especially now that I have advanced arthritis, two detached tendons and limited shoulder mobility that prevents high angle paddling. I think is a bit funny how most paddlers feel low angle paddling impedes straight tracking. I’m constantly told that I use a paddle that’s far to long to be efficient. The raw data could be absolutely wrong, but wouldn’t it be logical that the GPS data should be skewed in the opposite direction. When I calculate, based on time required to make a round trip, accurate to the minute on the clock, I find that my initial trip showed a GPD moving time of 2 hrs 15 minute when I started, but over repeat trips the time crept lower to 2 hrs even, then fell to 1 hr 45 minutes. I’ve calculated the error based on a 4.84 avg, if actual distance covere was 8.36, 8.46, 8.5 or 8.63 miles. Your general dismissal of the GPS data is more rhetorical than supported by analyzing the varisble. Read my posts, I can accept that the error is within a (+/-) .2 mph.
But let me help. Before I bought the GPS, I measured the distance of my test course using a 7.5 minute topo map from the NHS. I calculated the distance as 8.5 miles round trip. Using previous logs to extract GPS distances, as well as distance data from recent trip logs in the past 4 seasons (over 70 round trips), the extreme varied between 8.36 to 8.63 miles, with the value recurring most often being 8.46 mph, if I follow a strict turning point and return loop. So condider the variation to be GPS error. If the final avg speed of 4.84 mph actually involved travelled more or less distance than reported, how would the error change the calculation. In other words, how long would itvtake to cover those actual trip distances and still show a 4.84 avg speed.
4.84 mph over 8.36 miles is 1.72 hr or 1 hr 43.2 min
4.84 mph over 8.46 miles is 1.74 hr or 1 hr 44.8 min
4.84 mph over 8.50 miles is 1.75 hr or 1 hr 45.3 min
4.84 mph over 8.63 miles is 1.78 hr or 1 hr 46.9 min
Error is about 3.7 minutes which my other charts show an extreme difference of .1 mph (I’m not good at math so feel free to correct anything significant, or change other values, but please dont nit pick anything less than .1 difference, because I already conceded that I will tolerate a .2 mph error. I worked for a guy who said, “If you want to prove yourself right. Try to prove yourself wrong.” Unfortunately, I keep turning up results that seem insignificant. After sorting through information this time, I’m inclined to change my accepted rate of error as less than .1 mph rather than .2 mph. Unless you can “convince” me otherwise, I have nothing to add.i
How about yesterday’s trip, with GPS raw data showing 4.22 mph:
[There is a glitch in the GPS recording, but I won’t address that topic, unless you seek clarification; I doubt anyone else is reading, actually cares, or desire to prolong my getting to the conclusion]. Note that 4.22 mph is well below my high point for the season, having reached the GPS indicated speed of 4.71 mph within the fourth trip of the season:
In fact, yesterdsy’s trip places me back to where I started the season at 4.22 mph, on 17 May 23:
Notice how trips with the highest avg speed typically show the flattest track, even though I know the conditions are virtually within the same parameters, for wind and tide direction and wind speed. Another feature not apparent unless viewing the entire season is how the graph flattens and spikes become less extreme. Tracks also become more consistent and overlap, as a rule. There’s additional data in the app file that I can pull apart on a 65 inch screen that isn’t discernable on the small screen shot of a compressed phone screen.
You or anyone else reading my posts on the GPS are obliged to interpret the raw GPS data any way you choose. That doesn’t offend me. Early on, one member did generate a strong reaction from me by taking a cheap shot about members bragging about their speed. I regret that my reply was censored and removed and felt thst it was respectful, measured and appropriste. I hope nobody feels intimidated by the numbers posted above, they are far from braggadocio. In fact, they show that I’ve failed to meet my expectations. However, I find no shame in that either, because it’s the best I can do under the circumstance (laziness) rather than because the wind and tide, that always seems to be against me, for at least half the trip on my test course.
One member recently posted his GPS numbers on test runs for two of his solo canoes. 4.4 mph and 4.6 mph going on a track that included both down stream and back. As far as I’m concerned, that neutralizes any assist/drag, but I would not automatically assume it showed the canoes would preform equally over the same distance in flat water. It could mean he might be faster. He beat me in both canoes, and I respect the GPS enough that I know I was bested by a guy in a canoe living several hundred miles away. Ah, unfair
those canoes are race boats!!! See how easy that is.
I don’t have time for what if or whether we experienced a sun spot today. Instead, I rely on the overall trends. Rather than blame the GPS, it’s easier
quicker and more believable to make excuses. You can accept the my data as skewed and are free to interpret it. That simply means my data could be more favorable or lower. I hope that anyone interested in debunking the GPS myth finds my dissertations helpful. I’m sure the disinterested bypass the topic once my symbol pops up. No offense taken.
You guys aren’t reading what I type. I post the course track, explain that it divides equally the exposure to wind current, tides. I repeatedly say I limit my ksyaking to waves between 24 and 36 inches. I’ve posted live shots of my grand daughter paddling in my typical environment of about 18 inch waves. I post screen shots, describe the reported conditions, give wind direction and speed, based on my assessment using the Beaufort scale. You can clearly see a difference between graphs showing flat lines going into conditions and with. Yet you harp on what you know and experience, while rejecting ever word I say. Ive never paddled in any current stronger than 4 mph, never been and don’t want to ever face a 4 ft wave. I’m not paddling in any ocean, I paddle the tranquil waters of the Upper Chesapeake bay.
I have no issue with disagreement or anyone offering a counter arguement, but I have no doubt you guys haven’t even read my posts. For that reason, I do take offense to the replies.
I paddle fast enough to keep up but most if them are around my age.
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That works. Everything outside of being on the
water is icingbin the cake.
@Jyak they do. I had a different GPS about 5 years back that when paddling on a lake used to indicate I gained 8-10 feet in elevation while paddling on a lake. Consequently I didn’t rely on that data to calculate my racing time. The phone on the other hand was getting accurate information as it didn’t calculate any elevation change while on a lake. (By definition a flat surface.)
Elevation has no degree of accuracy on either my GPS or Phone app. I do not rely on it, and never have. I don’t need it, because I know the boat is at sea level. On the other hand, I’ve published distance, times, and averages as well as tracks. I’ve asked forum members to point out discrepancies, correct math errors or suggest a more accurate interpretation. So far, the only reaction is that elevation reading are spurious at best - it’s worth repeating that I don’t cite, reference or rely on elevation reading. The inference is that if elevation reading are so obviously wrong, then surface readings are ewually wrong. I’m still waiting for the experienced mariners to find the errors in my records.
The logs are for my use. I don’t get special privileges or recognition, stars next to my name, or a free cup of coffee. It doesn’t matter one way or another to me, whether anyone uses a GPS, believe the accuracy, or even cares about measuring trip length or speed. The information I provided is simply to help someone else assess the validity of his or her own GPS.
I’m not sure how anyone can attest to speeds or the advantage of one kayak over another, one paddle over another, or between paddle techniques. The obvious answer is to record the time to paddles a set distance. Well I’ve done that, but for some reason, if it aligns with the GPS readout, I hear the GPS can’t be relied on! That’s fine by me! The topic has my interest topped out and I thank contributors for helping me arrive at my conclusion.
Jay,
I would beg to differ on this. All commercial GPS units are ± 3 yard resolution. so your speed estimate by a GPS can be off by this much, granted it’s a negligible amount in the grand scheme of things. but it’s not necessarily your records, just the data filling it in is going to be off by some measurement. I have the same issue as everyone who uses GPS. (Even the military.) Although their gear has a GPS resolution of ±3 ft.
But for measuring performance, it really doesn’t matter how accurate the dip stick you use is, it more matters how consistent it is. Kind of like Dyno testing (they’re not really 100% accurate and will vary from machine to machine.) so it’s more important to use the same machine each time to know if you are improving or no.
I didn’t mean to disparage your record keeping, you know I know you know your shit I just meant to (as my neuro-atypical brain is wont to do) point out that the data going into your logs is not 100% accurate. But doesn’t matter as long as the equipment stays the same. 
Just as a FYI most commercial GPS implementation only uses 3-4 satellites, where as the military gear uses at a minimum 7.
@Craig_S the GPS is like any tool. Accuracy depends on the number of satellite queried, and the more satellite it queries, the longer it takes.
Confidence in the readings comes from interpreting the raw data, not simply relying on the screen display. I’m fairly confident that races rely in GPS to arrive at course distances, or they plot the start start points usin a National Geological Survey map or naitical chart. Once the distance is set, measuring error doesn’t matter because everybody in the race starts and stops at the same point. Recording time started and time it takes to finish is easy to measure to within seconds. The longer the distance, the greater the accuracy. At the speeds I paddling, it’s reasonable enough gmfor me to measure improvement of .1 mph by rounding off to about 3 minutes (+/-). So I know if I finish the 8.5 mile course in 1 hr 45 min or 1 hr 42 min, the difference is .1 mph. That’s what the GPS shows. I want to finish that distance in 1 hr 40 min. I know my calculations are under actual speeds, because I lose, as much as 90 seconds recording the actual start/stop times. Additionally, the GPS is still registering movement while I’m stopped on three water breaks that I limit to 30 second for consistency. That’s another 90 seconds. That’s between half a tenth to a full tenth of a mph to my advantage, so technically, 4.84 mph might be 4.89 mph or even 4.9 mph, possible close to 4.95 or 5 mph, but that’s a bit . . . I’m happy with 4.84 mph. I’m disappointed to say that my last trip finished ar 4.2 mph. That’s confirmed by the GPS, and consistent with time started to time stopped.
I have no objection to anyone rejecting the reliability of any GPS, nor do I feel slighted if anyone feels the speed they paddle isn’t of interest or of value. I’ve offered my methodology, so its open to correction, yet nobody has had the time or interest to counter the calculations. Anyone is free to doubt what I believe. I won’t win a free cup of coffee if anyone agrees with me. However, another paddler out there may be able to build on what I offered. No offense taken with disagreement, because Im satisfied with the results. It’s a lot more accurate than my WAG.
And sure enough more accurate than the seat of my pants Dyno.
as they say GPS, it’s close enough for government work. I think the discrepancy on solely relying on GPS alone viz, Distance over time. is about .01 MPH or less so again close enough.
but from what I see your math(s) are correct. And for measuring improvment it’s spot on. But granted it’s not swiss time.
CEGW is probably an accuarate moniker. The only way to put it in perspective is to compare it to other methods. I logged most of my trips from the beggining, and typically went from landmark to landmark, such as a point of land, inlet, launch or pier. For long trips, I put thin tracing paper over a 7.5 minute NGS map and put pin marks in the paper and turning points, then connected the lines and measured the track with a strip of paper, making a pencil mark on the strip at each turn. Then the total length of the tract was measured against the map’s scale and data recorded on the paper log strip.
Then I started using a GPS. Since I repeated those landmark to landmark trips often over the years, the distance remained constant and all I needed to do was change the start/stop times in the log, so I was able to verify the trips by retracing the steps and compare old speed distance calculation to the old recorded trips. All those trip notes were recorded in a note book with tides and conditions. I also recorded who I was with, the boats, and eventially added the paddles used.
(Battery Point is the tip of the peninsula on my test course, which is the point of a turn marked by down spikes where I take a water break; it’s actually a datum point for tidal measurements).
It might not be perfect, but it’s good enough for me to assess my progress. Casual GPS users can believe what others tell them about accuracy. I’m convinced the device I use is more reliable than most realize. I rotate through 8 rechargeable AA batteries. I also use the batteries in TV remotes and flashlights and had to buy a second set of 8 rechargables. I could get 2 trips on a recharge with the battery saver turning off the readout after one minute. I now just keep it on the entire trip and replace the batteries each time.
I hear the sage wisdom about speed depends on wind, tides, trip distance and currents. I agree. However, using the GPS over the years has enabled me to understand how conditions impact performance. Now with the app that graphs speed, I can compare a novice paddling, like my 11 yr old grand daughter, or a new adult paddler. I can also compare my earlier paddling into the wind/tide and agsinst it. It doesn’t take much imagination to figure it out. Years ago, I always finished the last two mile leg of a trip by all out paddling to burn off remaining energy. I could see that spike at the end peak before landing. Since looking at the graph and understanding energy output more precisely, my trips show a more consistent, tighter graph line with closer, tighter graph lines where I focus on strict form and a downward trend for the last 1/4 mile - burned out from anerobic output. The same is true about tracking. Its less about how accurate the track is as compared to improvement over the years.
I have 50 graphs of the same track. I’m comfortable interpreting the GPS and trust the stats, but that doesn’t mean anyone else has to. However, remember that CEGW is a step above a WAG.
A friend told me he wasn’t interested in speed. After I pressed him to check the time between a measured distance, the calculation showed he was paddling up against the hull speed and his time/distance calculation explained why he felt like the boat was climbing a hill and falling back. I think he’s a racer and doesn’t realize it. If my other GPS didn’t die, I’d send it to him to see what he can really do, but he enjoys looking around. So do I, but he’s probably a slow looker; I just look faster!