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accurately measuring water flow speed?


magnetman

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I'm thinking of going on some tidal creeks in my kayak. Will do some reconnaisance on foot first.

What's the best way to accurately measure the flow speed from the bank?

 

I was thinking of strapping an old smartphone to a piece of polystyrene, app loaded to record an average speed. Push it into the flow attached to a fishing line and reel, let it drift for a while then reel it in and look at the data.

 

Is this too complicated :rolleyes:

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Which app were you thinking of? They all seem rather inaccurate at low speeds over short distances?

 

I find walking alongside a stream useful in gauging the speed. If a brisk walking pace keeps you level with debris in it, the stream is about 4mph. If you can't keep up with the flow without breaking into a jog, then it's faster. If you can relax a bit and not have to walk 'fast', then its slower.

 

What sort of stream speed would have you deciding its too dangerous?

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Simple on the face of it but measuring out 500M of a twisty creek is more of a faff than the fishing line and phone idea!

Also depends on where said stick floats. The speed of the water over the ground is not the same the whole way across the width, not the whole length although that is probably not so material as an average speed is probably what is sought.

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Which app were you thinking of? They all seem rather inaccurate at low speeds over short distances?

 

I find walking alongside a stream useful in gauging the speed. If a brisk walking pace keeps you level with debris in it, the stream is about 4mph. If you can't keep up with the flow without breaking into a jog, then it's faster. If you can relax a bit and not have to walk 'fast', then its slower.

 

What sort of stream speed would have you deciding its too dangerous?

Its not so much about danger but working out the timing of a there-and-back trip using the top part of the tidal window. I have fitted a shurflo pump motor a shaft and a propeller to it ( :wub: ) so the kayak has power assist which means I can maintain a fairly constant speed without getting tired. I can calculate this speed by carrying out sea trials on the cut. The unknown factor is then the tidal flow which will have a fairly big effect on journey time.

 

I guess its probably fairly random and changeable according to the state of the tide and channel profile. I think your walking idea is good :)

 

And yes phones are not ideal for low speeds I have noticed that they seem to be a bit jumpy.

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No it's fairly well inland - barking creek and deptford creek and the river Darentt at Dartford are the places I am thinking about so not swept out to sea but out to the tidal thames which could be quite serious I suppose.

 

Edit for errer

Edited by magnetman
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I haven't looked closely at the apps available but I think there is one which gives speed v time of day (maybe a jogging thing or summat) so if it was done for say ten minutes the pushing out and reeling back in could be excluded when processing the data

 

Its too anoraky at the end of the day.

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Its not so much about danger but working out the timing of a there-and-back trip using the top part of the tidal window. I have fitted a shurflo pump motor a shaft and a propeller to it ( wub.png ) so the kayak has power assist which means I can maintain a fairly constant speed without getting tired. I can calculate this speed by carrying out sea trials on the cut. The unknown factor is then the tidal flow which will have a fairly big effect on journey time.

 

I guess its probably fairly random and changeable according to the state of the tide and channel profile. I think your walking idea is good smile.png

 

And yes phones are not ideal for low speeds I have noticed that they seem to be a bit jumpy.

The basis is that GPS is designed for measuring location, not speed. The latter is calculated on the basis of successive location readings. As with any differential, the result will always be much more erratic than the base data.

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That's interesting

 

I have two old marine gps units one made by sitex and the other by koden. I find they are very accurate at low speed with 0.1mph resolution whereas the speed display on my phone tends to jump around much more. I have an idea that phone gps is not quite the same as a real gps. Phone seems pretty good for car driving speeds but below about 5mph it doesn't seem to be able to display down to 0.1mph resolution for some reason.

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Notwithstanding your Shurflo arrangement, hasn't any other kayaker ever done this? I mean, your surely not going up xxxx creek without a paddle, are you? ;)

 

If the Sureflo packs in or runs out of power, you're just the same as any other bloke in a kayak, just with so much extra ballast. Icant believe that you're going to be the first kayaker up any creek in the UK and, even if you are, are you really sure you need this info to a level of accuracy greater than that provided by MtB's walking speed comparison? Apologies for appearing negative, but perhaps you should be asking this on a kayak forum?

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Notwithstanding your Shurflo arrangement, hasn't any other kayaker ever done this? I mean, your surely not going up xxxx creek without a paddle, are you? ;)

 

If the Sureflo packs in or runs out of power, you're just the same as any other bloke in a kayak, just with so much extra ballast. Icant believe that you're going to be the first kayaker up any creek in the UK and, even if you are, are you really sure you need this info to a level of accuracy greater than that provided by MtB's walking speed comparison? Apologies for appearing negative, but perhaps you should be asking this on a kayak forum?

You are quite right :)

 

(Note to self: stop adding pointless topics to the forum )

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That's interesting

 

I have two old marine gps units one made by sitex and the other by koden. I find they are very accurate at low speed with 0.1mph resolution whereas the speed display on my phone tends to jump around much more. I have an idea that phone gps is not quite the same as a real gps. Phone seems pretty good for car driving speeds but below about 5mph it doesn't seem to be able to display down to 0.1mph resolution for some reason.

Depends on what you mean by accuracy. The first important thing to do is distinguish from precision: a measuring device may well, depending on the technology, give a reading with a certain number of significant digits, but not be anything like that accurate. It may well give mph to 0.1 mph whilst being accurate to 1 mph. That is to say, with a reading of 2.6 mph, the actual speed may well be 2.1 - 3.1 mph.

 

The civilian GPS is specified as accurate to several metres (various numbers are quoted including a somewhat questionable 'average accuracy'.) Let us suppose that it is accurate to 5 metres (which is pretty good for position) What then happens will depend on how the signal is processed. Whether a smartphone or a specialist GPS unit, the data stream from the satellites will be the same but uses measurements from a varying number of satellites. The mph reading depends on how that stream is processed, after the location data is calculated.

 

It comes through pretty fast, too fast for many computing apps to deal with easily so some will be discarded. At 3 mph, a narrowboat travels around 1.5 metres in a second. If the speed is derived from positions that are 1 second apart then the possible error (assuming that successive positions have a random error) could be substantial (OK its too late in the evening to work it all out and I leave that as an exercise for the reader!) Of course, the data stream is much faster and the more data points then the more that the speed reading can be smoothed out. However, the more smoothing that is done over a longer time then the less responsive the device is to actual speed changes.

 

My own application is written in Visual Basic and reads the raw data (but cannot process every item in the stream) which does include a speed from the hardware device. If you look at it you will see that the speed varies very rapidly. For a canal-based application I then apply a longer term averaging which is still only stable to little better than 0.5 mph (based on readings taken at constant speed - at least in a narrowboat the actual speed under testing conditions can be held steady for a decent time to assess.

 

My overall conclusion is that, in a situation where it mattered, I would not want to claim my speed to better than nearest mph - however much I'd like to claim otherwise when someone unrealistically comments on our speed! It also takes around 15 - 30 seconds to show a real speed change.

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How about sit in you canoe not paddling and see how fast you go! Measure with a phone if you want, I think there are some boat speed apps out there, I have a wrist worn GPS designed for walkers / runners which is quite good. Once you've done your test paddle back against the current to where you started.

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That's interesting

I have two old marine gps units one made by sitex and the other by koden. I find they are very accurate at low speed with 0.1mph resolution whereas the speed display on my phone tends to jump around much more. I have an idea that phone gps is not quite the same as a real gps. Phone seems pretty good for car driving speeds but below about 5mph it doesn't seem to be able to display down to 0.1mph resolution for some reason.

People presume that GPS measures velocity by comparing the position of consecutive fixes but this is not the case for "proper" GPS. That measures velocity by means of Doppler shift of the satellite signals. I suspect phones use the consecutive fix method which, bearing in mind the fix scatter, is bound to make it jump around a bit.

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People presume that GPS measures velocity by comparing the position of consecutive fixes but this is not the case for "proper" GPS. That measures velocity by means of Doppler shift of the satellite signals. I suspect phones use the consecutive fix method which, bearing in mind the fix scatter, is bound to make it jump around a bit.

Looking at the data stream (at least on my GPS unit, can't say for everyone as the standard refers to the format) the speed value is highly erratic. However it is derived, I have to do some form of smoothing - I have tried several alternatives - in order to display (in my app) a reasonably stable figure. Even so it is not much use other than a general guide. Still interesting but I would not want to use it in a court!

 

According to various web sites, some systems use doppler and some use successive locations.

Edited by Mike Todd
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No it's fairly well inland - barking creek and deptford creek and the river Darentt at Dartford are the places I am thinking about so not swept out to sea but out to the tidal thames which could be quite serious I suppose.

 

There is a canoe club on Barking Creek. I'm sure they could advise you in great detail without any need for your own measurement contraptions.

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