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Thread: Laser Pool Measurement Device

  1. #21
    Very Active Member Chris Stevenson's Avatar
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    Re: Laser Pool Measurement Device

    Quote Originally Posted by Meegan Wilson View Post
    Let's stick to the 50m case, and make sure I've got it right:

    1. A 50 meter pool is nominally 49.9999 meters long. (Sure, it's weird, but 'dem's the rules!)

    2. If I can somehow delude myself into believing my measurements are accurate to 2.5mm, I cannot in good faith certify a pool measuring less than 49.9999+0.0025 = 50.0024 m.

    (The Leica's still in transit, but I'm guessing it will resolve only 1mm. If so, my rule must be modified: )

    2'. I cannot in good faith certify a pool measuring less than 50.003 m.

    3. An offending pool, measuring 50.002 m will be recorded as "50m, 0cm", but I must indicate somehow (with an asterisk like in baseball?) that the pool is not known to conform.
    You're making this way too complicated. The nominal length of a 50m pool is exactly 50 meters; you don't get to make it shorter by double-converting your units. Since "nominal" is a theoretical value, it has an infinite number of significant figures, so "exactly 50" means what it says.

    In any event, there is no point in using more decimal places than the precision of your measuring device, so ditch the 4th decimal place if your device is only precise to (say) 1mm.

    I believe you are also confusing measurement precision with "minimum distance." First secure a device that can measure the pool length with the required precision (5 mm). Once you have it, measure the length in every lane. If the touchpads are in place, then the measurement can't be less than 50.000m. If the touchpads are not in place, then the measurement can't be less than 50.010m if you plan to use one touchpad in competition, or 50.020m if you plan to use two.


    Quote Originally Posted by Meegan Wilson View Post
    But as it turns out, my understanding of these pocket-sized laser pulse time-of-flight devices was way too naive. My self doubt started when I realized that to resolve 1 mm required counting a clock ticking at 150Ghz or so. Now I'm sure there's a three-letter agency that can pull such counters out of their parts bin, but I doubt such a device could be powered very long by two AA cells. Google Scolar found me "Laser ranging: a critical review of usual techniques for distance measurement", Amann, et.al., Optical Engineering, Jan 2001, pp 10-19. For the time being, this paper is leaking out of some university library database: http://bit.ly/9wEq8j

    These little doodads may be made from cheese, but it's pretty darn sophisticated cheese.

    First, I had envisioned a sharp pulse, but the the pulse is quite smeared out (into a Gaussian shape) with no distinct features to use as a reference. But by comparing the outgoing and returning pulses one can normalize the two and trigger on any point on both normalized versions. Furthermore, this whole process can be magically automated.

    Second, a clock rate of a few tens or hundreds of Mhz will only allow the instrument to resolve to the tens of centimeters for one round trip. But the pulses are generated asynchronously from the timer ticks, and this introduces a kind of dithering that (statistically at least) allows a much higher resolution when the experiment is repeated a few thousand times (automatically, of course).

    That's probably why the phrase "2-sigma" appears in most specs; the measurement is accurate within 1.5mm with very high probability, but not with certainty.

    I'm still suspicious of the time base; instability there would introduce a systematic error increasing with the distance measured, just like the specs describe. If one could tighten up the time base, I can believe a sufficiently accurate measurement could be made, but I don't think these cheapo $300-500 units will cut the mustard unless modified.

    Have a look at Fred Piggot's contraption. The beam is high enough above the water that I think your worries of multi-path might be assuaged.

    But since the underlying accuracy of the rangefinder is in the tens of cm, do they meet the better-than-5mm accuracy requirement? Is "highly probable" sufficient? Or is "certainly" required? "Probably" would suit me, but I'm an engineer, and 3.97 is close enough for government work.
    I don't have the device so I can't answer your questions with certainty (no pun intended). But from your description, here's what I would say:

    -- don't confuse the uncertainty of the measurement of a single pulse with the overall uncertainty of the device. There is the effect of averaging, after all; for completely random error the precision scales with the inverse of root(N).

    -- don't sweat the time jitter either. While I am not familiar with this device, I have done a decent bit of work with pulsed lasers (nsec widths). If you split off a portion of the light and trigger your detection electronics off it then the time resolution is much better than the pulse-to-pulse time jitter.

    -- as far as "measurement certainty," there is no such thing. It is all about probability. So 2-sigma is fine.

    Again, you are making this too complicated. Look up the manufacturer's specified measurement accuracy and accept that it is correct. If it is 5mm or better, then you're ready to do your measurements. Hopefully they aren't too short.

    Note added in proof: I just scanned the Opt Eng paper. I assume that most laser devices for pool measurement are of the pulsed types. From what I can tell (although they don't quite state as much), optical triggering is indeed used. (I really see no other way to do it, and it is very easy to implement...a small glass slide, for example, reflects 5% of the pulse and transmits the rest, so that would work.) And while it is true that the statement "to obtain 1 mm accuracy, the accuracy of the time interval measurement should be 6.7 ps" is pretty daunting, the authors also state that "averaging enables millimeter or even submillimeter precision to be achieved." I assume a laser diode is used for typical pool measurements (YAGs would be overkill) and, with their high frequencies, the time required to achieve 1mm accuracy would be pretty short.
    Last edited by Chris Stevenson; July 31st, 2010 at 09:35 PM.

  2. #22
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    Re: Laser Pool Measurement Device

    Quote Originally Posted by Chris Stevenson View Post
    ...The nominal length of a 50m pool is exactly 50 meters; you don't get to make it shorter by double-converting your units.
    I didn't want to make it shorter. 50m makes my life much simpler. I thought USMS wanted it shorter. I inferred this from the pool length cert form you referred me to. Indeed, the error is down in the noise, but why confuse me with all this rounding-to-hundredth-inch smokescreen?

    Quote Originally Posted by Chris Stevenson View Post
    -- as far as "measurement certainty," there is no such thing. It is all about probability. So 2-sigma is fine.
    You know that, and I know that, but most civilians tend to see the marks on the tape as somehow "certain", and may feel uneasy about statistical methods used to reduce the error interval from a much larger value with Monte Carlo techniques.

    Quote Originally Posted by Chris Stevenson View Post
    I believe you are also confusing measurement precision with "minimum distance." First secure a device that can measure the pool length with the required precision (5 mm). Once you have it, measure the length in every lane. If the touchpads are in place, then the measurement can't be less than 50.000m. If the touchpads are not in place, then the measurement can't be less than 50.010m if you plan to use one touchpad in competition, or 50.020m if you plan to use two.
    English is my first language, and I was sure the newer form demanded my interpretation, but I am certainly willing to go along with yours if that is the accepted practice. Let me try one more time to see if I understand this practice which is certainly simpler to implement:

    1. By whatever (unbiased) method, half the pools measuring 50.000 meters will be in reality shorter; the other half will be longer, but that's OK because as long as we're within 5 mm it's good enough for government work. Regardless of reality, all such pools may be certified for competition.

    2. Practical advice: If your pool lasers out just a little short, you should try again next week because your luck may be better.

    3. Practical advice: Try when it's hotter or colder to see which makes the timebase in your laser gadget run faster and your pool measure a teeny bit longer.

    If everyone agrees on this methodology, I would advocate changing the "Measurement Procedures" blurb to:

    1. Drop the red-herring conversion to English units.

    2. Change "A pool must always be at least as long ..." to "A pool must always measure at least as long ...". To me, the word "be" connotes a much closer relation to reality.

    Jim
    Last edited by Meegan Wilson; August 1st, 2010 at 12:31 AM.

  3. #23
    Very Active Member Chris Stevenson's Avatar
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    Re: Laser Pool Measurement Device

    Quote Originally Posted by Meegan Wilson View Post
    You know that, and I know that, but most civilians tend to see the marks on the tape as somehow "certain", and may feel uneasy about statistical methods used to reduce the error interval from a much larger value with Monte Carlo techniques.

    English is my first language, and I was sure the newer form demanded my interpretation, but I am certainly willing to go along with yours if that is the accepted practice. Let me try one more time to see if I understand this practice which is certainly simpler to implement:

    1. By whatever (unbiased) method, half the pools measuring 50.000 meters will be in reality shorter; the other half will be longer, but that's OK because as long as we're within 5 mm it's good enough for government work. Regardless of reality, all such pools may be certified for competition.

    2. Practical advice: If your pool lasers out just a little short, you should try again next week because your luck may be better.

    3. Practical advice: Try when it's hotter or colder to see which makes the timebase in your laser gadget run faster and your pool measure a teeny bit longer.

    If everyone agrees on this methodology, I would advocate changing the "Measurement Procedures" blurb to:

    1. Drop the red-herring conversion to English units.

    2. Change "A pool must always be at least as long ..." to "A pool must always measure at least as long ...". To me, the word "be" connotes a much closer relation to reality.

    Jim
    Most people are pretty comfortable with the idea of averaging and intuitively trust an average over a single measurement, whether or not they've heard of the Central Limit Theorem. I don't think it's a problem.

    (In my mind, a slightly bigger quibble is the reference to a maximum tolerance of a measurement device. Tolerance -- maximum allowable deviation -- is a function both of the precision of the measuring device and how it is used. In the rule I think the maximum tolerance is more properly referring to the pool length (and its measurement), not the capability of the device. But it is just a small quibble and I'm not sure that anything is served in going into all that on the measurement form.)

    To answer your questions:

    1. You got it. Though I would point out that, because of how the rule is written, most certified pools have measured out at above 50.000 m. That means that -- assuming only random error -- the fraction of such pools that are truly shorter than 50 m is substantially less than half.

    Actually, the situation is even better than that because all lanes of the pool are measured and if ANY ONE of them is too short, the pool is not certified. So I think it is reasonably unlikely that a certified pool is in reality too short.

    When I first read the rule, the measurement scientist in me was slightly offended that it didn't just say something like "the pool measurement must be 50 m or longer, within the precision of the measurement device." Assuming an unbiased measurement procedure, such a situation means that certified pool could not be proven (in a statistical sense) to be shorter than the nominal length. But I suppose I see the value in the way it is written.

    2. I suppose, though you are biasing your measurement procedure by cherry-picking your measurements. For example, if you hired surveyors to do it for you (not uncommon) I am not sure they would cooperate.

    3. I don't think temperature variations over the range likely to be experienced at the pool would make a significant difference is measurement accuracy, but feel free to try.

    To respond to your suggestions:

    1. Some people use devices that measure in English units so I think that needs to stay. The uncertainty in conversion should be less than that of the measuring device anyway.

    2. I will bring it up. I do like your wording better, but I'm not sure the rest of the Records & Tabulation committee will agree. They may feel that your wording invites abuses (you know, like doing lots of measurements and choosing the one that gets the pool certified ).
    Last edited by Chris Stevenson; August 1st, 2010 at 07:18 AM.

  4. #24
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    Re: Laser Pool Measurement Device

    Quote Originally Posted by Jim impersonating Meegan Wilson View Post
    Or do you have ... ideas ... that would allow shorter pools to be deemed conforming?
    I wish I hadn't phrased it quite that way. I've been fighting your impression that I'm trying to bend the rules in favor of too-short pools. I'm only trying to understand common practice and avoid inflicting an overly-strict procedure. To probe what's acceptable, I tend to let my cynical side show through. Any "Practical advice: ..." I offered was in that vein.

    My real intention is/was to devise a system, packed in a foam-lined case that would fit in the trunk of a compact car. The system operation could be described in a half-page instruction manual so I can hand off the job to an amateur (or even a meet director) who would certify a pool. Other than initial testing and perhaps periodic checking of calibration, I hope to never touch the thing.

    (Since I no longer swim, I was only mildly amused at the UF-pool-too-short fiasco. My feelings were similar to the one I had for the Billy Martin/George Brett pine-tar incident. But I can assure you some other principals had a substantially more violent emotional response.)

    Quote Originally Posted by Chris Stevenson View Post
    (In my mind, a slightly bigger quibble is the reference to a maximum tolerance of a measurement device. Tolerance -- maximum allowable deviation -- is a function both of the precision of the measuring device and how it is used...)
    I wish you hadn't shared that thought. I was all set to report back that there was no way to make sufficiently accurate measurements with inexpensive pocket-sized devices, but the most-expensive Leica D5 barely squeaks in under the wire at 4.5mm.

    I had interpreted "measuring device" to mean "measuring system", including the jigs used to assure conformance to the rule at various depths, but parsing the procedure closely seems to confirm your interpretation. Only the specific component of the system used to perform the actual measurement is required to be accurate.

    Please understand, I'm not trying to rationalize sloppy construction of the other components of the system or abuse this error source to certify non-compliant pools, but only to point out that these (statistically-independent) error sources will degrade overall accuracy outside the 5mm limit.

    Unfortunately I have more work in front if me, if that is the intention of the rules and not just the ravings of some metrically-libertine tow-headed nutjob that swims fly fast. There's only room for one nutjob in this discussion, and I claim that position.

    Incidentally, I am told that neither Fred Piggot's contraption, which apparently hails from Michigan Masters, nor the improvements suggested by Bob Griffiths are widely known. I suggest these secret documents somehow be declassified and made available to this forum.

    Quote Originally Posted by Chris Stevenson View Post
    ...because of how the rule is written, most certified pools have measured out at above 50.000 m.
    Because of how the rule is written and because the danger of striving too hard for the ideal may subsequently entail corrective action involving jack hammers, dump trucks, cranes and multimillion-dollar contracts.
    Quote Originally Posted by Chris Stevenson View Post
    I do like your wording better, but ... the ... Records & Tabulation committee ... may feel that your wording invites abuses
    In "A pool must always be (measure?) at least as long ...", I'm sure they could wield a particular definition of "always" to smack down a cherry picker.

    Jim

  5. #25
    Very Active Member Chris Stevenson's Avatar
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    Re: Laser Pool Measurement Device

    Quote Originally Posted by Meegan Wilson View Post
    I've been fighting your impression that I'm trying to bend the rules in favor of too-short pools
    I don't have that impression at all. Fire away with your questions. Hopefully I can answer them eventually.

    Quote Originally Posted by Meegan Wilson View Post
    Incidentally, I am told that neither Fred Piggot's contraption, which apparently hails from Michigan Masters, nor the improvements suggested by Bob Griffiths are widely known. I suggest these secret documents somehow be declassified and made available to this forum.
    I don't know what these are. Can you elaborate?

    Quote Originally Posted by Meegan Wilson View Post
    In "A pool must always be (measure?) at least as long ...", I'm sure they could wield a particular definition of "always" to smack down a cherry picker.
    I'll put it on the agenda (which is pretty full right now but we'll get to it eventually ).

    I assume you are working with Meegan's daughter Marly on all this? She is Top Ten recorder of your LMSC and would be the one who is in charge of keeping records of certifications of all the pools where you hold meets. When you certify a new pool, please be sure to let Walt Reid know so he can add it to the national database:

    http://www.usms.org/~rectabs/poollengthdb.xls

  6. #26
    Very Active Member jroddin's Avatar
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    Re: Laser Pool Measurement Device

    Quote Originally Posted by swimmieAvsFan View Post
    just so you know, the current championship committee chair you're being snarky about is himself an engineer, and probably deals with more precise numbers on a daily basis than you do...


    Actually, Connie is a civil engineer and land surveyor which means she knows about 100 times more than I do about taking precise measurements. I design parts so I know how things are "supposed to be" but she works on the practical side of things.

    PS. I didn't interpret her comments as being snarky. She pointed out a naive mistake I made in interpreting the tolerance of the Leica Disto by taking what they said on their website (i.e. marketing) to be true instead of digging into the technical manual to find out the true limits of the device.
    Last edited by jroddin; August 3rd, 2010 at 07:38 AM. Reason: Added PS

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    Re: Laser Pool Measurement Device

    I submitted this stuff to Wikileaks, but they were too busy vetting other things so I got Meegan to more or less wrangle permission to release the hush-hush description of the laser/target holding apparatus I misattributed to Fred Pigott along with a critique and some suggested improvements from Bob Griffiths.

    In a series of emails, Fred denied any responsibility for the document and instead credited Frank 'Skip' Thompson for the concept. I've done a bit of editing, and without doubt I've introduced an error or two to both Skip's original and Bob's improvements. Sorry, Skip. Sorry, Bob.

    These older documents are mainly interesting from a historical perspective, because Bob's recommendations appear to have been accepted in their entirety by USA Swimming, and the resulting document is probably a better reference.

    Here's the old stuff: http://floridalmsc.org/forms/PoolMeasurement.pdf

    And here's the new: http://www.pacswim.org/certifyingpools.pdf -- See particularly Part 2 and page 9 where the contraption is professionally illustrated by someone with a good knowledge of Inkscape or Corel Draw. This is almost exactly what I had pictured in my mind after stealing Bob's ideas.

    Jim, impersonating Meegan
    Last edited by Meegan Wilson; August 5th, 2010 at 12:46 PM. Reason: Stupidity

  8. #28
    Very Active Member Chris Stevenson's Avatar
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    Re: Laser Pool Measurement Device

    Thanks, Jim. I skimmed both documents and they look useful, esp the USA-S one.

  9. #29
    Very Active Member Conniekat8's Avatar
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    Re: Laser Pool Measurement Device

    Quote Originally Posted by swimmieAvsFan View Post
    just so you know, the current championship committee chair you're being snarky about is himself an engineer, and probably deals with more precise numbers on a daily basis than you do...
    Jeff is also is also a very dear personal friend of mine.
    -Connie
    I'd rather be swimming http://www.mastersmvnswim.org/

  10. #30
    Very Active Member Conniekat8's Avatar
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    Re: Laser Pool Measurement Device

    Quote Originally Posted by jroddin View Post
    PS. I didn't interpret her comments as being snarky. She pointed out a naive mistake I made in interpreting the tolerance of the Leica Disto by taking what they said on their website (i.e. marketing) to be true instead of digging into the technical manual to find out the true limits of the device.
    Well, I wouldn't exactly call it a naive mistake. It's one of those things that one is not likely to know unless they are a land surveyor who has education about it and deals with those things every day.

    Best thing anyone outside of having a degree in Land surveying can do is fins a local Land Surveyor in a pnone book, tell them what they need to acomplis, what precision is needed (this is very important for the choice of methods and instruments) and ask for advice. Most Surveyors are likely to be quite happy to give advice on this. They're most likely not going to try and sell you anything, because coming out for an hour or two to mesure a pool is typically cost prohibitive for them.

    Lot of times Conuties and Cities also have a Land Surveyor on staff as a part of their Public Works office. They may be even more willing to offer advice. If the pools are city owned and run, there may be even a way to get them out there to do it for you. For example, if a City Parks and Rec. department contact the City Engineer....
    -Connie
    I'd rather be swimming http://www.mastersmvnswim.org/

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