I would like any suggestions on how to do this with a tape. (This is pretty non controversial, right?) I have spent a career trying to figure out how to measure things, and I am curious. From the comments in this group, the problems I mention below must have been solved.
First you stretch the tape along the side of the pool. That would work well, but how are you sure you are aligned with the walls, or that the walls are square? Also, you need to measure in several lanes, rather than the side.
So you get in the pool and hold the tape against the walls, 1 foot below water level. The problem is that the tape sags, and any measurement will overestimate the length of the pool by a significant amount. You could pull the tape tighter, but I suspect before the tape gets tight enough, your tape will stretch or break. (I used to assign that as an intro physics problem). I suppose you could build a support structure across the pool, but that would be a pain in the neck, and you would have to move it from lane to lane.
Getting the water out of the pool would make that support structure easier, but the water weighs a lot. It almost certainly causes the walls to bend outward, especially in the deeper pools. (How do the people who design above ground, temporary competition pools do it?) I don't know how much the walls will move, but how do you account for that? (and by the way, how do the masons make sure their plaster/tiling is correct in an empty pool?)
Here is an idea - you lower the water level by a foot or so, and build a *floating* support structure for the tape. I think that would solve most of the problems. Has anyone done that?
Former Member
Originally posted by Mark in MD
I wonder if someone will eventually come up with a method of measuring pools using GPS. With the technology getting more precise, could it be only matter of time? Just a thought . . . nothing more!
1. GPS dpesn't work indoors.
2. GPS is not more accurate then what was discussed here.
With a handheld GPS, even withthe selective availablity turned off few years ago, the $200 dollar handheld units are only good to +/- 1 meter.
Wanna het down to a few centimeters, you need about $ 40,000 worth of equipement, and it's even more difficult to use than the tape, and it has more quirks and restrictions than a laser.
Yeah, High precision GPS is something I use frequently and do a lot of measurement analisys and adjustments (GPS, and others). GPS would not be the method of choice for this.
It issue is not the technology. Thechnology to measure the pools accurately and precisely exists. It's how it's being used that introduces the errors in the measurements.
SONAR?????????????????????????????????????????We use sonar on well level detection for land drainage pumping stations......if its good enough for flipper.......:D
Originally posted by osterber
Well, I've measured a couple of pools with the Disto Pro, and signed my name to several pool length certification forms as fully believing that my measurements were accurate to the tolerances specified by USMS.
No, I'm not a certified surveyor, and never claimed to be. I feel a whole lot more confident with our laser measurements than I was with any metal tape measurements we did before.
-Rick
Confidence in measurements, as you describe it to me sounds very subjective.
Professionaly speaking, when it comes to having confidence in measurements, there is no room for subjectivity. There is a set of procedures that needs be followed to assure a certail level of precision, and the set of procedures depends on a desired precision.
At that point, something measured, you don't get just 'subjectiove confidence' you get mathemathically substantiated 'level of certainty' that can be certified to 95%, 90%, 80% (etc.)
As a surveyor, I can tell you that there are many situation in which I can get much higher level confidence measurements then with a laser. There are sutuations where it just the opposite, and the laser may be the equipement of choice.
As far as you signing of on the measurements would be like me signing off on someone's coaches certification...
You don't have the authotiry on the former, I don't have theauthority for the latter.
That's just another absurdity in the pool length certification rules, as written. They require extremely (and in my opinion very un-necessary) level of precision (which does not assure the accuracy), but on the other hanmd, they accept just about anyone signing off on the measurements.
Technically, it's ludocuous, and if anyone ever seriously challenges their time or a record based on the pool length, challenging the way USMS does pool certifications in court, or arbitration, by even a half baked professional in that field will be a child's play.
So, even if it is probably very far fetched, I do see a potential legal issue here.
Couple of questions...
You say that you are confident that your measurements are within the tolerances of what USMS requires? How is that?
What;s your level of certainty, and what is your error factor? Can you plot the error elopses? How many repetitions did you use in your mesurement? How did you interpret the repetions, did you average them out, or did you use least squares adjustment?
Do you know that as written, USMS rules don't allow for any tolerances in the measurements, event the tolerance that the manufacturer claims on the equipment that you use.
Did you know that the manufacturer's tolerance indicates the best case scenario, ideal conditions in a testing lab, and that in real live measurements you can expect that error to at least double.
Did you know that by signing a 'certification' you can be held legaly liable for the accuracy of the measurement that you're unable to substantiate. Did yuou know that by signing for a measurement you may end up being found as "practicing land surveying without a license" and that you could find yourself fined (In California for example) $500 dollars, or may be imprisoned for up to 30 days... and (not that it would matter to a lay person) get suspended from ever becoming a professional land surveyor.
Anyway, I'm playing a devils advocate here, and portraying the worst case scenario... my point being, the way the measurements are done within USMS right now, if any of it ever ends up in court, that's what you can expect.
Originally posted by Sarah H
I'm curious too.
The University of Minnesota hires someone for the big meets. They are "on call" for appropriate measurements. I was told that they take many factors into consideration including humidity and water temp. Hmmm, doesn't the pool either measure-up or not? We were also told that the going rate for the "expert" to come in for a Sunday Masters meet is probably around $700. Even if they are 7 X high, who wants to pay that???
If university of Minesota has an Engineering department, that teaches civil engineering (I'd guess they do), then they probably have a qualified professional on the staff, who is capable of measuring and certifying the pool. Heck, he can even do it as the part of the "Land surveying 101" class project.
We do it was laser here in New England. We picked up a Leica Distro Pro 4a laser, which has accuracy better than required by USMS rules. I put it together on a contraption that involves some magnetic levels and a cutting board, so that you can mount the laser (and a target) in the gutter, flush to the wall, on each end, and get an accurate measurement. We measured an 8-lane 50 meter pool in about 10 minutes total.
I don't have the device with me now... I should take some digital pictures of the rig and post them. The laser device itself cost about $600.
Don't try to use anything that's based on radio waves... those things, as far as I know, are barely accurate to an inch. (These are the $50 devices you find at Home Depot.)
-Rick
Originally posted by Conniekat8
And, in gheneral, I've noticed that the USA Swimming requires that the pool be certified by a professional land surveyor, or a civil Engineer qualified to perform Land Surveys (not all of them are).
In a nutshell, by making that requirement they got it right when it comes to getting the pool certified.
This do-it-yourself measuring th pool is tanamount to a lap swimmer claiming that he'll get to the olympics by doing laps, not realizing that that there's little (a lot) more to it than just swimming laps.
Suppose you are running a 3 day state meet in a pool with a bulkhead. To be correct you would have to measure the pool 4 times. Can you give us a very rough estimate on what it would cost to have a qualified Land Surveyer to do that many measurements?
Let's say it takes 15 seconds to swim across a 25 Yard pool... (we do that in practice on 25Y sprints)
15s/25Y=0.6 Yards per second
That converts into 1.8 feet per second.
Now, the times are measured in 1/100's of a second...
At the above velocity a swimmer will tavel 0.18 feet in 1/10th of a second
and will travel 0.018 feet (or 4.5 mm) in 1/100th of a second.
Measuring something to 0.02' or 0.01' in surveying is considered high precision, and for the USMS purposes would be cost prohibitive. Plus, with the touch pads there is a lot more slop than that just in the way they are mounted...
The real question is, considering th equipement used, the pools the touchpads etc, what would be reasonable uncertainty in the finishing times for the athletes. Is +/- 0.02" in time an acceptable uncertainty? +/- 0.2" would give a total of 0.04" in time, which would translate into a pool length tolerance of 4*0.018'=0.072 or around 18mm (almost 2CM). With the touchpads and a tape strapped across the pool, it's going tpo be very hard to determine the pool lenght (at the time the events had been swum) to that level.
Touchpads bow, bulkhead may shift, and there are errors in measurements, and I've seen the condition of the tapes that are often used, kinks, rust etc... I wonder if the tapes were to get calibrated, how they'd hold up.
Realistically, from what I've seen I would make an guesstimate that the measurements that are being perfoprmed right now are around +/- 2 cm (4cm or just under 2" total potential length error.)
But that's just the guesstimate. To be able to trell for sure, we'd have to have a set of comparative measurements done.
Anyway, the question remains, what is the acceptable uncertainty in the finishing times? That will determine how, and to what precision the pools need to be measured.
And yes, there will always be uncertainty in finishing times.
Sorry to burst some people's bubbles. Just because something is measured to 1/100th of a second, doesn't mean that the measurement is necessary reflective of reality, as we see it.
Hey Jeff,
Hypothetically speaking... Do you have access to a theodolite or a total statin or something similar, and if your local pool(s) needed to be measured, would you be able to donate that service?
What are the 2 man survey crew hourly rates in your area?
Short pool example:
If a 25y pool is ½” short…
Let’s take the example of a swimmer going 1:00.00 for a 100y race in this short pool. Therefore he/she is traveling 2 inches short of the full 100 yards (100 yards = 3600 inches):
(60 s)/(3598 in) = (? s)/(3600 in)
At this speed the swimmer’s time would be 1:00.03 if they had to swim the extra distance. Some of the USMS pools that are coming up short have been of the magnitude of ¼” inch so the extra time would be halved.
I'm not posting this to convey any opinions - just showing the math because the question was raised.
Jeff Roddin, P.E.
(Registered Professional Engineer, which means I may sign off on the USA Swimming pool measurement form since they require either a certified surveyor or a professional engineer.)
So you get in the pool and hold the tape against the walls, 1 foot below water level. The problem is that the tape sags, and any measurement will overestimate the length of the pool by a significant amount.
I know I'm dragging up a hella old topic here, but I did this just the other day.
The arc length of a catenary that is 75 feet of horizontal length with a 6 inch sag is 75 feet 0.1 inch.
easycalculation.com/.../cable-sag-error.php
so 6 inches of sag is only adds a tenth of an inch to the measurement, that is actually within the measurement error standard for the measuring device as indicated on the pool measurement certification form.
That's kind of a worst case, if you can convince Fred to stand in the middle and hold up the tape level so it doesn't sag as much, you'll do even better.
I linked to this old thread while reading the current one on "pool length" and could not resist offering a simple solution to the problem. I have built more than a hundred houses over the years and currently oversee construction for Habitat for Humanity-Maui. Our new homes are usually simple rectangles and we follow the industry standard of setting up batter boards off each corner. The batter boards are all set to the same level and enable us to pull masons line to outline the perimeter of the building. It is simple to "tune up' these strings to ensure squareness and length of sides, sometimes to "fanatic out" we will even take into account the thickness of the line, for example using the inside of the string as our guide.
Seems like measuring a pool would be similar. the goal being to set two parallel lines directly over the most inward part of each of the opposing walls. Then just measure the distance between the strings making sure the tape measure runs square to the lines and does not sag, not difficult since we could pull the tape on the deck on either side of the pool.
