The Art of Pool Length Measurement

Thanks, NASTISWMR - I'll check out Lowe's or Home Depot first. $30.00 sounds like a bargain. (By the way, who are you? You must be either Doug, Melissa, Cheryl, or ***?) Phil, the new rule states that a pool with a movable bulkhead must be measured at the beginning of the meet and again at the conclusion of the meet, just in case someone tightens the lane lines during the meet.

Thanks Anna, I knew that . . . :o Michael - The pool will contract and expand with temperature also. I do not know if it will expand or contract at the same rate as the tape, however. Anyway, if the pool water is maintained at competition temperature (and the tape in or on the water) it should not be affected by temperature too much. The measurements should really be done with the pool full (is that required?) because the pool will be a different length if the pool is full of water, than when it is not. This would be particularly true for deep pools. It is good that the rules specify a length and a way to measure it. Suppose you measured you pool to be 50.000 - 0.000 m following the rules with a steel tape with 0.005 resolution. Then someone else come along with a super-duper laser system with a 0.001 m resolution, and measures the pool to be 50.998 m. No conflict - the second system has a better resolution and the measurements are consistent. Is the pool still legal? You bet, because the method of measurement is defined. That is why this question, how to measure the length? while geeky, is still important.

I have heard of a couple of cases where tightening the lane lines cause the bulkhead to bow, making the pool too short, especially in the center lanes. If the bulkhead tends to do that, the pool should be measured after the lane lines are installed, every time.

For reasons that should be obvious, I need to not comment about most of what goes on here because one never know when one would be asked to intervene or interpret but I felt a story here was useful. For a number of years now, I have been officiating at the USA-S Sectional meet that one and now both of my daughters have been competing. A couple of years ago, I was pulling duty as the turn judge at the turn end during the 1000 Free on the first night of the meet. The meet was being conducted in a 50 meter pool with two bulkheads set at 25 yards. I sat down on the corner of the pool and looked down to watch the turns and quickly noticed that the bulkhead was far from being straight. In fact the bow in the center of the pool was on the order of 4-5 inches! (In the 1000 that amounts to over 13 feet difference.) At the time I was not one of the assigned crew, so I got the attention of one of them and explained what I saw. Now at least half of the event had already been contested. No one did anything and I didn't push it that night. The next morning, I went up to the Meet Referee and simply said "You have a problem". After he said "I do?", I brought him over and showed him the bulkhead. We then scrambled to do what we could. The first thing that happened was the double lane lines were reduced to single lane line that were only as tight as they needed to be. That took care of about half of the buldge. Prelims were run in that configuration. Between prelims and finals, the maintenance crew came in with two come-alongs, broke open the back of the bulkhead and pulled the two bulkheads together in order to straighten the competitive one. The meet then proceeded with what was close to a regulation course. Things like this happen and to this day, I wonder how many NCAA cuts were made in the center lanes of that pool.

This subject is aptly titled. We have been discussing the "art" of pool length certification. After measuring a 25 yard course with steel tape measure it became obvious to all involved that a laser device has several advantages (accuracy, speed, etc). It's also pretty hard to imagine measuring a 50 meter pool with a 200 foot steel tape and keeping it tight even and just on top of the water so that the measure is accurate. So, NEM is seriously considering purchasing a laser device. One NEM found a: The DISTO Pro4a. Specs says it is +/- 1.5mm. The USMS rules allow for +/- 0.005 m which is 5 mm. $725. Others seem available within the allowable tolerance for a few hundred dollars less but we are inclined to get the slightly more expensive one. Another NEM wondered how one mounts both the laser and "target" such that they are directly above the wall. We strongly suspect that others out there have experience with laser devices and this might be of general interest as we come into the LCM season and I can't imagine many of you meet directors are relishing the idea of measuring a LCM pool with a steel tape. Any experience with laser measuring devices and how to use them properly?

more

Wayne, I am curious as to the physics behind the inability of a laser to measure over water. The laser is a parallel beam of light with very little divergence, especially over the distances we are talking about. Therefore, there should be no reflection off of the water. At least this should be the case with the beam going from the laser to the other end. Now, maybe there is a problem if the target causes the beam to diffuse but given that the instrument averages multiple measurements and the water is not likely to be still (to the same degree as the beam is coherent) then the noise should average out. Did he/she give you an explanation? Leo

I don't know anything about these laser sighters, but I do know a little physics. The speed of light depends on the medium it passes through (air, glass, water, etc.). This will cause all sorts of interesting phenomena. Think of a mirage on a hot day. :cool: So it sounds like the argument is that the optical properties of the layer of air just above a body of water are different enough than the surrounding air (due to temp, humidity, etc.), that it can throw off these instruments. Or maybe I'm barking up the wrong tree. :) (Connie, how would you get a more accurate distance measurement? You mentioned higher-end non-laser based instruments. I ask, because the definition of a "meter" used to be a physical bar of platinum stored somewhere, and is now based on a wavelength of light.)

Originally posted by breastroker SPMA discussed this with a mechanical engineer who knew about laser measurements, and they quickly added that measurement over water was impossible. Reflections from the water supposedly ruin the accuracy. I have not looked at the laser device web sites listed, do they mention measurement directly over water? The engineer added we should use a surveyor, only $300-$500 per pool. SPMA has used a special steel tape that uses a tension device to determine droop and maintain accuracy. The tape is over 20 years old, and very high quality. I guess we got our monies worth out of it:p Hi Wayne, I'll have to talk to you and Steve about this in private.... Actually, I already started dialoging with Steve about this... :)

Unfortunately, this discussion is split between two threads. So to answer questions on both: The reality of what we are trying to do is that we cannot build pools that are either precise or accurate enough to say that the difference in time that we measure (0.01 second) is meaningful (if we assume the USA-S maximum length of +0.03 meters) for anything other than a sub 20-second 50-yard freestyle. So given that, what do we do? The best we can or get faster :) I have done the math and actually wrote a white paper that we used in our discussions about this issue. Part of the problem is that we have a lot of history from USA-S. By trying to do our best, we attempt to eliminate pools that are without a question short. Most errors using a tape will cause the measurement to be longer than reality and therefore few pools should be falsely eliminated. There are two obvious exceptions to this: a bad tape and measurements in a high temperature. The former should be checked if one fails a pool and the latter can be corrected if necessary. As far as the errors the other way, a sagging tape was mentioned. At the 50-meter distance, the amount of sag necessary to make a significant difference is actually quite a bit (> 0.3 meters). Ignoring that the sage is not triangular (which means that the calculation overestimates the error), the error is less than 4 mm. With half that sag in a 25-meter pool, the error is under 2 mm. Someone stated that USMS has no tolerances. The tolerance is -0.00 meter. That means that the pool must be less than 0.005 meters short. We set no maximum (after some debate) because we did not want to exclude any more pools than necessary. Lasers have always presented a dilemma. Some bodies want to use them exclusively and I have advised them otherwise. I know that USA-S has used them for certifying a number of pools used in their championship and grand prix meets. Personally, the work required to set one up makes me want to go buy a tape. I attended the "Our Kids Initiative" meeting last weekend in Indianapolis. In attendance were representatives from NFHS, NCAA, YMCA, NISCA, USA-Swimming and USMS. This was our first year being invited to this three-year-old gathering. The first topic on the agenda (at least the real part) was certification of pools. After the mess in Virginia Beach last year, it has become a major topic. Unfortunately, USA-Swimming doesn't have to worry too much since they don't set too many records. These other organizations only worry about the "big meets" and they do go out and spend the money to have a professional do the measurements. This includes measuring the bulkhead for every session if necessary. So the bottom line here, IMHO, is that there are no simple answers even if you define what the true goal is. There are no acceptable answers to some of the questions. Leo