PDA

View Full Version : How do you measure the length of a pool?



Philip Arcuni
February 12th, 2002, 01:46 PM
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?

GZoltners
February 12th, 2002, 02:03 PM
How about measuring along the top of a competitive lane line? Those get cranked pretty tight and are also floating.

How about let the measurement instrument sag and use hyperbolic functions to determine the length? You can measure the drop from vertical using a laser and a ruler.

Swim fast,
Greg

jim thornton
February 12th, 2002, 02:26 PM
Phil,

I recently was asked to give a toast in celebration of the immortal memory of Robert Burns, poet laureate of Scotland. In the course of my research, I discovered that Burns--the so-called Ploughman's poet who spent a good deal of his life as a farmer--came up with a classic method of measuring the weight of a hog. I realize this is somewhat different from measuring the length of a swimming pool, but perhaps his method might be somehow adapted to the latter pursuit.

In any event, here's what Burns suggests:

(1) Get a perfectly symmetrical plank and balance it across a sawhorse.

(2) Put the hog on one end of the plank.

(3) Pile rocks on the other end until the plank is again perfectly balanced.

(4) Carefully guess the weight of the rocks.

Sarah H
February 14th, 2002, 10:08 PM
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???

sparx35
May 17th, 2004, 05:19 PM
this should have been done when built..the pool regs require this....seek the installer...!!
ps sorry this reply is a few yrs late....ive bin outta town...:D

nyswim
May 17th, 2004, 05:36 PM
Actually I saw them measuring the pool at Nationals. It appeared that they were floating the tape on some sort foam blocks in the lane and then pulled the slack out to take the measurement. I only saw them in the end lane, not sure how (or if the do) measure the center also.

knelson
May 17th, 2004, 06:38 PM
Step one: throw away tape.
Step two: buy laser.

Conniekat8
May 17th, 2004, 08:00 PM
Originally posted by Philip Arcuni
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?

Well, It just so happens that I'm a Land surveyor (in addition to being a civil Engineer), we;re experts, and the last word on doing theese things (according to the laws and most civil codes, and according to the Professional Land Surveyors Association etc...).
I've already told the guys at the SPMA I'll look at the requirements, and write down the procedure and recommend how it should be done etc...
I'd love to write more about it, but I'm short on time right now.

Conniekat8
May 17th, 2004, 08:03 PM
Originally posted by knelson
Step one: throw away tape.
Step two: buy laser.

Nope. No Lasers.
For one, you can't do it accureately enough with a home depot laser... There is pretty severe signal dispersion and mis-reflection near water. You get a ton of false readings.

You may have meant the EDM (Electronic distance meter) which works based on radio waves... It's very different from a laser.
But, no need to buy a $50,000 dollar piece of equipement.

Conniekat8
May 17th, 2004, 08:06 PM
Originally posted by GZoltners
How about measuring along the top of a competitive lane line? Those get cranked pretty tight and are also floating.

How about let the measurement instrument sag and use hyperbolic functions to determine the length? You can measure the drop from vertical using a laser and a ruler.

Swim fast,
Greg

Each tape, when properly tensioned has a factor for it's own 'catenary sag' that can be calculated.
The problem with tapes around pools is that you can't tension it properly, and what you end up with by just hand pulling is anyone's guess.

If it's to be done with a tape, ot should be done with 4 range poles, an offset tape (layiong flat on the side of the pool, an no worries about the sag) and couple of right angle prisms... But, again, it has to be done properly.
it's called 'station and offset' measuring, and it would be much more precise and accurate than what I see being done now.

Conniekat8
May 17th, 2004, 08:13 PM
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.

aschueler
May 17th, 2004, 09:05 PM
Cool discussion -- I am learning a lot. Keep it going :)

Howard
May 18th, 2004, 07:15 AM
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?

osterber
May 18th, 2004, 09:48 AM
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

Conniekat8
May 18th, 2004, 01:48 PM
Originally posted by Howard


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?

It depends on what part of a country you're in, as their hourly rates vary drastically, depending on a local economy and local real-estate values.

For the measurements, the stuff a surveyor would have to do is pretty basic (compared to the other stuff they normally do), they would probably charge you about an hour to do this, it would be two guys for an hour... (it would take less then that, but most would charge a minimum of an hour) each time they come out.
You want to hire someone who is close, not someone who would have to drive two hours to get to the pool.

In Southern California, in LA area, a 2 man crew wouldcost around $180 per hour, 200 miles away, the prices can be down to a $100/per hour for a 2 man crew.
In other parts of the country, with different rates, it could be as low as $50 per hour for a 2 man crew.

If it's on a weekday, near the end of the day, and you make friends with a local surveyor, they may be able to just swing by and do the measurememnts really quickly for you... as a donation, or as a favor.

Conniekat8
May 18th, 2004, 02:08 PM
Originally posted by osterber
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

I'm quite familiar with Leica products, as well as with Geodimiter, Trimble, Carl-Zeiss and many others.
This is a typical piece of equipement that a surveyor would bring with him:
http://www.servco1.com/con_ts.htm
or:
http://www.servco1.com/theodolites.htm or: http://www.servco1.com/ts.htm

'those things based on radio waves' you're talking about are sophisticated pieces of equipement that run in the range of $ 50K to 80K or more, and are used for high precision surveys. Not something that you pick up at Home Depot, not something that's even sold there.

It's Disto-Pro (drop the "r"), as in Distomat-Pro, which is a Low end construction grade piece of equipement.
http://www.auto-met.com/leica/distopro.htm
(still, better than the tape) based on the written USMS requirements of the required precision of the measurement, a construction grade instrumen't won't do. Then again IMO, those requirements need to be rewritten... By a professional.
I read and re-read and re-read the USMS rules on the measurements, and I can guarantee you that the device you have does not fit the rules... Even though the device may be adequate for what USMS realistically needs, it's the way the rules are written.

Back to the Disto-Pro, still, you have the issue of laser near the water, and also, even though the equipment claims to be +/- 1.5mm, depending on how you line it up, you're going to get bigger discrepancies than that.

Howard
May 18th, 2004, 02:40 PM
Originally posted by Conniekat8


It depends on what part of a country you're in, as their hourly rates vary drastically, depending on a local economy and local real-estate values.

For the measurements, the stuff a surveyor would have to do is pretty basic (compared to the other stuff they normally do), they would probably charge you about an hour to do this, it would be two guys for an hour... (it would take less then that, but most would charge a minimum of an hour) each time they come out.
You want to hire someone who is close, not someone who would have to drive two hours to get to the pool.

In Southern California, in LA area, a 2 man crew wouldcost around $180 per hour, 200 miles away, the prices can be down to a $100/per hour for a 2 man crew.
In other parts of the country, with different rates, it could be as low as $50 per hour for a 2 man crew.

If it's on a weekday, near the end of the day, and you make friends with a local surveyor, they may be able to just swing by and do the measurememnts really quickly for you... as a donation, or as a favor.

The added cost of up to 700+ per meet might be enough to keep a smaller local meet from actually being held. I don't know who wrote the original rules but it may be that they didn't want to write them in such a way that the meet host was going to have to face the choice of having the meet or having the pool measured. On the other hand, given the way it's written and how hard it is to accurately measure with a tape it's questionable if the results are worth the paper they are written on.

Conniekat8
May 18th, 2004, 03:35 PM
Originally posted by Howard


The added cost of up to 700+ per meet might be enough to keep a smaller local meet from actually being held. I don't know who wrote the original rules but it may be that they didn't want to write them in such a way that the meet host was going to have to face the choice of having the meet or having the pool measured. On the other hand, given the way it's written and how hard it is to accurately measure with a tape it's questionable if the results are worth the paper they are written on.

You got my point there!!!
One of the things I have to do, that I haven't gotten around to yet is figure out just how much time difference an inch or half an inch or few milimeters make, time wise at certain swimming velocities, and from there come up with a reasonable set of tolerances in the pool length measurements.
Measurements will NEVER be exact, there is always some slop in them. The important part is to come up with an acceptable +/- tolerances. Yes, that would mean that some of the finishing times cary a certain level of uncertainty.

There is a a large volume of literature written on 'error theory' and how to make adjustments for them etc... Something I had to study at length for the business I'm in.

One of toughest thing for 'lay' people to understand is that the measurements and results will never be 'exact', but that they're still good, and comparative.

Explaining all this in detail can be a very lengthy process... I wouold like to, but I really can't afford the time to get into an in depth discussion about it here. I'm sorry.

Phil Arcuni
May 18th, 2004, 03:47 PM
A long time ago I participated in this discussion, arguing that an excessive attention to pool length was both unnecessary and harmful to the availability of pools and competitions.

Boy, did I get slammed!

I followed that up by arguing that measuring a pool to the accuracy desired was both difficult and expensive, and I implied that almost everyone was unqualified to do it (nothing has changed my mind on that, either.)

Once again, I was at my most controversial (evidently (and except for the very long ago 'America' thread).)

After the pool length controversy, I have seen at least one local pool no longer able to host sanctioned Masters events, even though it had been doing so for at least 20 previous years (and I think USS continues to hold meets there.) Every local pool that hopes to host a meet has been measured with a steel tape by local USMS officials.

I now take the attitude that most people seem satisfied with the method of measurement, and there are pools that can still host meets. The last thing we need is to remeasure pools and either disqualify more pools or make the whole process more expensive.

This is one case where the right way is the wrong way.

Phil Arcuni
May 18th, 2004, 03:55 PM
You got it, Connie, but this is a very unresponsive group to any even-not-very sophisticated uncertainty analysis. Believe me, I tried.

And you are also right in your intuition that it does not make much of a difference -- even in the most extreme case the effect is in the thousandths of a second.

Mark in MD
May 18th, 2004, 04:16 PM
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!

Conniekat8
May 18th, 2004, 04:22 PM
Originally posted by Phil Arcuni
You got it, Connie, but this is a very unresponsive group to any even-not-very sophisticated uncertainty analysis. Believe me, I tried.

And you are also right in your intuition that it does not make much of a difference -- even in the most extreme case the effect is in the thousandths of a second.

Yeah, error theory can be tough even for a fair amount of left-brainers to wrap their mind around. With right brainers, you can pretty much forget it.

It does not make much of a difference, probably till you get to around an inch...
Then again, if they want to be that accurate, then they have to go away from the touch pads, and come up with some sort of a photo finish... Of course, that wouldn't work with flipturns, would it??? ;)

The more I think about how it all works... Even though there is a big disparity (or in my professional opinion HUGE disparity) between what the rules say, and how it's actually being done, they're probably fine - talking practically.
When you look at it theoretically, it's all goofy.... but hey, it works.

Watching it, for me it's got that nails on the chalkboard effect, so, if or till they ask me to help out, I just have to turn the other way.

You know, it's like watching a lap swimmer with a snorkle, paddles and fins doing laps with open turns, and calling it competetive training and saying they're gonna get a fastskin to help them get faster.

I'm also willing to bet that most of the bulkheads get places in a much mpore accurate position than, with current methods being used, are able to measure them.

Conniekat8
May 18th, 2004, 04:27 PM
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.

osterber
May 18th, 2004, 04:46 PM
'those things based on radio waves' you're talking about are sophisticated pieces of equipement that run in the range of $ 50K to 80K or more, and are used for high precision surveys. Not something that you pick up at Home Depot, not something that's even sold there.


Those things based on radio waves in the consumer market are what I'm referencing. Products such as this:

Irwin Laser Tape at Amazon.com (http://www.amazon.com/exec/obidos/tg/detail/-/B0000ALR30/qid=1084912880/sr=8-1/ref=pd_ka_1/103-7736714-2324606?v=glance&s=hi&n=507846)

Those _are_ around $50, and yes, they _are_basically worthless for anything more detailed than an inch or two.



Back to the Disto-Pro, still, you have the issue of laser near the water, and also, even though the equipment claims to be +/- 1.5mm, depending on how you line it up, you're going to get bigger discrepancies than that.

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

sparx35
May 18th, 2004, 05:05 PM
SONAR?????????????????????????????????????????We use sonar on well level detection for land drainage pumping stations......if its good enough for flipper.......:D

Conniekat8
May 18th, 2004, 05:37 PM
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.

Conniekat8
May 18th, 2004, 05:40 PM
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.

jroddin
May 19th, 2004, 11:37 AM
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.)

Conniekat8
May 19th, 2004, 01:06 PM
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?

Maui Mike
March 3rd, 2013, 12:49 PM
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.

Kevin in MD
March 3rd, 2013, 09:23 PM
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.
http://easycalculation.com/analytical/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.