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Why is measuring a swim course so difficult?
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 2009-06-29 9:40 AM |
Veteran  196   Arlington, VA |  Subject: Why is measuring a swim course so difficult?I'm new to triathlons, but it seems as though half the race reports I read have complaints that the swim was either too long or too short. And it's not just rinky-dink local triathlons, but large races such as the Longhorn 70.3 a year or two ago. I'm familiar with the USATF road running calibration procedures using a calibrated course and bicycle. For a swim, it seems to me that's it's rather simple to go into Google Earth and stake out a course using the path function to within centimeters of the actual distance. You then take the corner points and end points, download them into a handheld gps, grab a boat and buoys, and drop the buoys into the lake at the correct spot. Obviously there's a slop factor of slight drift of the buoys due to currents and anchor ropes that are longer than the lake is deep. But as a neophyte, I think I could layout a 1500m swim course within 1% or 2% on my first try. It should be easier than measuring a biking or running course because there are no curves and shortest possible distance issues to deal with. Given the repeated problems with swim course length, I'm somewhat nervous about the Nation's Tri this fall in the Potomac. They use two mirror image courses, so one group heads upstream first and returns downstream to the start, while the other group heads downstream and then returns upstream. What are the chances that the courses are anywhere close to each other? I'm not fast enough to be going for prizes, but I still want an accurate race measurement. Edited by kcb203 2009-06-29 9:41 AM |
| 2009-06-29 9:55 AM in reply to: #2249865 |
Cycling Guru 15134 Fulton, MD | Subject: RE: Why is measuring a swim course so difficult? Buoys move after they are placed often. GPS systems are not THAT accurate in the big picture and there is a margin of error built into them. For Nations, it is easier to measure because it is a point to point and you can measure on land adjacent to see what the distance will be. Just go into your race knowing that EVERYONE is swimming the same distance (assuming you swim the course straight and are not all over the map). So you are still going to put up the same sort of number proportionately as you would in other races in relation to the competition. |
| 2009-06-29 10:00 AM in reply to: #2249865 |
| over a barrier | Subject: RE: Why is measuring a swim course so difficult? I think generally think they're pretty accurate within 100yds or so....I don't buy race reports claiming swim distance was X off.....I'm sure there are some outlies and I know of one RD that doesn't even try to measure the course but the vast majority are pretty accurate. |
| 2009-06-29 10:01 AM in reply to: #2249865 |
Master 2407 Bellevue, WA | Subject: RE: Why is measuring a swim course so difficult? There's probably a lot of non-obvious challenges and problems to course layout that us Monday morning quarterbacks don't see from our armchairs. But I'm fairly skeptical of all the race reports that say the "swim was long" when a lake course is involved. I'm aware that buoys move more than you might think (anchor dragging and all that), but still. Since you're new to triathlons, you've probably not heard the story of the first Ironman Californa in 2000. This was a a full length Ironman, taking place around Camp Pendleton, with the swim a 2 loop affair in Del Mar Basin. The Navy measured the course. If you ask a Navy team to measure out a 2.4 mile course, that's exactly what you'll get - a 2.4 nautical mile course. 600 yards longer than it should have been. Edited by brucemorgan 2009-06-29 10:02 AM |
| 2009-06-29 10:05 AM in reply to: #2249923 |
Champion 6046 New York, NY | Subject: RE: Why is measuring a swim course so difficult? running2far - 2009-06-29 11:00 AM I think generally think they're pretty accurate within 100yds or so....I don't buy race reports claiming swim distance was X off.....I'm sure there are some outlies and I know of one RD that doesn't even try to measure the course but the vast majority are pretty accurate. go look at the Patriot long course website. The swim was 1 mile not 1.2 due to measuring difficulties - that is for those who swim straight. Me? I took a whole tour, and swam AT LEAST 1.2. Even said hi to the kayakers. |
| 2009-06-29 10:10 AM in reply to: #2249865 |
Veteran 183 Grand Rapids, MN. | Subject: RE: Why is measuring a swim course so difficult? I live on the bike portion of our local tri, and regularly swim in the lake. They have actual cement blocks they leave in the lake to attach to, and it is the same every year because of this. And times totally depend on the weather conditions, and I guess how straight you swim. When it's choppy and prevailing winds west to east are present the times are always better. We fight the wind going out a little, but on the way in it's like riding surf and you fly. Yet people still assume something was different. |
| 2009-06-29 10:44 AM in reply to: #2249865 |
Extreme Veteran 3177 | Subject: RE: Why is measuring a swim course so difficult? To me, distance does not matter until I learn to swim arrow straight. Once that happens then really, who cares. Every course, even the same course on different years, will be different. As long as everyone there is swimming the same course, that day, I am fine with a course long or short. |
| 2009-06-29 11:07 AM in reply to: #2249865 |
Master 1651 Breckenridge, CO | Subject: RE: Why is measuring a swim course so difficult? kcb203 - 2009-06-29 8:40 AM For a swim, it seems to me that's it's rather simple ... I agree with you. I think for some reason it's accepted within triathlon for the distances to be greatly "estimated". In the pre-GPS days it was difficult to accurately measure courses but not any more. For a swim course all you need is a hand-held GPS (to get w/in 50' or so) and two compass bearings off a couple points ~90 degrees apart to double-check the corners. Bike/run courses are easy now with tools like BT's Route Tracker. Bike courses are the hardest since you're restricted to roads and racers prefer loops to out-and-backs. If you're trying be exactly accurate (like the 5430 Boulder Peak), you probably end up with funky (annoying) little out-and-back spurs off your loop to get the distance exact. Also, I have no proof but I also think that some RDs make the distances short on purpose so that the particpants will be happy with their times and therefore look back favorably at the race. Don't underestimate the promo value of everyone's oly PR being at your race... In terms of swim distances being long or short I know with 100% confidence it happens often. I swim straight and I know exactly what pace I'm swimming in open water with my wetsuit/effort. Since I've been back into tri's (04/25/08), I've done seven tri's. Two (29% and listed below) were grossly off on the swim distance. It's a small sample and those two could be anomolies but I don't think so. "Tri-the-creek Spint 2008", the swim was so long that I beat guys barely in the OA (because I'm swim strong) that I'd normally come in about 3-4 minutes behind in a sprint. The course was at least 200y long (+25%). "Longhorn 70.3 2008" the course was grossly short so that my pace was at 1:10/100y (start to beach) which is at least 10 seconds too fast. But as Daremo said, in your race since they'll have groups swimming different courses and they can measure both distances on land then just line up the buoys between two points on each side of the river, it's a no-brainer. I'd bet they're taking great care to make sure the courses are the exact same distance. |
| 2009-06-29 11:51 AM in reply to: #2249865 |
Elite 3088 Austin, TX Gold member | Subject: RE: Why is measuring a swim course so difficult? kcb203 - 2009-06-29 9:40 AM Obviously there's a slop factor of slight drift of the buoys due to currents and anchor ropes that are longer than the lake is deep. But as a neophyte, I think I could layout a 1500m swim course within 1% or 2% on my first try. It should be easier than measuring a biking or running course because there are no curves and shortest possible distance issues to deal with. Give it a try. First, even the most accurate GPS technology, WAAS, is accurate only to within 7 meters 95% of the time. Garmin themselves only claim an average accuracy of 3 meters for devices using it. If you're using straight satellite-based GPS, accuracy is 15 meters on average. For a simple out and back Sprint course, you're almost certain to be off by a minimum of the 1-2% you claim. Add another turn buoy for a triangular or rectangular course and you've increased your margin of error. Second, you might be surprised by how much the buoys actually drift. You may be able to anchor the buoys to the bottom of the lake, but if you're using a large, visible float, you better anchor it really damn well. Tying it to a concrete block isn't going to do the job. There's a lot of lateral force generated on them from even a moderate breeze. Oh, and if you manage to get them anchored well, you're still screwed if the wind shifts, because you're anchor is going to pull free. The winds were pretty strong the morning of the Longhorn race you mention and the buoys drifted quite a bit between when Keith and his crew positioned them and when the race started. |
| 2009-06-29 1:17 PM in reply to: #2250296 |
Extreme Veteran 417 Davidson | Subject: RE: Why is measuring a swim course so difficult? "First, even the most accurate GPS technology, WAAS, is accurate only to within 7 meters 95% of the time. Garmin themselves only claim an average accuracy of 3 meters for devices using it. If you're using straight satellite-based GPS, accuracy is 15 meters on average. For a simple out and back Sprint course, you're almost certain to be off by a minimum of the 1-2% you claim. Add another turn buoy for a triangular or rectangular course and you've increased your margin of error. Second, you might be surprised by how much the buoys actually drift. You may be able to anchor the buoys to the bottom of the lake, but if you're using a large, visible float, you better anchor it really damn well. Tying it to a concrete block isn't going to do the job. There's a lot of lateral force generated on them from even a moderate breeze. Oh, and if you manage to get them anchored well, you're still screwed if the wind shifts, because you're anchor is going to pull free. The winds were pretty strong the morning of the Longhorn race you mention and the buoys drifted quite a bit between when Keith and his crew positioned them and when the race started." Dennis, I tend to agree with the OP. It really wouldn't be too much trouble to lay out a pretty accurate course especially if WAAS is available. Even my handheld GPS unit with WAAS capability gives me an accuracy of 7'-8'. I have complete confidence that I could set out a course for our local sprint tri that would be accurate within 50' in less than an hour with my handheld GPS unit, a boat, and two others to handle the weights, tethers, and floats. I agree that bouys can drift, IF they haven't been properly secured to adequate weights. Also, you can't set the tether at a length that leaves no slack to account for variations in lake levels or tidal effects, so you are going to have at least a little bit of bouy drift from the position where the weights were dropped. By the way, using an anchor would be the wrong thing to do since an anchor relies upon a sideways pull to set and hold its position on the bottom. Depending on the type of anchor and bottom conditions, it may require a 8 or 10 to 1 scope, the ratio of depth to lateral distance from the anchor point. That sort of ratio would clearly create all sorts of inaccuracies for use in a triathlon since the bouy would have to be 8 or 10 times the water depth away from the anchor. And you are right, if the wind shifts, the bouy will swing to a new position and probably drag the anchor at least a little bit until it sets again. Regardless, the truth is everyone is swimming the same distance, so it shouldn't be a big issue until somebody drowns in the last 200 meters of a course that was 300 meters too long. Then, that 'hold harmless' clause in the registration materials will get fully tested. If I were a race director, I would just go ahead and set it up to be as accurate as reasonably possible to make everyone happy. At least you would have a defensible position if you ever got called out on it or, heaven forbid, you had to testify as to how you laid out the course. JMHO, Greg |
| 2009-06-29 1:30 PM in reply to: #2250657 |
Veteran 196 Arlington, VA | Subject: RE: Why is measuring a swim course so difficult? It really wouldn't be too much trouble to lay out a pretty accurate course especially if WAAS is available. Even my handheld GPS unit with WAAS capability gives me an accuracy of 7'-8'. Furthermore, the accuracy of GPS on an open lake is better than typical because it almost certainly has a clear view of the sky with no obstructions. Also, the errors will usually be offset in the same direction, so the overall error is about the error for one buoy, not a cumulative error. |
| 2009-06-29 2:10 PM in reply to: #2250709 |
Elite 3088 Austin, TX Gold member | Subject: RE: Why is measuring a swim course so difficult? kcb203 - 2009-06-29 1:30 PM It really wouldn't be too much trouble to lay out a pretty accurate course especially if WAAS is available. Even my handheld GPS unit with WAAS capability gives me an accuracy of 7'-8'. Furthermore, the accuracy of GPS on an open lake is better than typical because it almost certainly has a clear view of the sky with no obstructions. Also, the errors will usually be offset in the same direction, so the overall error is about the error for one buoy, not a cumulative error. Wrong on both counts. |
| 2009-06-29 2:42 PM in reply to: #2250853 |
Champion 8540 the colony texas | Subject: RE: Why is measuring a swim course so difficult? considering there other thread about people swimming straight, is the reason I believe that most people think the swim course is long IMHO of course... I"m amazed at how many people during a swim that are swimming perpendicular to me ?????? or are waaayyy off.. It's rare that I hear people say a swim is short... There is one local group that has a long sprint/Oly where they admit the swim is a bit short. And Like someone pointed out if the wind kicks up or shifts you can have some drift of the bouys. I think some RD's will actually use a lazer measurement just before the start to double check also. |
| 2009-06-29 3:32 PM in reply to: #2250853 |
Extreme Veteran 417 Davidson | Subject: RE: Why is measuring a swim course so difficult? dgunthert - 2009-06-29 3:10 PM kcb203 - 2009-06-29 1:30 PM Wrong on both counts.It really wouldn't be too much trouble to lay out a pretty accurate course especially if WAAS is available. Even my handheld GPS unit with WAAS capability gives me an accuracy of 7'-8'. Furthermore, the accuracy of GPS on an open lake is better than typical because it almost certainly has a clear view of the sky with no obstructions. Also, the errors will usually be offset in the same direction, so the overall error is about the error for one buoy, not a cumulative error. Dennis, With all due respect, I am not new to GPS. I have been using it for a lot of years in aviation and WAAS capabilities were developed specifically for use by aircraft to make precision approaches to airports without expensive ground based radio navigational transmitters. As an instrument rated pilot, I can tell you that with a WAAS signal, it is absolutely possible to position a bouy within 7'-8'. That degree of positioning is absolutely vital when you are landing an aircraft in minimum visibility conditions on a runway that may only be 10' wider than your wingspan. The WAAS corrected GPS guidance is also tested and certified by the FAA before the approach can be published for use by the public. So, if a WAAS corrected signal is available, any RD could lay out a very accurate swim course. Even if a WAAS corrected signal was not available, I dare say any RD could make their course much more true to the published distance if they took the time to do a little planning and use a handheld GPS unit. Greg |
| 2009-06-29 5:15 PM in reply to: #2249926 |
Extreme Veteran 419 San Jose, CA | Subject: RE: Why is measuring a swim course so difficult? brucemorgan - 2009-06-29 8:01 AM But I'm fairly skeptical of all the race reports that say the "swim was long" when a lake course is involved. I'm aware that buoys move more than you might think (anchor dragging and all that), but still. If you were in a late wave at Boise 70.3 this year, you had a chance at a long swim because the 2nd turn buoy broke free and floated away  It took them a while to collect it, deflate it, and replace it with one of the regular buoys. In the meantime a lot of people (who were already getting tossed about by horrendously bad chop) swam in the direction they thought was correct while the buoy floated away from the course. |
| 2009-06-29 7:15 PM in reply to: #2249865 |
Regular 108 | Subject: RE: Why is measuring a swim course so difficult? This was addressed in the latest newletter from Max Performance:
Swim |
| 2009-06-29 7:25 PM in reply to: #2251488 |
Elite 4564 Boise | Subject: RE: Why is measuring a swim course so difficult? muppetdog - 2009-06-29 4:15 PM brucemorgan - 2009-06-29 8:01 AM But I'm fairly skeptical of all the race reports that say the "swim was long" when a lake course is involved. I'm aware that buoys move more than you might think (anchor dragging and all that), but still. If you were in a late wave at Boise 70.3 this year, you had a chance at a long swim because the 2nd turn buoy broke free and floated away It took them a while to collect it, deflate it, and replace it with one of the regular buoys. In the meantime a lot of people (who were already getting tossed about by horrendously bad chop) swam in the direction they thought was correct while the buoy floated away from the course.That was so annoying. I couldn't understand why the 2nd length of the square seemed like it was long. |
| 2009-06-29 11:03 PM in reply to: #2249865 |
| Member 44 Calgary | Subject: RE: Why is measuring a swim course so difficult? In the oil and gas pipeline industry, sub meter accuracy GPS is very commonly used for locating specific sites. Some time sub foot accurarcy GPS meters are used when that level of detail is required. Granted, these devices are not your garden variety Garmen GPS units, but, my point is, GPS can be accurate to less than a foot. |
| 2009-06-30 10:33 AM in reply to: #2251748 |
Master 2407 Bellevue, WA | Subject: RE: Why is measuring a swim course so difficult? JoshR - 2009-06-29 5:25 PM muppetdog - 2009-06-29 4:15 PM brucemorgan - 2009-06-29 8:01 AM But I'm fairly skeptical of all the race reports that say the "swim was long" when a lake course is involved. I'm aware that buoys move more than you might think (anchor dragging and all that), but still. If you were in a late wave at Boise 70.3 this year, you had a chance at a long swim because the 2nd turn buoy broke free and floated away It took them a while to collect it, deflate it, and replace it with one of the regular buoys. In the meantime a lot of people (who were already getting tossed about by horrendously bad chop) swam in the direction they thought was correct while the buoy floated away from the course.That was so annoying. I couldn't understand why the 2nd length of the square seemed like it was long. I was in the next to last wave at Boise (we started at 2:50), and yes indeed the leg to the 2nd turn seemed very long. The turn was more like 120 degrees, not 90 degress, and after it I seemed far, far to the left of the buoy line heading back. I thought it was me since I have a habit of zig-zagging. |
| 2009-06-30 11:26 AM in reply to: #2249865 |
Champion 5782 Northridge, California | Subject: RE: Why is measuring a swim course so difficult? Just because someone can lay out an accurate swim course doesn't mean they actually will. We had the USAT SW Regionals at a local tri on Sunday and the swim course was--I'd put a large sum of money on it--very short. Every race report and post regarding the swim has mentioned this, one way or another. My time was more than 8 seconds per 100 yds below my best previous and equalled my best speed for a 20 x 50 interval training session. No way I held that speed over 1500m in the ocean, wetsuit notwithstanding. With a swim split that included a 2 minute plus run to the timing mat in deep sand over a dune, my pace for the swim segment was sub-1:30/100. No way. (And I finished only 181 out of 422 on the swim. At 1:29/100? I don't think so.) And I got an accurate split of my actual time in the water: If it was actually 1500m of swimming, then I was at 1:21/100. Never have come anywhere close to that in training. As I mentioned everyone else had basically the same response. So, even in a USAT Regional Championship event, it seems pretty clear you can get the swim distance wrong. The second sprint I ever did: Advertised as 300m, but was very obviously (from the times of the entire field) actually 400m. That seems beyond just difficulty in accurately measuring...that seems like an RD who just plain forgot what distance they advertised for the swim on their website. Edited by tcovert 2009-06-30 11:29 AM |
| 2009-06-30 11:44 AM in reply to: #2249865 |
| Alpharetta, Georgia | Subject: RE: Why is measuring a swim course so difficult? When people say "everyone is swimming the same distance so why does it matter?" it irks me. I will (and have on this site before) be called anal or Type A or whatever about this and to "get over it", but whatever. |
| 2009-06-30 11:49 AM in reply to: #2253236 |
Sneaky Slow 8694 Herndon, VA, | Subject: RE: Why is measuring a swim course so difficult? lisac957 - 2009-06-30 12:44 PM When people say "everyone is swimming the same distance so why does it matter?" it irks me. I will (and have on this site before) be called anal or Type A or whatever about this and to "get over it", but whatever. I tend to agree with this... the day of the race, insofar as competition against other people in the race is concerned, yes, you are all swimming the same distance. But to me, it doesn't matter if I finish 50/100 or 48/100 AG on the swim, it more matters if I beat my last swim time, etc. I'm out there to try and improve. If I finish 50/100 in one race, and finish 48/100 the next, does that mean I improved? Who knows? But if I swim 2 minutes faster, that likely means I improved. If the distance is off, I have no idea. |
| 2009-06-30 11:57 AM in reply to: #2250296 |
Pro 6011 Camp Hill, Pennsylvania | Subject: RE: Why is measuring a swim course so difficult? "Give it a try. First, even the most accurate GPS technology, WAAS, is accurate only to within 7 meters 95% of the time..." That is the case with regular run-of-the-mill handheld GPS, but not with all GPS. I work for a construction company, and we have a GPS surveying department. Depending on time of day and satellite locations, accuracy is .1 (1/10th) foot both horizontally and vertically for our equipment. For a 1.2 mile swim, this is an accuracy of 99.997%. IMO, a RD who was concerned about accuracy could get a local surveyor to volunteer a few hours of time to help them place the buoys in exchange for being listed as a race sponsor. Edited by TriMyBest 2009-06-30 11:58 AM |
| 2009-06-30 12:14 PM in reply to: #2253015 |
Elite 4564 Boise | Subject: RE: Why is measuring a swim course so difficult? brucemorgan - 2009-06-30 9:33 AM JoshR - 2009-06-29 5:25 PM I was in the next to last wave at Boise (we started at 2:50), and yes indeed the leg to the 2nd turn seemed very long. The turn was more like 120 degrees, not 90 degress, and after it I seemed far, far to the left of the buoy line heading back. I thought it was me since I have a habit of zig-zagging. muppetdog - 2009-06-29 4:15 PM brucemorgan - 2009-06-29 8:01 AM But I'm fairly skeptical of all the race reports that say the "swim was long" when a lake course is involved. I'm aware that buoys move more than you might think (anchor dragging and all that), but still. If you were in a late wave at Boise 70.3 this year, you had a chance at a long swim because the 2nd turn buoy broke free and floated away It took them a while to collect it, deflate it, and replace it with one of the regular buoys. In the meantime a lot of people (who were already getting tossed about by horrendously bad chop) swam in the direction they thought was correct while the buoy floated away from the course.That was so annoying. I couldn't understand why the 2nd length of the square seemed like it was long. I was in the last wave and there was no 2nd red buoy when I was swimming. I also felt like no matter how much I was correcting I could never get going in a straight line. It was very annoying. |
| 2009-06-30 12:20 PM in reply to: #2253236 |
Champion 8766 Evergreen, Colorado | Subject: RE: Why is measuring a swim course so difficult? lisac957 - 2009-06-30 11:44 AM When people say "everyone is swimming the same distance so why does it matter?" it irks me. I will (and have on this site before) be called anal or Type A or whatever about this and to "get over it", but whatever. I agree, size length DOES matter! If the course is long (not typical) it benefits the swimmers. If the course is short ( more typical) it benefits the non-swimmers. Hence why I measure improvement by where I am in my AG...because you can never guarantee the swim distance will be accurate. But if you finished 50/100 one year and 20/100 the next year...you improved! Your age group will always be a bell curve of swimmers. Yes, some events attract a higher caliber of athlete, but really...the swim will always be a bell curve. Also irks the CRAP out of me when people go, "oh! I had such a great swim!" when in reality, the swim was short...credit where credit is due, people! If you've never hit that pace in a pool, the swim was short. |
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