Measuring slalom course

[RachelShort1]

Can someone please tell me the best option for verifying the turn ball distance is correct on each of the turn balls in our course? Obviously surveying is an option but I am hoping there is a simpler way of doing this. Any advise would be great!

[Ilivetoski]

I’m not a TC by any measure, but I am 99.99% sure surveying is the only way or I would have seen a lot less of it

[thompjs]

You can pull a tape for a rough idea. To ensure record tolerances you need a survey.

If you site has a recent shot on Google Earth you can check for gross errors with the ruler.

I've used it on my lake, which have very accurate courses and I get within 0.1m most of the time.

 

 

[mwetskier]

you can build a floating measuring stick using lengths of white plumbing pipe. you should either cap the ends to seal the air inside or slide some small diameter swim noodles inside so it will float.

 

The total length from end to end should be 41' 5.5" (+or - 4.5"). This is the distance from the outside boat guide to the skier ball. distance between centers of boat guides should be 7' 6.5" (+ or - 4.5"). distance from nearest boat guide to skier ball should be 33' 11" (+ or - 4"). While not accurate down to the absolute hair it will be close enough to be in spec.

 

[MNshortliner]

Another question, are the measurments on tournament courses true to actual course demensions or are they minus the 4 1/2” for the ball to be as close to the center of the course as possible but still within the allowable tolerances?

[Horton]

Surveying is not nearly as daunting as it sounds

[mwetskier]

@Horton -surveying is not particularly daunting, but if you're not a surveyor or good buddies with one it *can* be expensive. figure around 9 or 10 pairs of ski gloves worth.

 

[vtmecheng]

The PVC pipe idea is good. If you go that, get the stuff that's foam core instead of solid. It will weigh less and float enough that you probably won't need the pool noodle.

[LeonL]

To repeat from another thread, measurements are to be made as close to actual as possible. Any tolerances are just what means, tolerances. To set up a course intentionally to narrow tolerance is in violation of the rules.

[Justin_C]

If you're concerned about your turn balls having moved over time you might want to be careful using your boat guides to measure off of. If your turn buoys have moved there's a good chance your boat guides have too. If you look straight down my course it looks straight as an arrow but there are a couple of guides that have moved forward and back a touch.

[klindy]

The hard part isn't course width. You can use a fiberglass tape or a rope with marks on it and get a good enough measurement to know how wide the course is. The harder measurements to get are the up/down course and diagonal measurements. While surveying the course the diagonal dimensions are often the ones which are out of tolerance and most difficult to correct. Measuring the center of the gates to #1 is the hardest of all since there's no buoy in the middle of the entrance gate.

[DW]

We have used a marked stainless cable, not great but it does work. One length is ball to ball (long), one is gate to gate, another is width from both boat guides and another is previous boat guide to ball. Basically we triangulate various elements to do our best to verify ball position. Cable stores nicely, I do like the floating PVC option better.

[PatM]

@Edbrazil shot my course a couple of times. He made sure what is important (the turn balls and the entry and exit gates) was correct. The inner boat guides are not as important as long as it is over all close.

[Horton]

@mwetskier all you need is a laptop and a rented surveyors equipment. You can do it with two people in a couple hours. Someone with experience can probably do it in a half that time.

[Justin_C]

@Horton what gear do you rent? GPS based stuff or older line of sight stuff? (I clearly only have a surface level knowledge of the process, my brother's the engineer/surveyor.

[mwetskier]

@Horton -you left out the most important part -knowing *how* to use the rented survey equipment. my smart phone is smarter than i am so i'm still trying to learn how to use that. pretty sure i don't have enough room left in the noggin to take up amateur surveying too. just sayin

[Horton]

@mwetskier I put in an RC course 25 years ago when I was a college kid. Is easier than programing a VCR.

 

@justin_c That is a question for a currently experienced TC. What I have used is digital but non-GPS gear.

[eleeski]

Having set and surveyed lots of courses, a properly used long tape measure is incredibly accurate. My 100 meter tape makes it easy.

 

Finding the center between buoys might add the most error if you are working in the water. I use a rope or pipe. Lots of swimming is needed and hopefully the water is warm.

 

Survey equipment is easier but if you aren’t perfect errors are possible. Maybe the modern stuff is better than my older transit. The tape was several cm more accurate than my transit.

 

Eric

[klindy]

@Justin_C the surveyors equipment doesn't need to be too fancy. But an EDM (Electronic Distance Measurement) integrated helps a lot. The hardest part is knowing when you're making a mistake. Unfortunately you need to do a few to realize somethings off and you need to correct or reshoot. For example, swapping the even/odd boat guide buoys when you right the numbers down, etc.

 

Second hardest is using and understanding the survey spreadsheet (Dave Clark's) or survey program (Lion). Entering the data is easy. If somethings off, understanding what you need to do to correct it can be difficult.

 

It's all doable but it helps to have a few under your belt to get proficient at it.

[Edbrazil]

Many Technical Controllers can survey and have the gear. They will be lots less expensive than a survey company. Who may be learning on the job, so to speak. Options are a 3-point survey or a 1-point survey. For the latter, you need a "Total Station" instrument, which has a distance meter plus the angle measuring. Angles should be taken to 10 arc seconds or finer. The newer survey instruments may have "reflectorless" laser capability, and can read distances directly off an object. Depending on distance and color of the object. The distances involved in surveying a SL course can stretch their range. You also have to compensate or buoy diameter. Definitely, some learning needed.

 

Generally, best to get a Technical Controller to do it. Maybe for travel expense, plus ski rides, plus a little more, like a tournament entry. To locate TCs, go to:

http://www.usawaterski.org/pages/offdirectory.asp