Straight sections of a cross country course can be measured very accurately with laser measuring devices. The condition of the terrain will have no effect on the measurement. The problem is turns and curves which can be measured with a steel tape or cable. Final distance is not precise, but more than adequate for cross country.
Laser Measure wrote:
Straight sections of a cross country course can be measured very accurately with laser measuring devices. The condition of the terrain will have no effect on the measurement. The problem is turns and curves which can be measured with a steel tape or cable. Final distance is not precise, but more than adequate for cross country.
We are discussing practical methods here. No one is going to measure the straights with a laser, and then measure the turns with a tape. Once you've got your tape, you just measure the whole thing and you're done.
Laser Measure wrote:
Straight sections of a cross country course can be measured very accurately with laser measuring devices. The condition of the terrain will have no effect on the measurement. The problem is turns and curves which can be measured with a steel tape or cable. Final distance is not precise, but more than adequate for cross country.
The is a difference between straight and flat.
DiscoGary wrote:
The problem with the wheel was that we got significantly different calibration numbers on the grassy parts of the course versus the firm dirt parts. The On Course Ratings guys warned us that the time of day would change the wheel measurement as well because of dew on the grass in the morning, etc.
How many attempts were made measuring with different amounts of pressure applied to the wheel on these different surfaces? Sounds like user error to me.
Thread Follower wrote:
DiscoGary wrote:The problem with the wheel was that we got significantly different calibration numbers on the grassy parts of the course versus the firm dirt parts. The On Course Ratings guys warned us that the time of day would change the wheel measurement as well because of dew on the grass in the morning, etc.
How many attempts were made measuring with different amounts of pressure applied to the wheel on these different surfaces? Sounds like user error to me.
No attempts were tried with different amounts of force. If you can produce a method which will allow a wheel to be accurately calibrated for a XC course, then please verify it and report back here. I'm sure everyone who has read this far would love to know how you did it. I am in that group.
Thread Follower wrote:
DiscoGary wrote:The problem with the wheel was that we got significantly different calibration numbers on the grassy parts of the course versus the firm dirt parts. The On Course Ratings guys warned us that the time of day would change the wheel measurement as well because of dew on the grass in the morning, etc.
How many attempts were made measuring with different amounts of pressure applied to the wheel on these different surfaces? Sounds like user error to me.
Keep in mind that a calibrated Jones counter on a bike only got within 20 feet of the tape. The bike should have had plenty of pressure applied consistently.
DiscoGary wrote:
....
Keep in mind that a calibrated Jones counter on a bike only got within 20 feet of the tape. The bike should have had plenty of pressure applied consistently.
Out of curiosity, how many times did you measure the course with the steel tape? If so, how close were the measurements?.
I can see potential user error in any method, but close replicated results do mean something. Error could be just hard to measure via replication. Has the course been measured recently by this method?
is this the group that measured your course? http://oncourseratingsystems.com/
DiscoGary wrote:
We are discussing practical methods here. No one is going to measure the straights with a laser, and then measure the turns with a tape. Once you've got your tape, you just measure the whole thing and you're done.
I wouldn't say laser is impractical, particularly on a course like McAlpine where you have some significant straights. It's fast, accurate, and if you put down survey markers at each end you have a nicely documented segment that can be revalidated (and reused in the event of course wear or construction).
Electronic measurement is approved by USATF, and is a great way put set up a permanent calibration course. The measuring team for the Atlanta Olympics used laser measurement to lay down the tangents for the blue line (and they also had a team that did the overall measurement over those tangents with bikes (yes, multiple measurers)).
Even in cases where it's impractical to measure the majority of the course electronically (too much of a PITA when a lot of turns) it's a wonderful tool for laying out the fair start boxes.
DiscoGary wrote:
Thread Follower wrote:How many attempts were made measuring with different amounts of pressure applied to the wheel on these different surfaces? Sounds like user error to me.
Keep in mind that a calibrated Jones counter on a bike only got within 20 feet of the tape. The bike should have had plenty of pressure applied consistently.
To be within 20 feet over a three mile course is very accurate. The difference easily could have been variation in speed since the Jones Counter is not accurate with speed changes; or errors with the tape on the turns, which would be easy to do. Either way, both are impressively accurate measurements for a cross country course.
Remember there is a difference between straight and flat.
I recall the blue line not being the SPR in Atlanta, but just a course guideline.
Replicate wrote:
DiscoGary wrote:....
Keep in mind that a calibrated Jones counter on a bike only got within 20 feet of the tape. The bike should have had plenty of pressure applied consistently.
Out of curiosity, how many times did you measure the course with the steel tape? If so, how close were the measurements?.
I can see potential user error in any method, but close replicated results do mean something. Error could be just hard to measure via replication. Has the course been measured recently by this method?
The course was measured once. If we did it again I would ask them to measure it from finish to start to see how close the two measurements would be.
Keep in mind that we were measuring many other course configurations, in many different modes on that 85 degree day, so some things didn't get done that we wished would have been done after the fact.
As far as I know, no one has measured it with a tape since then.
Thread Follower wrote:
is this the group that measured your course?
http://oncourseratingsystems.com/
Those are the guys and I encourage anyone with detailed questions or concerns to ask them directly. They are the original source and I am giving you information based on what I saw them do. They will know the answers to the tension questions.
Replicate wrote:
DiscoGary wrote:....
Keep in mind that a calibrated Jones counter on a bike only got within 20 feet of the tape. The bike should have had plenty of pressure applied consistently.
Out of curiosity, how many times did you measure the course with the steel tape? If so, how close were the measurements?.
I can see potential user error in any method, but close replicated results do mean something. Error could be just hard to measure via replication. Has the course been measured recently by this method?
Great point, I also saw an article comparing various tapes and there was some issues between the tapes, some short, small long. Add in not properly stretching a tape, placing the tape a little too far out from all of the curves, being off by a half and inch at some of the connecting segments, etc, and it would be very easy to add in 20' in 50 -100 tape pulls over a cross country course.
They said they used a FIBERGLASS tape in the video, but it looked more like nylon. They did not tension the tape and it looked to be 50m, not 100m as you had mentioned in a previous post. Flags were also layed flat for the second guy to start the next measurement, again plenty of room for a half inch or more error on wear on the flag to place the tape.
Thread Follower wrote:
So on a bike, how close do you think you can get two measurements/rides of a 5k road course? Why do you feel you couldn't get two similarly close measurements with a wheel?
I've been within 1/2 count (2 inches) over a 10k course, and a number of times within 1 inch on 1 mile courses that I've measured for my personal training. However many times, I always take the shortest measurement.
A bicycle keeps a constant pressure on the tire, and you have the bicycle to assist a straight line. A wheel by itself has no constant pressure, and it is not possible to have one by a person walking along with it. Also a wheel by itself has a much greater tendency to wobble back and forth.
Thread Follower wrote:
DiscoGary wrote:Keep in mind that a calibrated Jones counter on a bike only got within 20 feet of the tape. The bike should have had plenty of pressure applied consistently.
To be within 20 feet over a three mile course is very accurate. The difference easily could have been variation in speed since the Jones Counter is not accurate with speed changes; or errors with the tape on the turns, which would be easy to do. Either way, both are impressively accurate measurements for a cross country course.
Agreed.
The reason the USATF Jones counter guy was so disappointed is that 20 feet over 3 miles isn't good enough for USATF certification, and that's the kind of accuracy he was hoping to get. That would have been about 2 meters if I remember correctly. When we finished he was convinced that the tape was the gold standard and that the bulk of the 20 feet was due to his bike, and it wasn't because of user error either. He was qualified, motivated, and meticulous, and it still wasn't good enough for him.
It might be good enough for others though, but keep in mind that this was on a hot dry day when the ground was firm. Almost ideal conditions. It's easy to see how other conditions might introduce errors for the bike where the steel tape should be immune.
J.R. wrote:
Thread Follower wrote:So on a bike, how close do you think you can get two measurements/rides of a 5k road course? Why do you feel you couldn't get two similarly close measurements with a wheel?
I've been within 1/2 count (2 inches) over a 10k course, and a number of times within 1 inch on 1 mile courses that I've measured for my personal training. However many times, I always take the shortest measurement.
A bicycle keeps a constant pressure on the tire, and you have the bicycle to assist a straight line. A wheel by itself has no constant pressure, and it is not possible to have one by a person walking along with it. Also a wheel by itself has a much greater tendency to wobble back and forth.
Is there constant pressure on a bike(front tire) while going up and down hills? Can you maintain the same speed up and down hills.
Can you maintain a smooth line while climbing a hill on a cross country course?
Are we sure that the Jones counter guy road the exact same course as the tape method guys? How many rides did the Jones counter guy do?
Do you recall the tape length that was used?
Correction, always take the longest measurement when measuring.
Tape, calibrate, measure, calibrate.
When calibrating, the least counts are most accurate.