One topic we didn't get to at the RRTC meeting was the difference we should allow between the two tape measurements of a calibration course. There is nothing in the manual or on the application about what an allowable, or even an expected or reasonable, difference is.

What are people's thoughts?
Original Post

My thought is, if there is a difference greater than 1/4", then proper tension was not consistently applied. There is no way to know which measurement was closest to accurate, so there should be at least one more measurement of the entire cal course, paying closer attention to tension on the tape.
A calibration course is very important as to having an accurate race course, hence we need to have a general rule here. As Duane stated, 1/4 inch may be too large. I would as Duane stated do it again. My feeling is a difference of no more than 1/8 inch would be acceptable.
Are you saying 1/4 inch or 1/8 inch difference between the two measurements of a 1000-foot or 300m cal course? That's a difference of only 0.002%.
My surveying text (“Surveying” by Charles B. Breed) says “competent tapemen can usually obtain precisions of 1/5000 or less…”

In the layout of two calibration courses associated with the measurement of the 1996 Olympic Marathon in Atlanta, eleven teams of two experienced measurers measured the lengths.

Raw measurements of the two courses were:

West course
Minimum – 480.205 m
Maximum – 480.440 m
Range – 0.23 m
1/5000 = 0.09

East course
Minimum – 480.87 m
Maximum - 481.09 m
Range – 0.22 m
1/5000 – 0.09

In my own experience I am happy to get within 2 or 3 cm for two measurements. All it takes to produce a big difference is for the sun to come out on one measurement and be overcast on the second.

The 1/5000 level of precision would suggest that 10 cm in 300 m or 4 inches in 1000 feet is not unreasonable. Most of the time this will be achieved.
I am surprised to read requirements of agreement to 6mm or even 3mm. Is this intended for a 300m long calibration course or a 50m tape length?

For a 300m cal course, 6mm is 6 parts in 300,000 or 1 in 50,000. That is 50 times smaller than our SCPF of 1 in 1,000. The class 2 steel tapes which we use are only accurate to 1 part in 5,000, so this requirement would be 10 times more accurate than the steel tape markings made by the manufacturer.

Of course I may have misinterpreted Mark's post, perhaps he meant how much difference would be acceptable in one tape length of say 50m. In this case 6mm would be 6 parts in 50,000 or about 1 in 8,300. If that is what Mark meant, then, yes, I would agree that a remeasurement of that 50m interval would be worth doing.

The tapes I have used typically stretch about 6mm to 8mm when you apply 50N force, so basically I am saying that we need to put roughly 10lb force on the tape but it does not have to be precise. However, I do use a spring scale to get it accurate and even then I can sometimes get 6mm difference over the 6 taped lengths of a 300m cal course, but this is much more than adequate for bike calibration.

I once checked a 400m cal course and found the measurer had it short by 100mm. That was due to a mistake he had made and needed correcting. 3mm or 6mm forget it.

EDIT: Mark and Pete posted while I was writing. I agree with what they say.
If the pros ("Surveying" noted by Pete above) recommend 1/5000, why would we recommend something for more demanding than that? 2.4-inches difference between 2 measurements of 1000-feet each seems like a very easy tolerance to meet. 1/8-inch seems nearly impossible, especially for a new measurer or for a measurement by a single measurer. Also, what does it mean (as Mike notes) to have a tolerance smaller than the accuracy of the measuring instrument?

I've measured my share of calibration courses and would probably remeasure if I had a difference over 1-inch for a 1000-foot course, unless there was an appreciable temperature change between measurements. This is based only in my experience almost all the calibration courses I've measured have a difference between measurements of less than 1/2-inch.
I have been able to achieve less than 1/2" difference between two 1,000 foot measurements, when using a spring scale to maintain 10 lbs tension on the tape. I have had another person hold the other end, so there is less variance from placing nails and doing it all solo.

If you had a difference of two inches in two 1000-foot measurements, which is the more-accurate measurement? Split the difference, and maybe have the course short an inch? If so, the Jones is not precise enough to register the 1" difference. If you calibrate to 1/2 Jones click, then you may see a difference. That would magnify 5 times over the course of a mile, and 26 times over the course of every marathon measured using that cal course. 7" times 26 miles is 15 feet. That is a material difference, even though our SCPF is 38 feet.

I personally would rather have more precision in my calibration courses than in my race courses, since my race courses are based on my calibration course. I use an electronic counter, so I use 1/100 of a wheel rotation for my "clicks", which is more precision than a Jones will give. Therefore, that 1" makes a material difference in my measurements. I use the Jones for verification clicks, but the electronic is more precise. (That much precision is not needed, but since it is available at no extra effort, I don't mind using it.)
What is the electronic counter you use, Duane? Other than precision, are there reasons you prefer it to the Jones?