Calibration Course Temp Correction

Jeff, I think the answer is "it depends".

I have a flat tape. I measure the road temperature, as I think it has more impact on my tape's temp. than the air does. It also depends on if your tape is in the sun for the entire measurement process. If so, it is likely to be closer to the pavement temp than the air temp.

Since the adjustment is normally less than an inch, and a Jones click is about 3.5 inches, the entire adjustment is a moot point, unless the temp is more than 30° plus or minus of 69° (if I recall correctly about the adjustment midpoint).

That's my take on it.

Thanks Duane, I greatly appreciate your thoughts on this.

I've been steel-taping and running comparisons this weekend. I now believe that you are correct: The pavement temperature appears to be closer to tape temp than the air temperature in the range I have been working in this week.

My newest toy is an infra-red laser sighted touch-less temperature gun. It really does measure the "pavement temperature", and I'm imagining that I can also get a reasonable tape temperature with the same device.

Previously, I had followed the manual:

"... set out a thermometer on the pavement and shaded from the sun".

I've been doing that for years, but that procedure appears to yield a figure closer to "air temp" than the pavement temp mid day, based on this weekends numbers using both thermometer, and infra-red gun. In the summer sun, the street gets really hot!

My new question: Is it appropriate to use this type of device (the infra-red gun thermometer) for this purpose? The immediate result will be to create calibration courses that are almost a couple inches shorter after temperature correction than my original practice...

JJ

The temperature of the tape is influenced by that of the surrounding air, by the temperature of the pavement against which it lies, and by the solar radiation it gets from the sun.

Nobody, to my knowledge, has ever been able to pin down an exact value for the temperature of the tape. It’s likely that during the taping process the tape temperature will fluctuate as the local conditions change a bit, and may be different at different points along the tape’s length.

In reading about this subject years ago I came across reference to a device which contained a small thermometer which clipped to the tape, indicating its temperature. I have been unable to find a reference to it again.

Surveying does not measure as we do. In “plane surveying” as it’s called, all measured distances are the horizontal distance between the measured points, not the undulating distance along a road between two points which may not be at the same elevation. Before electronic distance measuring devices, the taping team had to keep the tape level. This required them to keep the higher end on the ground, and the downhill end held level with the uphill end. A plumb bob was employed to get to the mark on the ground from the mark on the tape. In my surveying class we students found this to be a difficult procedure.

Also required was a correction to allow for the sag of the tape.

We’ve got it easy, believe me.

So what’s the temperature of the tape? Best we can do is a ballpark. What to do on a blazing hot day? Is the tape really as hot as the surface of the black asphalt? I suspect it’s a bit less, but don’t really know. Seems to me the thermometer laid on the pavement is as good as we are likely to get.

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As a non-engineer, I am not well equipped to determine how much difference in length the tape temp vs. air temp vs. road temp makes. Jeff's temp gun readings from the tape itself would seem to me to potentially provide the most accurate data for laying out a cal course, since it is the change in the length of the tape we are ultimately concerned with. But the steel tape correction table (or formula) we use is based on air temperature, right? As Pete states, even tape temperatures can vary over the course of a cal course setup.

When I measured a ~ 1,000' cal course with John Sissala years ago, we measured on a road that was partially shaded. The pavement in the sun was hot. The pavement in the shade was noticeably cooler. We ignored the road and tape temperatures an used the air temp for the adjustment, as our procedure specifies. If John and I had taken the temperature of the tape every so often, in different places on the tape, would this have been likely to provide more meaningful data for this cal course layout? Would Duane's practice of using the road temperature for adjusting for a flat tape be good enough? What if the road temperature varies from place to place along the cal course by say, 25 degrees F or more? With a temp gun, we should be able to measure tape stretch more accurately, thus determining more accurately the exact length of the cal course we establish. Is this moot, as Duane says, unless the tape temperature is 30 degrees F or more higher or lower than 69 F (I thought this was 68 F)?

The chief question I have is compound: 1)how much stretch or contraction in the tape occurs when the actual tape itself changes temperature during a cal course layout; 2)how should this tape length fluctuation dictate the magnitude of the mathematical adjustment we make - at what order of magnitude should the tape mean temperature and the tape temperature fluctuation range (vs. air temperature) necessitate an adjustment calculation - if any?

Is this a "The quest for perfection is the enemy of good" -type consideration, or is there cause to suspect that using tape temp rather than air or pavement temp may cause a non-trivial difference in the measurement of race courses, particularly longer courses?

Knowing that Dave Katz's steel tape he had checked out for the London Olympic Marathon was determined by NIST to be off - by ~ 1/2 inch, wasn't it? - I am wondering if the manufacturing accuracy of any particular steel tape is more germane to this discussion than the steel tape temperature(s) during a cal course layout. And, in what subsequent course measurements are these considerations likely to constitute non-trivial deviations from acceptable course measurement accuracy?

I use a non contact digital thermometer.They are available at most big box home improvement stores or Harbor Freight.
As soon as I finish using the tape, I role it up and test the temp at several points around the reel. I figure this is the best way to get the actual temp of the tape as it was being used.
You will be surprised at the temps you can get on a warm but sunny day.

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Course Adjustment Contest.
I just laid out a Calibration course. Apologies to Bob Baumel for using feet but this is a courtesy for a measurer who prefers feet to meters.
Six lengths of 165 ft. plus one length of 89 ft. The 89 ft was added to move the South end out of broken pavement.
This renders a raw length of 1079 feet exact. The ambient air temp is 80F. The road temp is 95F. The tape length immediately upon completion rolled up on the open reel is 98F.
Forget the actual adjustment length to achieve 1000 ft.
What temperature do I use to calculate it's effect on the steel tape? 80F?, 95F, 98F?

I vote for 80F. Best to always use the coldest temp the tape might have been during the measurement to minimize the possibility that you'll end up with a cal course that is shorter than the stated distance.

I vote for following the manual and using the road temperature. If the manual was followed and the tape was dragged on the road between measurements, it is probably pretty close to road temperature. Did I mention the manual?

My vote is in favor in favor of the road temperature, if taken on the pavement and shaded from the sun. I also prefer feet to meters.

I also vote for the road temp.

But, to support Mark, if we use 80°, and the course was measured to 1,000 feet (my table is set up for this length), we shorten the endpoint of our course by .9288". If we use road temp, and the temp is 100°, we shorten our course by 2.4768". IF the tape was really closer to 80°, then our cal course would be short of the 1,000' desired length.

HOWEVER, on my bike, a Jones click is 3.445". From 32° having an adjustment of 2.78" to 100° having an adjustment of 2.477", the temp adjustment is less than a click on a Jones counter. Yes, it is more than half a click, which translates to 5 x 1.75" = 8.75" per mile, or 19' in a marathon.

In my mind, only a marathon or longer course will be impacted by a temp-adjusted calibration course, if it was measured at freezing or 100°. The difference between an 80° adjustment and a 100° adjustment is less than half a click, and therefore won't be registered in calibration rides.

Just my ramblings on a summer night.

I appreciate all your opinions and comments.
I did use 95 degrees to adjust the course.
My thinking was simple. The concern is in regards to the temperature effect on the tape. This cal course was in full sun. The road temp was what I assumed the tape temp would be since it had been laying on the road surface for the last half hour. The few degrees increase in the actual tape temperature once rolled onto the reel are really not significant but interesting. I will watch this in the future.
I work to get cal courses as accurate as possible since the cal course is the foundation of any measured course. I prefer to believe that errors should be made on the side of a longer rather than shorter race course. The SCPF and common practice of using larger constant take care of any necessary "fudge factor" so I don't like disregarding the tape temp over shaded road or ambient temperatures. That being said, the reality is that the difference in using 80 degrees over 95 degrees would make the course effectively 1 inch longer. Extrapolated to Full Marathon length, the difference is less than 12 feet.

Can anyone show me a where .9288" is on a tape? This is why I prefer to work in meters!
(And why I try to lay out cal courses when it's 68F)

Wow, I was out of town and missed this great conversation.
Jim: I agree that .9288" is a weird distance to find but we should remember that surveyors use (or used to, when they actually did this) tapes marked in "tenths" (of a foot) rather than inches. The translation of .9288" would be 0.0774 feet. On a surveyor's tape you can read to hundredths and interpolate to thousandths, and fairly accurately measure 0.077 feet. (close enough, to put it unscientifically). I prefer meters as well but my 300' decimal feet tape is way sturdier than my 60 meter tape, much more resilient when cars drive over it etc.

The only way to settle some of these questions is by repeated measurement under different conditions. AJ Vanderwaal, who taught me a lot about all of this, says that you actually get better agreement in hot sunny conditions when you put the thermometer right out in the sun where the tape is (this is where the sun is beating down on the tape). I've never been nervy enough to use that method; instead I've always looked for a place where the thermometer can be in the shade, if only the shade from something I put down.

The very best time to lay out a cal course is during the night when you don't have to worry about the effect of the sun, and when temperatures are much stabler. But that's not always convenient.

I also was out of town. My vote is for the road temperature.

As for using feet or meters, I vote for meters.

Wonderful job, guys. Now I am more confused than ever. Not that this requires any particular effort by anyone.

Duane, based on what you say, I wonder how it is we bother to correct for tape/road/air temperature at all. Setting up a cal course with a 98 degree F tape might make a course long, but only trivially so, it seems.

Bob: if our tapes are likely to be + or - 0.5" short or long - and we do not know how short or long - does it nevertheless make sense to adjust for tape temperature?

Mike - what are you taking away from this can 'o worms discussion?

"Mushroom Boy"

When the manual was written using the temperature of the road was the closest approximation of the temperature of the tape that could be easily obtained. Now that we have an easy way to get the temperature of the tape directly, why wouldn't we use it?

That being said, I tend to believe that we should never shorten a cal course, mostly because there will be someone out there who shortens a course by an inch when they should have lengthened it by an inch. It's an easy thing to confuse.

I agree with Mark. In fact, the Manual states that no adjustment is necessary for temperatures above 68F. Since the difference between tape, road, and air temperatures is far less at 68F or below, isn't this whole discussion fairly moot?

Lyman, I appreciate your honest evaluation of the discussion. My take on the discussion is that as a group we are doing what we have always done. That being expressing opinions on a given topic that are all valid but really don't generate large differences in the final product. In doing so we make it appear complex to anyone not intimately involved in the discussion.
My opinion is that we continue to adjust all calibration courses using the existing formula. That means shortening cal courses when the tape temperature exceeds 68F. In my opinion, tape temperature is the same as road temperature.
I will continue to use a non contact digital temp meter to determine the road temp. and use that value for adjustment. Using any thermometer on the road and shading it from the sun is probably next best. In the past I have looked for shady spots to check temps. This would not necessarily be an accurate method.
All that being said, the differences amount to minor deviations in the final course length. The discussion while being thought provoking in attempts to home in on an absolutely accurate method really have a minor impact on the final race course distance.
I like my method since it's MY method, and it feels the most right to me. Reading the ambient temp from my cars dashboard would be less accurate but not so much as to cause me to lose any sleep over it.
This is road course measurement. There is NO absolute accuracy, only reasonable accuracy can be achieved.

I think my take-away on this is that variations in the length of steel tapes as manufactured may potentially have more impact on the "real" length of any given cal course than temperature during cal course setup.

Any comments? Anyone?