How exactly is a "measured mile" measured?

[Seldom Seen Slim]

Anecdote offered for the sake of the discussion:

A handful of years - (10++?) one of the companies represented in the local engineering club's roster put on a presentation on using GPS for surveying.  One of the principals on the board is a "LS" - the next notch up from regular licensed surveyor,  I guess, and therefore knows his stuff.  Anyway, he reported that the GPS was still kinda new to them - and the industry - so on one long line -- about ten-fifteen miles -- they used both GPS AND traditional surveying chains and stuff, just for the sake of comparing.

In that long line -- they came out dang near exactly identical.  As I remember, Gerry reported that the difference in the two methods, after all those miles of hills and trees -- was about the diameter of a DIME.

And that was a bunch of years ago when things weren't as developed as they are now.  Geez, maybe that GPS stuff will work some day soon, hey?

[wobblywalrus]

Lets say the timing should be accurate to 0.01 second.  A bike or car traveling at 60 mph moves 0.88 feet in 0.01 seconds.  The distance marks should be within a few inches to get accurate times.

A guy or gal traveling at 400 mph travels 5.87 feet in 0.01 second.  The timing marks need to be within a couple of feet to provide plenty of accuracy.

A skilled operator with a GPS setup made for accurate work should be able to set the timing marks up plenty good for the 60 mph guy and much more accurate than needed for Poteet and Main.

Fortunately at Bonneville there are plenty of GPS satellites in the line of sight.  This is a big help.









   

[tortoise]

Lets say the timing should be accurate to 0.01 second.

Based on what?

 A bike or car traveling at 60 mph moves 0.88 feet in 0.01 seconds.  The distance marks should be within a few inches to get accurate times.

A guy or gal traveling at 400 mph travels 5.87 feet in 0.01 second.  The timing marks need to be within a couple of feet to provide plenty of accuracy.  

A "couple of feet" error in a measured mile is .038% . That's .15 mph error for a 400 mph run: way too much. Records are posted to thousandths of a mph.

[wobblywalrus]

A record measured to the thousandth of am MPH would need the course laid out so the mile marks are within the distance the bike or car would travel in one thousandth of a second.  0.001 second x (60 miles / hour) x (1 hour / 60 minutes) x (1 minute / 60 seconds) x (5,280 feet / 1 mile) = 0.088 feet  The vehicle would travel 0.088 feet in 0.001 second.  That is a little more than an inch at 60 mph.

Today I asked our survey crew about GPS accuracy.  They said the typical hand held one you buy at the store is good for 20 or 30 feet accuracy.  This can be increased by using an antenna.  They have a survey grade GPS setup that costs $20,000 to $30,000 dollars typically.  It can locate a point within a tenth of a foot accuracy.  This is 1.2 inches.

The typical way of doing these measured miles is by an electronic distance meter.  These are fairly cheap and give good accuracy.  Measuring them out in short segments like 200 or 300 feet helps to take into account the curvature of the earth.

"Hire a registered land surveyor and tell him/her the accuracy you need and they will lay the course out.  The precision needed to time a vehicle to the nearest 0.01 second is a bit much for the average person.  Some real good equipment and skill is needed."  This was the final advice the survey crew gave me.  This advice is even more applicable for a course laid out to time vehicles to the nearest 0.001 second.

   

[dw230]

You have to factor in the margins required by the FIA/FIM, much closer than you states. Did you know that the Rice Brothers, Allen and james, who are the officail timers for SCTA, BMST, FIA and FIM are certified to set the track? They use modern laser technology equipment.

DW

[tortoise]

You have to factor in the margins required by the FIA/FIM, much closer than you states.

Appendix D of the FIA international Sporting Code.

"D5.1.2 Measurement. The length of the Course must be
measured and duly certified to within 1/10,000 of its length."

That's 6.336" for a measured mile.

Go figure.

[TrickyDicky]

A record measured to the thousandth of am MPH would need the course laid out so the mile marks are within the distance the bike or car would travel in one thousandth of a second.  0.001 second x (60 miles / hour) x (1 hour / 60 minutes) x (1 minute / 60 seconds) x (5,280 feet / 1 mile) = 0.088 feet  The vehicle would travel 0.088 feet in 0.001 second.  That is a little more than an inch at 60 mph.
   

A record is not measured to 0.001 mph.  Although record speeds are reported to three decimal places.  The measured parameter is time, which may be recorded to many decimal places but is then truncated to 3dp for (FIA) record purposes.

This means it is not possible for many mph numbers to be reported as record speeds.  For example:

• If the time recorded is 8.999 seconds, then the speed is 400.044 mph
• If the time recorded is 9.000 seconds, then the speed is 400.000 mph
• If the time recorded is 9.001 seconds, then the speed is 399.955 mph

So speeds of 400.001, 400.002, etc. are not possible using the (FIA) rules for calculating average speeds.

The above analysis does not apply to SCTA record calculations.

[tortoise]

A record is not measured to 0.001 mph.  Although record speeds are reported to three decimal places.  The measured parameter is time, which may be recorded to many decimal places but is then truncated to 3dp for (FIA) record purposes.

Hmm . . .
So if you're timed at 7.2009 for the mile, which figures to 499.9375 mph, you'd be credited as going 500.

[TrickyDicky]

A record is not measured to 0.001 mph.  Although record speeds are reported to three decimal places.  The measured parameter is time, which may be recorded to many decimal places but is then truncated to 3dp for (FIA) record purposes.

Hmm . . .
So if you're timed at 7.2009 for the mile, which figures to 499.9375 mph, you'd be credited as going 500.

Yep.  It needs someone smarter than me to explain why this is considered to be the "correct" way to do the calculation.

[Glen]

A record is not measured to 0.001 mph.  Although record speeds are reported to three decimal places.  The measured parameter is time, which may be recorded to many decimal places but is then truncated to 3dp for (FIA) record purposes.

Hmm . . .
So if you're timed at 7.2009 for the mile, which figures to 499.9375 mph, you'd be credited as going 500.

Yep.  It needs someone smarter than me to explain why this is considered to be the "correct" way to do the calculation.

SCTA/BNI does not round up numbers.It's always been that way.
I was the chief timer for many years and that's the rule.

I also remember where a chain was used to mark off the miles, as I remember it was 100 feet long with links marked for the 1/4 and the feet in miles/ft. Took a long time to measure but we only had one course. 

[wobblywalrus]

Use of the GPS is what I am typing about.  This is a complex subject and there is much more to it than this.

Setting points with the GPS is dependent on the proper equipment, skill, and calibration.  The end product is two points located on the salt.  The big question is, are they within the allowed error tolerance?  One can assume if the equipment and operation is adequate that the points are correctly placed.  This is an unproven assumption.

Chaining or EDM are surface measurements.  The crew proceeds from the first point to the second.  They set that point.  Then they measure back to the first.  The difference between the two is closure.  The closure needs to be within the allowed tolerance.  One has been mentioned in a previous post.  One problem is if the instrument is out of calibration.  Then, using the same instrument to go both there and back gives good closure but the point is set at the wrong distance.  We can go out with the EDM and come back with the chain.  A consistent difference between the two instruments readings indicates a calibration problem.  Or, the EDM can be checked across two points of known distance.  This last is easier.  We have two markers set in concrete in the parking lot and we know how far they are from each other.  We check the EDM calibration every morning.

In summary, a GPS that is accurate enough to set points is pretty expensive and needs to be operated by a professional.  There is no good way to check the closure unless the distance is measured by calibrated instruments.  Heck, if the distance measurement tools are needed, they can be used to mark off the course and there is no need for the GPS.
 

[tortoise]

Use of the GPS is what I am typing about.  This is a complex subject and there is much more to it than this.

Why bother?

Heck, if the distance measurement tools are needed, they can be used to mark off the course and there is no need for the GPS.
 

[fastman614]

You guys are making this sound extremely complicated. I was of the belief that the timing equipment will measure elapsed time to 1/10,000 of a second. With electronics of as far back as 30 years ago, in industry, I know it was possible. Then the elapsed time is factored to the measured distance and the average speed over that distance is what the given number represents. The terminal velocity speed is a timed reading, in order to give a mph speed "accurate" to one thousandth of a mile per hour would need to be timed to (at least) four decimal places...
   

[typo41]

Ok Guys, and Gals

I did not read through all the posts, sorry. But here is how the Rod Rider Racing Club lays out the 1.1 mile course for El Mirage, and have been doing it for over 20 years.
We have three vehicles with electronic foot distance counters in side. We start at the start line with two vehicles at the course edge and move towards what we plan as the course and finish. Every 20 feet we drop a cone. Once we reach the 'finish line' we mark it for timing lights location. We also wheel the spread for the course and wheel to the safety cones on the edge. 
While one part of the crew lines up the cones from start to finish, another part of the crew lays out the Patrol positions so that a different club can lay out the side cones.
Now for a two day event, we have to make sure the course we 'slide' over to is good enough to race upon. And at the end of the first day we move one line of cones over to the next course, making sure they are line up for the racers.
Some have tried to gps the course and like mentioned the finish line was off from 10 to 20 feet.
The counters are checked every meet by driving a known 1000 foot course to make sure of any changes to the machine or tires. The machine run off a magnet on the drive shaft.

Now, exact? As possible. But by having the same group with the same equipment every-time one has a better chance of being consistent. 

[Seldom Seen Slim]

Tony, I think that's a great concept - the way you measure and lay it out.  By having the same crew do it the same way for years -- you've at least added greatly to the "repeatability" of the measurement even if the absolute accuracy is not within microns or whatever.

Your chances of getting it close to exactly the same each time -- are better for the practice you get.