Author Topic: How exactly is a "measured mile" measured?  (Read 34643 times)

0 Members and 1 Guest are viewing this topic.

Offline TrickyDicky

  • Sr. Member
  • ****
  • Posts: 607
Re: How exactly is a "measured mile" measured?
« Reply #75 on: February 07, 2016, 05:32:27 AM »
Malcolm, I think tortoise's point was that if you apply the above rules then 7.2009 rounded off becomes 7.200 and, hey presto, 500 mph.

The figure of 7.2009 is never 'seen' by the FIA timekeeper as the rules are written. The fourth decimal place is just ignored.

Not sure how you could ever arrive at the average or mean time being 7.2009.




OK, using the rules as written you could arrive at a mean time of 7.2005 seconds.

Andy Green's outright record works like this. The calculated mean time was 4.7185 seconds but this is truncated to 4.718 before converting to a speed. Thus the quoted record speed is slightly faster than his "real" speed.

Offline tortoise

  • Sr. Member
  • ****
  • Posts: 685
Re: How exactly is a "measured mile" measured?
« Reply #76 on: February 07, 2016, 02:46:56 PM »
Effects of truncating 4th digit after the decimal point.

I hear Mr. Green's shooting for 1000 mph. A scenario:

His first run , if recorded to .0001 second, would have been 3.6009, but is recorded as 3.600 second.
Second run, if recorded to .0001 second, 3.6019, recorded as 3.601 second. Mean of 2 recorded times is 3.6005 second, recorded as 3.600. Record set at 1000 mph. Mean of 2 times as read to .0001 seconds, 3.6014, calculates to 999.611 mph.
« Last Edit: February 07, 2016, 09:48:14 PM by tortoise »

Offline TrickyDicky

  • Sr. Member
  • ****
  • Posts: 607
Re: How exactly is a "measured mile" measured?
« Reply #77 on: February 07, 2016, 04:58:49 PM »
Effects of truncating 4th digit after the decimal point.

I hear Mr. Green's shooting for 1000 mph. A scenario:

His first run , if recorded to .0001 second, would have been 3.6009 mph, but is recorded as 3.600 second.
Second run, if recorded to .0001 second, 3.6019, recorded as 3.601 second. Mean of 2 recorded times is 3.6005 second, recorded as 3.600. Record set at 1000 mph. Mean of 2 times as read to .0001 seconds, 3.6014, calculates to 999.611 mph.

Your logic and arithmetic are correct tortoise.

I suggest that the way to think of it is this: imagine a stop watch that ticks once a second. Start the watch at zero; then after one second has elapsed the watch clicks over to show one second; after two seconds the watch will click over to show two seconds; etc. So when the time that has really elapsed is between n and n.99999999999 seconds the watch will show n seconds.

The FIA watch effectively ticks every one thousandth of a second.

As your example shows the overall effect is always to exaggerate the quoted speed slightly. I suppose that if Andy's 'real' average speed was 999.611 mph then it is a near certainty that he would have exceeded 1,000 mph at least some of the time. Plus the rules are the same for everyone.

Offline tortoise

  • Sr. Member
  • ****
  • Posts: 685
Re: How exactly is a "measured mile" measured?
« Reply #78 on: February 07, 2016, 06:13:37 PM »
I suppose that if Andy's 'real' average speed was 999.611 mph then it is a near certainty that he would have exceeded 1,000 mph at least some of the time.
I object. Witness's comment is irrelevant and immaterial. The records in question are for average speed over a prescribed distance.
. . . the rules are the same for everyone.
Just so. The exercise shows, though, that the possible error, (if we choose to call it an error), is greater than many would intuit, when seeing speeds quoted to the thousandth.
Quote from: tortoise
 Not a big deal, just a curiosity.
« Last Edit: February 07, 2016, 08:03:37 PM by tortoise »

Offline TrickyDicky

  • Sr. Member
  • ****
  • Posts: 607
Re: How exactly is a "measured mile" measured?
« Reply #79 on: February 11, 2016, 05:59:34 AM »
tortoise and I seem to have killed this thread almost stone dead.  :-(

Getting back to the original point, two questions that intrigue me are:

  • Suppose you are confident your measuring system is accurate to +/- 6 inches.  Does your licensed surveyor lay out a course that is 1 mile +/- 6 inches (and thus risk that the "measured mile" is actually slightly shorter than that) or does he add 6 inches margin for error?
  • Is the course re-measured after a record has been set, to check for any changes that might have occurred since it was laid out?

Reading about the early days (pre-1920 say), there are hints that distance measurements were sometimes fairly lax.  For example, a crowd dissatisfied with the speeds being achieved before lunch might move the finish line to "increase" the speeds being achieved later in the day.  :-o :-o

It couldn't happen today of course.   :| :-)
« Last Edit: February 13, 2016, 04:16:14 AM by TrickyDicky »