I rode the wrong route for a certification last year and need to fix it.  Google Earth says it was about 8 m long the way actually run.  Since the course was originally certified as a 5,000 meter course, can I use a percentage difference to adjust the Start?  And that means NO calibration or recalibration.

Procedure: First, ride a part of the wrong route twice establishing beginning and ending points.  Then ride the correct way twice. For safety sake, ride segment of wrong route one more time. If that agreed within .0008 of the worst of the first two rides then there is no reason for concern with tire expansion or deflation.  Then multiply the % difference of the shorter rides by 5000 and adjust.

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I'm not understanding the need to do any percentage calculation. Ride the original (wrong) route twice. Ride the new (correct) route twice. Add (or subtract) the difference you find between those two routes to the start. What am I missing?

If you do not calibrate, how do you know what the distance is?

Oscar

You do everything in counts. If the new course is 150 counts shorter than the original course, add 150 counts to the start.

Agree that would be really close.  But, how many absolute inches is each of those counts worth?  It would be a lot different if those counts were 2.75" each versus  4" per count. I have bikes that will do either. The amount added could vary quite a bit.

It doesn't matter how many inches each of those counts is worth. If you ride the old and new courses and find the new course is 150 counts shorter, then you use the same bike on the same day to add 150 counts to the start. It's not really close. It's exactly the same. You removed 150 counts by following the new course, and you added 150 counts to the start to make up for it. You never need to know how much that change was in feet or meters.

But, you are not riding the WHOLE COURSE which has a know distance of 5000 meters, just a segment which has an unknown distance.   Divide the two segments and that gives you a fraction to add or subtract from 1 and multiply by the known 5000 m.

And furthermore, now you've got ME wondering!

OK, now I see what you are saying. The difference in counts would be the difference in distance as long as you just used counts.  So just back off or move up the start or finish.   But you could still do this without any calibration.

Yes, you always do this type of adjustment without calibration.  In this scenario, calibration means nothing.  Absolute count comparison is the only method needed. Mark explained it very will in his post.  Just think of each click as a link in a chain.  Get the length of the chain set on the adjusted portion, and take the chain to the Start (or Finish).  Use to move the location.  Done.

Even more embarrassing than not checking my "percentage change" theory is that I moved a finish using "counts only" just that last spring.  Never translated that it would also apply to mid-course then difference at S or F.