I am measuring a 1/2 marathon and the measurement is short by 800m. the start and finish is sort of fixed. how do i add to the turn around point.
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Original Post

If you already have a turning point, all you need to do is move it 400 meters farther out. This will lengthen the course by 800 meters.

If you don’t already have a turning point, it gets more complicated. Let’s see what others have to say.
Here is a way to add a turn-around point for 800 meters addition to the course:

1) Find a side road that is long enough.
2) Make a mark just before the side road. Call it point “A.”
3) Lay out 400 meters from point “A” to point “C.” Mark “C” temporarily.
4) Lay out 400 meters from “C” to a new point “B.”
5) Measure the distance from “B” to “A”
6) Add half of “B to A” to temporary mark “C.” This will be your turn around point.
An 800 meter adjustment at the turnaround makes a BIG difference to the length of the km / mile split at that point, in fact this is half a mile! If a measurement is off by that much, then I suggest that re-marking the km / mile markers after the adjusted turnaround would be in order so that runners don't get upset when their split times get messed up.