World Archery rules specify the tolerances for the distances to the targets in this rule:
188.8.131.52. The field of play shall be established with corners of 90 degree angles.
Each distance shall be accurately measured from a point vertically beneath the gold of each target face to the shooting line.
The tolerance for distances is at 90/70/60m ±30cm; at 50/40/30m ±15cm; at 25/18m ±10cm.
So while the the short distances have tolerances that are pretty tight, the longer distances can be significantly out.
I was wondering exactly how much difference it could make.
If you consider 18 and 25 meters, the arrows are travelling reasonably flat. I don't think the average adult archer who can reach 90 meters is going to have much of a problem getting to 25.
But what sort of arrival angle is there in an arrow that can only barely get to 90 meters?
To illustrate, we can think of an arrow descending at 45 degrees. (Because it's handy in terms of relationships. )
If the bow is sighted perfectly to hit the middle of the x at 90 meters, if it's 30cm forward of exactly 90m, the arrow is going to land 30cm high. That's a lot of points dropped.
Of course, we can go to the extremes because the tolerance is plus or minus 30cm, so there's a potential move of 60cm. On a 120cm face, that can turn an x into a 1 at a 45 degree arrival.
45 seems a bit extreme. It's the black angle depicted on the protractor below.
How about 27 degrees? I've made it in red. It seems a bit more reasonable. Most people never think about the angle that their arrows are falling out of the sky, because they're sticking out of the target mostly horizontally.
I'll try to get some photos of arrows arriving at different distances in future, so we'll be able to measure it reasonably accurately.
27 degrees arrow arrival is the point where you'd have a 15cm change in zero at the maximum 30 cm tolerance from ideal.
But if we take the entire size of the 10 ring on a 120cm face, we get 12cm. Presuming that we're precisely dialled in at the middle, we've got 6 cm to play with.
If I feed 30cm long and 6cm high into a right angle calculator.... we get an arrival angle of 11.3 degrees.
Heres's roughly 11.3 degrees. Is anyone willing to bet that their arrows at 90 are coming in flatter than that?
Realistically of course, this all means very little because most often, you're going to get sighters at 90m and that's well and truly enough time to get yourself dialled in 6cm or so.
But you don't necessarily always have to start at 90.
Someone truly evil can quite legally shoot a reverse 1440. You'll start at 30, with tight distance tolerances and a flatter shooting bow.
What it could do though, is throw your sight marks out if you were using a computer program to calculate your sight adjustments. You'll have checked them all of course.
Everyone checks all distances... don't they?
We take for granted that the clubs accurately mark the distances, but how accurate is the average archer in positioning target butts to a line?
It's also not like we align the middle either. Most target butts lean back a bit and the tendency is to line the wheels up with some kind of line on the ground.
I would expect that it's not unreasonable to get things within 20cm most of the time. Today I decided that I would do some random checking because I finally have a convenient measuring device. I found a Leica D5 Disto for a ludicrously cheap price and put a call out on facebook for responsible people to tell me "No. Don't buy it"
So yeah. It appears that I have no responsible friends.. and at least 14 of them who I can consider bad influences.
I went out to Tuggeranong Archery Club with the Disto, a tripod and a plumb bob. If I extend the back of the Disto it gives me an edge to hang the plumb bob from and the Disto then adds the length of the bracket to the measurement.
I positioned the plumb bob smack in the middle of the line and then turned the camera on. There's no chance in hell of seeing the dot at 90 meters. It also very conveniently gives me the ability to take a photo of the measurement.
This particular Disto is rated accurate to the millimetre. I'm happy with whoever put the 90m target out.