mils

Certain compasses (especially military issued) use MILS instead of degrees.  They have the advantage of ease at estimating object distance/size.
 
There are 6400 mil dots per circle. Why 6400? For several reasons.  Using a bit of high-school trigonometry, we can get the following equation
 
2pi * 1000 / 6400 ~= 1.0
 
in other words, two objects 1km away separated by two mil dots are about 1 meter apart.
 
Coincidentally, it is much easier to divide a circle (or anything) into factors of 2 than other numbers.  With 6400 marks, we can divide the circle by 2 for 6 times, so that each slice of the pie is 1/64 of the circle. Then it will be relatively easy to cut 100 marks along the edge of that tiny pie.
 
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on a compass with smaller dial (such as a hand-held lensatic compass), only 320 dots can fit, therefore
 
2pi * 1000 / 320 ~= 20
 
which means if two objects 1km away are separated by two dots, they are 20 meters apart.
 
 
 
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