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# Cailleuxs roundness index formula watch

1. Question for Geographers really..

I know its 2r/a * 1000 where r = radius of sharpest angle of the sone and a = longest axis however do you have to use mm or cm for r and a values?

2. any1 ? :-/
3. (Original post by CHL)
Question for Geographers really..

I know its 2r/a * 1000 where r = radius of sharpest angle of the sone and a = longest axis however do you have to use mm or cm for r and a values?

I'm a Physicist but I can answer your question using a purely mathematical approach. Because you are dividing r by a, as long as they have the same units it won't matter which ones you use (cm, mm, m, km, inches, miles, AU, light-years etc - although some of these would be pretty silly units to use!). All you are interested in here is the ratio of 2r to a.

For example
Suppose r = 30 cm = 300 mm and a = 15 cm = 150 mm

Then, with r and a in cm:

2r/a * 1000 = (2*30)/15 * 1000 = 4000

With r and a in mm:

2r/a * 1000 = (2*300)/150 * 1000 = 4000

These results are the same.
4. but the equation is only spose to give a value between 0 and 1000 - where 1000 is perfectly round .. :-/ ?
5. ahh ive see what youve done and i understand now

you wouldnt of known it but r will never if ever be a larger figure than a, thanks tho mate - I will use mm for both
6. Sorry, it will give a value between 0 and 1000 as long as 2r<=a. I didn't know that it had to be between 0 and 1000 because I'd never seen the equation before (My knowledge of Geography is a GCSE A* that I got in 2001 - therefore limited). The statement about the units is still true though, just make sure that 2r is less than or equal to a.

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Updated: March 13, 2004
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