Why is circularity measure and perimeter not accurate??

[moka]

I've noticed that the new analysis measurement functionon of PS CS5 seems to be inaccurate.

I'm trying to do a research study which requires measurement of circularity. The formula 4pi(area/perimeter^2) gives the ideal circle as 1.0. This is the formula given in the adobe manual.

However, when i try to draw a constrained circle, which gives me equal diameters in all axis, the circularity does not equal 1.0. The area measurement seems to be correct, but the perimeter that it calculates is also inaccurate.

I've tried changing the size of the circle, smoothing the edges etc, but the closest the value gets to is 0.9.

If anyone knows of a solution or why the perimeter and circularity measurement is wrong, I'd really appreciate it!!!

[the cute]

maybe you have to personalize your measurement in the analysis menu
and here is a tutorial about that :
http://www.youtube.com/watch?v=kUDQaNu3fHE
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