# What property does this equation calculate?

It's pretty difficult to Google for the meaning of a formula.

This is the equation, it has to do with ellipses and GIS coordinates.

$$\nu =\frac{ a} {\sqrt{(1 - (e^2 \cdot \sin(\varphi))^2)}}$$

$a$ is an ellipsoid axis.

$\varphi$ is geodetic coord latitude in radians.

$e^2$ is eccentricity squared.

I see it all over the code I'm porting and would like to separate it out but I can't figure out what to call the function!

Edit: On page 38 of this PDF the equation and its context is described.

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I believe it describes a band of some kind. $a<\phi<b$ –  Fred Kline Sep 23 '12 at 18:01
$3.14159 n-0.443255<\phi<3.14159 n+0.443255$ –  Fred Kline Sep 23 '12 at 18:14
where $n \in Z$ –  Fred Kline Sep 23 '12 at 18:20