How to find the radius (in meters) of an overlay item in MapView

I am attempting to program an app for Android that depends on the user being within a certain radius of a set location.

What I would like to know, is how to calculate (and store) the radius in meters of the circle that the user draws onto the map overlay. For example, the user zooms to whatever level they want, touches a point and adjusts a slider to increase or decrease the radius.

I want to find that radius in meters, rat开发者_如何学运维her than pixels over the projection.

Thanks.

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You can calculate the difference in distance between 2 locations/points by using geocoder