I have my x y and z coordinates.

Once again, this is not my game.. this is for GTAVC.

I need to find out if a set of XYZ coordinates is within my radius of like 20 feet. I have my xyz coordinates.

Before you answer, you should know GTA's wacky coordinate system.
X = North/South
Y = East/West
Z = Up/Down (jumping increases your z coords..)

So is there some kind of formula that I can use PI and makybe cosine/sine to determine this? thanks