Why the value of 'g' on …

Acceleration due to gravity on the surface of earth g = GM/r², where r is the radius of the earth, M the mass of the earth and G the universal gravitational constant.
In the above expression for g, G and M are constant. But r varies due to the shape of the earth. We know that the shape of the earth is not a perfect sphere. It is flattened at the poles and bulged at the equator. So the polar radius is smaller than the equitorial radius. Hence g is slightly more at the poles than at the equator. 
Thus the value of g on earth is maximum at poles and decreases as we go from poles to the equator and is minimum at the equator. also g decreses as we increase or decrease the distance of a body from the centre of the earth. 
Also the value of g decreases as we go above the sea level and again decreses deep iside the earth.