We know that g=GM/R^2. Mass of earth and G are both taken as constants. so g is inversely related to R^2 (R=radius of earth) 1.So as R is more at equator g is less but vice verse in poles. 2.if we ascend to higher places then we will increase the distance from us to the center of earth so the new R'=R + the height attained by us, so again more R so g will be less . 3. if go underground also g is affected.