Gravity Near the Earth's Surface

               m m_E                       m_E 
mg = G ——— thus g = G ——
               r_E²                          r_E²

Calculate the acceleration of gravity at the top of Mt. Everest which is 8.8 km above sea level. 
Earth's radius is 6380 km.  Earth's Mass is 5. 97 x 10 ²⁴ kg.
  

             m_E                                                         5. 97 x 10 ²⁴ kg
g = G ——— = 6.67 x 10 ^-11 N m² kg ^-2 —————————————  =  9.76 m s ^-2  
             r_E²                                                 ( 8.8 x 10 ³ m + 6.38 x 10 ⁶ m)²

Factors that effect g on Earth.

Earth is not a perfect sphere.

Rocks of different densities.

Fluids and gases trapped underground.

  

Gravimeters can measure gravity anomalies on the order of 1 part per 10 ⁶ to 10 ⁷ in the value of g. 

They are used to find mineral and petroleum deposits.