Is gravitational force constant all over earth?
For the most part, yes; the weight of an object is equal at any point on the Earth as measured by any standard scale. However, using a gravimeter it is possible to detect minute differences in gravitational force. These differences are due to differences in topography or rock density at the site of measurement. For example, a gravimeter will yield a slightly lower gravity measure if read above a granite slab (low density) than it would over a basalt slab (high density), providing that both slabs are at equal altitude and are thick enough to be detected by the gravimeter (often on the order of 10s-100s of meters thick). Measuring gravity is how many features below the ice on Antarctica have been mapped, and is often how oil fields are discovered.
