The least important things sometimes, my dear boy, lead to the greatest discoveries.The Doctor (William Hartnell) Serial: The Space Museum Episode: The Dimensions of Time (1965).

Every planetary body (including the Earth) is surrounded by its own gravitational field, which exerts an attractive force on all objects. Assuming a spherically symmetrical planet, the strength of this field at any given point is proportional to the planetary body's mass and inversely proportional to the square of the distance from the center of the body.I'm beginning to have my doubts.

The strength of the gravitational field is numerically equal to the acceleration of objects under its influence, and its value at the Earth's surface, denoted g, is expressed below as the standard average. According to the International Bureau of Weights and Measures, under the International System of Units (SI), the Earth's standard acceleration due to gravity is:

g = 9.80665 m/s2 (32.1740 ft/s2).

I know that explanation is simplistic but gravity is expected to be fairly constant on the surface of the Earth.

This week I checked out the Cosmos Mystery Area near Mount Rushmore and was astonished by one thing in particular. The pendulum.

I pushed the pendulum myself and the force required to push it to a 45^{o} angle towards the wall from it's natural point was less than the force required to push it to a 30^{o} angle away from the wall. The first 15^{o} was fairly easy but the force required increased dramatically after that point.

I hate to speculate but I will. The cabin is on a hillside and there are cliffs to one side and gullies towards the other. The mountains on one side are denser than the air on the other and with much greater mass on one side it there is a greater attraction in that direction.

The pendulum naturally sets 10 to 15^{o} off true zero. Once the pendulum is pushed passed true zero the gravitational attraction of the Earth adds to the force pulling from the mountains and makes the pendulum much harder to push.

I believe this and other similar areas deserve more attention than the blanket dismissals I have seen on many websites discussing the phenomena.

Update:

h/t to

h/t to LaFeminista from the Comments.

Recent analysis shows a much higher surface gravity variation.

Via Science Daily. Data from The Western Australian Centre for Geodesy.