Gravitational field lines are always pointing towards the center of mass. Magnitude of gravitational field is proportional to the number of field lines per unit area through a surface perpendicular to the lines. Therefore, g is larger when lines are close together and smaller when they are far apart.
Gravitational field strength close to the Earth can be considered as uniform as the Earth surface is relatively flat at a point.
Variation of gravitational field strength on Earth
- Earth is not a perfect sphere (ellipsoid, flattened at the poles). Gravitational field strength increases(at sea level) when one moves from equator to poles.
- Density of Earth is not uniform.
- Earth is rotating about an axis passing through the poles. Gravitational Pull on a body at equator has to provide for the centripetal acceleration required for rotation. Hence the measured gravitational acceleration is smaller.