Free-air anomaly is the difference between the _____ gravity and theoretical gravity at a point.

The free-air anomaly is specifically calculated as the difference between the observed gravity and the theoretical gravity at a given location. The observed gravity refers to the actual measurements obtained from gravity surveys, while theoretical gravity is derived based on models that account for the Earth's shape and gravitational field, which consider factors such as latitude and elevation.

In this context, the concept of "observed" gravity indicates the direct measurement taken in the field, representing the real-world value at that specific point. This measurement can be influenced by various local geological structures and other factors that are not accounted for in the theoretical calculations. Measuring the difference helps geodesists understand variations in the Earth's gravitational field and can be crucial for applications such as geophysical surveys and tectonic studies.

The other options, while possibly related to gravity concepts, do not accurately define the relationship needed to determine the free-air anomaly.