Longitude can be accurately used as a scale to measure distance only at which location?

Longitude lines, also known as meridians, converge at the poles and are farthest apart at the Equator. This means that at the Equator, each degree of longitude represents a consistent measure of distance—approximately 69 miles (111 kilometers). As you move away from the Equator towards the poles, the distance represented by a degree of longitude decreases significantly. At the poles, longitude lines meet at a single point, making it impossible to use them as a scale for measuring distance.

Thus, the most accurate location for using longitude as a scale for measuring distance is the Equator, where the distance between degrees remains constant. At other locations, such as the North Pole or South Pole, or even the Tropic of Cancer, the effectiveness of longitude as a distance measurement diminishes.