Because the Earth is nearly spherical, the shortest distance between two points on the surface of the Earth is approximately the distance along the arc of a great circle passing through both points. A great circle is a circle on the surface of a sphere centered on the sphere's center. Since the Earth is not a perfect sphere, this distance is only approximate, but ignoring hills and valleys, the approximation is always within a few percent of the true distance.

A Google map is a Mercator projection of the Earth's spherical surface onto a flat surface. A straight line segment on this projection does not follow an arc of a great circle. Instead, the line segment follows a rhumb line. A rhumb line marks a path of constant compass bearing, so if one travels across the Earth's surface in a constant compass direction, five degrees south of east say, s/he follows a rhumb line. Before the advent of computer navigation, rhumb lines and the Mercator projection were widely used in navigation, because following a path of constant bearing is relatively easy. Following an arc of a great circle involves no simple use of compass bearing.

If two points have the same longitude or lie along the Earth's equator, the circular arc and the rhumb line between them are identical, but if two points have very different longitudes at higher latitudes, the circular arc and the rhumb line between them can be very different and have very different lengths. The difference between the great circular distance (the shortest distance) and the rhumb line distance between two points can exceed 30%.

The red path illustrated above joins Anchorage, Alaska to Glasgow, Scotland. This straight line path on the Mercator projection follows the rhumb line joining the two locations. The blue path follows the great circle joining the locations. The circular path appears much longer on the map surface, but this greater length reflects the extreme distortion of the Earth's surface created by the Mercator projection at high latitudes. Greenland appears larger than the United States on the map but is truly much smaller. In fact, the blue path is much shorter than the red path along the Earth's surface. The distance between Anchorage and Glasgow along the blue path is approximately 6670 kilometers. The red path is approximately 8470 kilometers long, 27% longer than the blue path.

For a shorter path at lower latitudes, the great circular and rhumb line paths differ much less. The paths joining Spokane, Washington and Thunder Bay, Ontario are approximately 2090 and 2100 kilometers long, respectively. For locations separated by less than a few hundred kilometers, the circular arc and the rhumb line are hardly distinguishable, and the difference in length is negligible. Below, 111.6 kilometers separate Greensboro, NC and Raleigh, NC by both measures.

Airliners typically fly the shortest distance to a destination. A flight from New York to London follows the blue path below. The flight travels up the east coast of the United States and over Newfoundland rather than flying "straight across" the Atlantic, because the path up the east coast is truly shorter.