As the crow flies is a phrase that often comes up when geocachers discuss distances; Especially when they consider the distance from a given point to a geocache location.
When geocachers talk about distance between two points they talk about two different kinds of measurements.
The first is the distance required to travel between the two points. This distance is the length of the trail or the distance required to drive from one place to another on the roads. The nature of the world is that there are obstacles between points. Navigation requires deviation from a straight-line course to avoid these barriers.
These barriers are why turn-by-turn navigation is necessary when driving between two pints. It accounts for the viable path between two points rather than considering a straight-line distance between them.
What As the crow flies Means
The second distance that geocachers talk about is a point-to-point distance or a straight-line distance. Often we think of this as a presumed straight line between the two points.
This imaginary line is the distance we would travel if there were nothing in our way.
Most frequently geocaches will refer to this as the distance to the cache “as the crow flies,” and sometimes they will say “crow miles,” “CROMS,” or “chromes,” to shorten the phrase.
Great circle distance
The reality is a little more complicated than this. The shortest distance between two points on the surface of a globe is not a straight line. It is a curved line. But we measure the Earth along the surface of a geodesic sphere.
Instead of a straight line through the sphere’s interior, the line curves along its surface. This line in curved, rather than Euclidian, space is a great circle distance or orthodromic distance.
Every set of two points on a sphere that is not opposite one another traces a unique great circle that encircles the globe. An arc of the circle is the shortest orthodromic distance between those two points.
GPS receiver units take the calculations for great circle distance into account when providing the distance to a cache from the geocachers current location.