Distance
The distance in a geographic context is a measure of the length between two points on the surface of the earth. It is always dependent on a metric, ie. the way of measuring size. For example, distance can be measured in a straight line on an area considered flat, or by taking into account the roundness of the earth (orthodromic distance). The distance measured along a network of paths will have a different value than a straight path (for example, we call the distance measured in a orthogonal plane in a city the Manhattan Distance).
http://en.wiktionary.org/wiki/Manhattan_distance
http://en.wikipedia.org/wiki/Taxicab_geometry
Distance can be measured in meters, kilometers, miles, etc., but also in travel time or energy to get from one point to another.
In a flat space that is without constraints, isotropic in the sense that measured distances are the same in all directions – and thus in a rather abstract universe – Euclidean distance is used. The distance between two points is calculated using the Pythagorean theorem:
