The ratio between the distance separating two points on a map of the earth's surface at a standard latitude (or latitudes) and the distance between the corresponding points on the earth's surface itself. For most map projections, this factor is a slowly varying function of latitude, and for synoptic- chart base maps it is usually of the order of 10⁻⁶ to 10⁻⁷. The scale factor is sometimes defined as the reciprocal of the above ratio. Compare conformal map.