True Scale at:
Parallel through center , between standard parallels
Lines on the map, straight or not, with constant, linear scale (with length proportional to that of corresponding lines on Earth) are lines of true scale or standard lines. Map projections with a well-defined, nontrivial set of standard lines are sometimes referred to as equidistant.
Meridians are equally spaced Parallels get closer near poles. Parallels are sines. True scale at equator. History
Invented in 1772 by Johann Heinrich Lambert with along with 6 other projections. Prototype for Behrmann and other modified cylindrical equal-area projections.
Each parallel is a circular arc of true scale. The scale is also true on the central meridian of the projection. The projection was in common use by many map-making agencies of the United States from its proposal by Ferdinand Rudolph Hassler in 1825 until the middle of the 20th century. ...
After that, the two graphing papers are superimposed, providing a scale model of the landscape, or rather the targets in it. The true scale can be obtained by just measuring one distance both in the real terrain and in the graphical representation.
In conformal projections, large land masses are distorted while small shapes, local scales and relative angles in the large land mass are preserved. Equidistant maps show ~ between one or two points and every other point. Different maps for different purposes.
9996012717 (as opposed to the more normal scale of 0.9996 for the Transverse Mercator projection): this results in distances along the northing 180,000, almost exactly half way across Great Britain, being to ~.
See also: What is the meaning of Meridian, Map, Projection, Parallel, Map Projection?