This famous cylindrical projection was originally designed by Flemish map maker Gerhardus Mercator in 1569 to aid navigation. Meridians and parallels are straight lines and cross at 90° angles. Angular relationships are preserved. However, to preserve conformality, parallels are placed increasingly farther apart with increasing distance from the Equator. Due to extreme scale distortion in high latitudes, the projection is rarely extended beyond 80°N or S unless the latitude of true scale is other than the Equator. Distance scales are usually furnished for several latitudes.
Meridians are straight and parallel.
Parallels are straight and parallel.
Meridian spacing is equal and the parallel spacing increases away from the Equator. The graticule spacing retains the property of conformality. The graticule is symmetrical. Meridians intersect parallels at right angles.
Linear scale is true along the Equator only (line of tangency), or along two parallels equidistant from the Equator (the secant form). Scale can be determined by measuring one degree of latitude, which equals 60 nautical miles, 69 statute miles, or 111 kilometers.
Best suited for navigation. Secant constructions are used for large-scale coastal charts. Commonly used for nautical charts. Examples are the charts published by the National Ocean Survey, US Dept. of Commerce.
This projection can be thought of as being mathematically based on a cylinder tangent at the Equator. Any straight line is a constant-azimuth (rhumb) line. Areal enlargement is extreme away from the Equator; poles cannot be represented. Shape is true only within any small area. It is a reasonably accurate projection within a 15° band along the line of tangency.
Rhumb lines, which show constant direction, are straight. For this reason, a Mercator map was very valuable to sea navigators. However, rhumb lines are not the shortest path—great circles are the shortest path. Most great circles appear as long arcs when drawn on a Mercator map.
In the figure above, all angles are shown correctly, therefore small shapes are true (that is, the map is conformal). Rhumb lines are straight, which makes it useful for navigation.