Maps of relatively small regions of the Earth such as a state or a province accurately show places, distances, and directions. For larger regions of the Earth such as a hemisphere, or, for the whole Earth, a map cannot show all distances and directions accurately because the Earth is round, whereas maps are flat. Depending on the intended use of the map, different mapping techniques have been invented to show some aspects such as distances realistically at the expense of other aspects such as direction or the relative size of land masses and bodies of water. A Mercator map is a technique where compass directions are shown as straight lines. It helped sailors map routes to distant places and became the standard for world navigation.
A cylinder can be cut and smoothly rolled out into the flat surface of a map. Due to this property, different globe-to-cylinder mappings have been invented by people over the past centuries. In a simple mapping method, one wraps a transparent sheet of paper into a cylinder around the equator of a globe. Then while keeping the North Pole of the globe pointing straight up, one can view all points of the globe horizontally, and mark them on the paper.
In this mapping, longitudes are represented as equidistant vertical lines, and latitudes as horizontal lines. East-west distances are accurate at the equator, but expand everywhere else. For example, Greenland looks enormously wide. In fact, the North and South poles are stretched out along the length of the map even though they really are mere points. This is true for all cylindrical mappings where the cylinder connects to the equator of the globe.
In the mapping described above, north-south distances are accurately represented. As east-west distances vary from the equator to the poles whereas north-south distances do not, such a map causes difficulty while traveling to distant places because the route one should take is not always shown as a straight line. In 1569, Flemish geographer and cartographer Gerardus Mercator invented the Mercator map where the routes of any given compass direction where shown as straight lines. The Mercator map was considered a significant invention as it came at a time when shipping and nautical navigation was gaining importance.
Mercator stretched north-south distances to match the east-west stretch at any given latitude. This made the distances, and hence the scale, identical in all directions around any point. Nevertheless, areas away from the equator, such as Greenland, looked huge because they were vastly wide and vastly tall. In spite of this, Mercator’s method of mapping was very useful to sailors.
A Mercator map is useful in depicting the regions from the equator to the area about the Arctic and Antarctic circles. Beyond that, the distortion in distances and areas is extremely large. Nowadays, airplanes are able to follow shortest routes across the globe; however, such routes do not utilize constant compass directions, but constantly curving routes called "great circles." Still, a Mercator map is very popular for showing areas and countries near the equator. Mercator maps are often used by established online map software as the calculations for zooming in and out can be done speedily.