Mercator projection

The Mercator projection is a conformal cylindrical projection in which meridians are equally spaced vertical lines and parallels are horizontal lines spaced so that the scale expansion in longitude is matched in latitude, preserving angles and rendering every rhumb line as a straight line. That property makes it the standard projection for navigational charts because a course laid off with parallel rulers is a constant true course. Its cost is gross area distortion toward the poles, so high-latitude work and great-circle planning use the gnomonic projection instead.