This map is based off the Universal Transverse Mercator. The Universal Transverse Mercator (UTM) conformal projection uses a 2-dimensional Cartesian coordinate system to give locations on the surface of the Earth. Like the traditional method of latitude and longitude, it is a horizontal position representation, i.e. it is used to identify locations on the Earth independently of vertical position.
Or, in other words, this map was created without the ridiculous imaginary curve that is never witnessed.