It is not possible to take a small part of the Earth’s surface and lay it flat – it would be similar to trying to lay a piece of orange peel flat on a table: some of the peel will break. The same problem arises when making a flat map of the Earth’s surface.
To overcome this and create the maps we use for navigation, the Earth’s surface is projected onto a flat surface. There are a few types of projection used:
A lambert conformal conic chart can be thought of as a cone placed over the top of the Earth. A light inside the Earth then projects the Earth’s surface onto the surface of the cone to create the chart.
The chart will effectively be the area between the two latitudes that touch the surface of the cone (the lines of latitude drawn in this diagram).
This makes the Lambert Conformal Conic chart most accurate for areas in the mid-latitudes.
The map created by a Lambert Conformal Conic projection will depict the lines of longitude converging towards the pole (as they do on Earth).
This means that a straight line drawn on a Lambert Conformal Conic chart will be a great circle.
The straight line (great circle) will cross each meridian at a different angle, so the direction measured (°T) will change depending on where it is measured on the chart.
The Transverse Mercator can be thought of as a cylinder surrounding the whole Earth, touching it along the equator. A light inside the Earth projects the Earth’s surface onto the surface of the cylinder to create the chart.
The chart created by a Transverse Mercator projection has parallel lines of longitude and latitude. The lines of longitude are equally spaced but the lines of latitude are not.
This creates an accurate chart at the equator but causes a lot of distortion closer to the poles (this is the reason Russia and Canada appear much larger on a map than they are in reality).
A straight line drawn on a Transverse Mercator chart will be a rhumb line.
The straight line (rhumb line) will cross each meridian at the same angle, so the direction measured (°T) will be constant no matter where it is measured on the chart.