The diagram at left depicts a human head drawn with true shape and area on one map and then what happens as the image is projected onto a conformal and onto an equal area map. The first projected map is a Mercator, which is a conformal map (we will look at the Mercator in greater detail later). Notice that the BASIC shape of the head is held true, but the area is severely distorted. In the second instance of projection, an equal area map is shown. The area of the head is now correct, but the shape has been severely distorted. Cons: The basic con is that a single cone cannot show the entire globe. They will typically show a little more than a hemisphere. Also, conic maps show only a sector of a complete circle…they don’t develop into a complete circle. Great Circles and Great Circle Routes. A great circle is the smallest possible ‘hoop’ that would fit completely around the globe. There are an infinite number of such ‘hoops,’ and they may be oriented any which way imaginable. This means that any two points on the surface of the globe may be connected by moving one of these ‘hoops’ until it passes through both points. An important fact is that the shortest distance between any two points on the surface of a globe is a great circle route. A great circle route may be determined for any two points on a globe just by connecting the points with the shortest possible piece of string as shown in the following diagram. Ideally, then, a navigator would like to follow a great circle route because it would minimize travel distance, time, and cost. We will find, however, that great circle routes are seldom straight, navigable lines! We will need a trick to make the Mercator work. 1. Any straight line drawn on the Mercator is a Rhumb line or Loxodrome. This means that any straight line may be navigated upon. 3. Only meridians and the equator are also the shortest distance between any two points since they are all equivalent to great circles. 4. All shortest distance paths other than meridians and the equator are curved so they are unnavigable. Note that a straight line is drawn between Portland and Cairo. Since this a straight line, it is a Rhumb and therefore navigable. This path would, however, not be the shortest distance between the two cities. If you had a globe in front of you, you could trace this path and see that you would have to travel along the ‘fatter’ part of the globe…that is nearer the equator. There is a second line on the diagram, a curve. This is actually the shortest distance between Portland and Cairo as it is a great circle route. If you tried connecting these two cities on a globe with string, this path would require the least amount. The curved path is not a Rhumb or Loxodrome so one could not navigate upon it. Why then is the Mercator such a good map for navigation? The following diagram shows how to navigate using the Mercator. If the curved path is the shortest distance, but can’t be navigated upon because it is curved, then why not approximate the curve with a series of straight, navigable, segments? A navigator would take a heading along the first segment and fly for a certain period of time and then adjust course to follow the second segment and so on. This is how to navigate using the Mercator. Cons:1. If you look at the previous diagram, you will see that the poles are not visible. This means that the Mercator still cannot show the ENTIRE globe. 2. Although this is the map you have worked with most in your school career, it has lied to you!! One of the worst characteristics of the Mercator, being a conformal map, is that it severely distorts area. Note the apparent sizes of Greenland and South America. If you only studied the Mercator, and haven’t looked at a globe, you would come to the conclusion that Greenland is as large or larger than South America. This isn’t the case in reality. South American is actually 8 times the size of Greenland!! Take a look at the following diagram. The map on the left is a Mercator…looks familiar doesn’t it? This is the map you have seen most of your life and it is responsible for giving you the impression that Greenland is so big. Now look at the Equal area map on the right. The shape of Greenland looks strange, but its area is now correct. Source.