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Letter from Stephanie: Maps

I'm going to clear some space on my work table to write to you–today it is covered with maps!

I'm using the maps because we just finished collecting several more cores and images of the seafloor. Maps supplement my journal as a place to record observations and other data; I use maps to record sites where we collected data, and to record patters we observed.

Which type of map do I use for my work? Well, there are several map projections from which I can choose, but I often use Mercator maps because the directions and distances can be measured easily and fairly accurately.

Why don't I use a globe? Globes are not only expensive–they're a little difficult to carry around! It's kind of hard to fold a globe, don't you think? I also need a BIG map on which to record all of my data points and projections because I want to be able to show locations very precisely; I would need a gigantic globe to be able to cover that kind of area. And of course it would be pretty hard to plot points on a globe–the pencil slides around so easily in high seas!

All of this brings us to a problem faced by cartographers (people who make maps and map projections): How do you take a landmass that exists on a sphere, such as our Earth, and make it flat without distorting the edges or the area?

Cartographers have made many attempts to address this problem; there are over 100 different map projections, some of which have been in use for over 2,000 years. Each attempts to represent the sphere of the Earth on a flat surface. Each fails in a unique way, distorting either distance, direction, area, or scale (or a combination).

Translating a globe onto a flat surface always requires some compromise. Cartographers must choose which features they will preserve and which they will distort. Decisions must be made about the placement, size, scale of features, and about the distance and angular relationships between features.

Each type of map, or projection, has a different approach toward managing the placement, size, and scale of physical features, as well as distance and angular relationships between features. Some maps preserve the areas (actual size) of the continents, but their shapes end up becoming distorted. Some maps are better for preserving directions or distances. Here's a breakdown:

  •  A map preserves distance when distances from the center of the projection to any other place on the map are correct.
  •  A map preserves direction when azimuths (angles from a point on a line to another point) are correct in all directions.
  •  A map preserves the scale when the same scale can be used at any location on the map. (Extra definition–a map is conformal when the scale at any point on the map is the same in any direction; but this may mean the scale changes from place to place.)
  •  A map preserves area when all the regions on the map have the same proportional relationships as those same regions on Earth. In other words, if Antarctica is two times the size of X, it will appear to be twice as large on a map that preserves area.

The different map projections portray this information in different ways, so some maps are more appropriate for specific purposes than others. Map makers need to know how the map will be used in order to determine the best type of map to create. They can then decide to make the map so that directions are correct and the user can plot a course directly on it; a map with distance preserved, for example, would be most appropriate for someone trying to chart the progress of an expedition.

That brings us back to my favorite map! The Mercator map was made by–guess who? Mercator's full name was Gerardus Mercator (1512-1594). He was a Flemish cartographer, geographer and (in his spare time) mathematician. He created the Mercator map projection in 1568. In Mercator's projection, lines of latitude and lines of longitude are straight lines that intersect at right angles. The lines of latitude are increasingly spaced toward the Pole. This means that the scale of the map changes from the equator toward the Pole, but conformality is preserved. Mercator was a map revolutionary–his map really advanced the art of representing the spherical Earth's surface on a flat paper.

Mercator maps are still used today, especially for navigation on the ocean. Because of the way the map is projected, all straight lines on the map are lines of constant direction. A sailing route between two locations can be drawn as a straight line and a ship can follow a constant compass bearing along the line.

The only problem is that those lines aren't truly straight on the globe! Corresponding lines on the Earth would curve as the Earth curves. As a result, Mercator maps greatly distort the areas toward the northern and southern extremes of the map. On a classroom Mercator map of the world, for example, fifteen degrees of latitude near the equator may be two inches apart; at the poles fifteen degrees of latitude may be five inches apart! This means that on a Mercator map of the world, Greenland looks about seventeen times its size, relative to land masses around the equator, which are represented accurately.

Mercator was obviously a pretty smart guy–even he knew that his own maps were ineffective in the polar regions; he used azimuthal-equidistant maps for the Poles as insets on his 1569 maps of the world. Azimuthal-equidistant maps are a type of Polar projection. They are convenient for showing the polar regions of the globe with minimal distortion. They were used long before Mercator, as early as the fifteenth century.

For my work here in Antarctica, I choose the map projection I use according to my specific needs. Mercator maps are not very good for showing the Antarctic continent from a world view because they distort the land in polar regions, making Antarctica look a lot bigger, relative to other continents, than it really is. But Mercator maps are good for small regions, like my study area, because we can use them for navigation. Polar projections are good projections for seeing the shape of Antarctica. When I can use them, of course, globes are the very best because there is little distortion of any type!

I guess it's time to get back to my maps before I forget what I'm doing. Of course, with my maps, I won't forget where I am! Enjoy your exploration!

American Museum of Natural History

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Maps and Directions

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