Earth science is a very map-oriented discipline, because geologists are always having to view and think about the disposition of rock bodies across the landscape. This section provides just a little elementary material about maps. Cartography, a well developed discipline in its own right, is the study of maps as maps.
People have been drawing maps to show the relative disposition of things on the earth's surface for a very long time. The accuracy of maps has improved as the ability to locate things relative to one another (which in one way or another always involves what could be termed surveying) has improved.
One unavoidably troublesome aspect of map-making has been (and continues to be) the need to transform spatial relationships on the surface of a nearly spherical body, the earth, into spatial relationships on a flat piece of paper, the map (Figure 3-34). The only way around this problem is to draw the map on a small globe, but the obvious impracticality of carrying around and using a globe- shaped map make that expedient not very useful. Various kinds of projections, or ways of systematically distorting a segment of a spherical surface to make it fit onto a plane, have come into use. Each such kind of projection has its advantages and disadvantages. For geologic maps of fairly small areas you don’t need to worry about the problem of projections, but for maps of whole countries and continents, or even of the states of the U.S., you do.
Aside from the fundamental requirement of representing accurately the spatial relationships in accordance with a given kind of projection, there are just a few elements essential to all maps:
- A scale, which expresses the ratio of a given horizontal distance on the map to that same horizontal distance on the actual land surface. This can be expressed as a numerical ratio, or it can be drawn as a labeled scale bar on the map, or (preferably) both.
- A north arrow, an arrow somewhere on the map, usually in the margin, that shows the direction of true north.
- A key or legend that explains to the user of the map all of the various symbols that show features on the map.
All maps have to have a scale. The scale of a map is the ratio of the distance between any two points on the map and the actual distance between the corresponding points on the Earth's surface. It’s usually expressed as a ratio, called the representative fraction, one divided by some number, like 1:25,000 or 1/25,000. It can also be expressed as a verbal statement: for example, one inch equals one mile, or one centimeter equals one kilometer. Most maps also show the scale graphically, by having a horizontal bar at the bottom of the map with tick marks along the bar labeled with the distances represented by the positions of the tick marks along the bar.
Here’s something really tricky to remember about maps scales: A large-scale map is one that uses a relatively large distance on the map to represent a given distance on the Earth's surface
A small-scale map is one that uses a relatively small distance on the map to represent a given distance on the Earth's surface.
This is confusing, because a small-scale map tends to cover a relatively large area of the earth's surface, and a large-scale map tends to cover a relatively small area of the earth’s surface!