Understanding map scale

Scale is the relationship between the size of features on a map and the size of the corresponding objects in the real world. Scale is commonly expressed as a ratio, or representative fraction, such as 1:24,000. This scale means one unit on the map is equal to 24,000 units on the earth. Another way of thinking about it is that the objects on the earth are 24,000 times larger than the features on the map which represent them.

You will commonly see references to two types of maps: large-scale and small-scale. Generally, a large-scale map, such as a map of city streets or a building plan, covers a small area in more detail. A small-scale map, such as a world map, covers a large area in less detail.

GIS maps are dynamic—you can change the scale to see more or less detail as desired.

The map on the left is displayed at a larger scale than the map on the right. On the left, you see a small area of the earth's surface (the city of San Diego, California) but features such as roads are visible. On the right, you see a bigger area of the earth's surface (the continental U.S.), but with less detail.