Scale refers to the relationship or ratio between a distance on a map and the distance on the earth it represents. Maps should display accurate distances and locations, and should be in a convenient and usable size. To accomplish this, maps need to show information proportionately smaller than the real-world objects they represent. This proportion is referred to as a map scale, or the relationship between distances on the map and distances in the real world.

Map scales are generally expressed in one of three formats:

• representative fraction
• graphical scale
• verbal-style scale

In a representative fraction or ratio, the number to the left of the colon (the denominator) represents a distance on the map, and the larger number to the right (the numerator) represents the actual distance on the earth's surface [See Figure 5]. The units of distance are not included because they are the same on the map and in the real-world, regardless of what they may be (ex. inches, centimeters). The advantage of a representative fraction or ratio is that any unit of measurement can be used (Monmonier, 1988).

On a graphical or barscale, the length of the scale represents a certain length of the earth. The units of measurement are provided. In this example, metric and English units are both given [See Figure 6]. The advantage of a bar scale on a map is that it can be easily enlarged or reduced without affecting the accuracy of the scale, assuming the reproduction is enlarged or reduced by the same factor in both horizontal and vertical directions (Monmonier, 1988).

Verbal scales are a simple translation to a usable unit of measurement, and can be as simple as "1 inch on the map is equal to 2000 feet on the ground." This type of scale is rarely used on maps, yet many maps are designed in such a way that one commonly used unit on the map will equate to another commonly used or easily estimated unit on the ground (Monmonier, 1988).

Large scale vs. Small scale

Given a map scale ratio or fraction, the larger the second number, the smaller the scale (or less detail in the map) [See Figure 7 and Table 3].  A map of 1:50,000 is of larger scale than a map of scale 1:1,000,000. In other words, a map which has a larger scale will represent features on the map larger. When choosing a map scale, one must sacrifice the amount of area covered in order to obtain greater detail.

Large Scale	Small Scale
Figure 7: Illustration of different map scales

Table 3: Representative distances for map scales:

(SIC 2002)

A GIS does not use a specific scale per se, because its maps can be enlarged, reduced, and plotted at many different scales other than the scale of the original data. However, all maps need to be converted to the same scale in order to be layered.