8.3 What Makes a Map?
The purpose of a map is not limited to navigation. Maps are used in the geosciences to show data or information in a spatial context. Maps are used to convey information about where observations were recorded in the field, or to show a geologist’s interpretations of the materials exposed or deposited on the Earth’s surface. Geologists often use topographic maps as a base on which to display the geological information or data they wish to share. A topographic map is a graphic two-dimensional representation of the three-dimensional surface of the Earth. The features shown on a topographic map may be divided into three groups:
- Relief – hills, valleys, mountains, and plains
- Water features – lakes, ponds, rivers, canals, swamps, and streams
- Cultural features – roads, railways, buildings, and land boundaries
You were introduced to some important concepts about maps in the Google Earth Tutorial and Lab 1 Exercises. You learned that all maps are drawn to scale; that is, a designated distance on the map is equal to a corresponding real distance on the actual surface of the Earth. You also learned that locations on maps are specified using grid systems that might include the geographic or UTM systems. This lab will build upon those concepts to give you a deeper understanding of what elements make a map and what information you can convey in a map.
Map scale
Map scale refers to the fixed ratio between the distance you measure on a map (or in satellite imagery or air photos) and the actual distance on the ground, in the real world. You were introduced to the concept of scale at the beginning of the term, when you completed the Google Earth Tutorial and Lab 1 Exercises. In that tutorial, you used a scale bar to understand distances in Google Earth. Scale bars, also called graphic scale, are one way to represent scale on a map. Graphic scales are used to quickly determine the distance between two points on the map, by comparing the length you measure on the map with your ruler to the scale bar. In Figure 8.3.1 below, each of the long bars on the scale is 2 cm long and represents a distance of 1 km on the ground. Let’s say you measured the distance between two points on a map to be 5 cm. By comparing that length to the scale bar, you can see that 5 cm on the map represents 2.5 km on the ground. Note that the scale bar may be distorted by your screen if you are reading this on an e-reader, smartphone or computer depending on your display settings.
Another way to represent map scale is using a representative fraction (R.F.). The representative fraction is a ratio written in numbers on a map. Two of the common scales for topographic maps in Canada are 1:250,000 and 1:50,000. A R.F. of 1:250,000 means that 1 unit of measurement on the map represents 250,000 of the same unit of measurement on the ground. You can use any unit of measurement (centimetres, inches) as long as you are consistent! A map with a scale of 1:250,000 means that 1 cm