Overview of Topographic Maps
In this chapter you will learn how to read and use topographic maps. This is a critical skill to prepare you to learn about more complex geologic maps.
Map Parts, Orientation, & Scale
A map is a plan view (viewed from above, also known as map view) representation of an area on Earth’s surface. Topographic maps are maps that illustrate the topography (vertical shape, such as hills) of the mapped region. Geological maps are maps that illustrate the rock types, rock ages and other geological features of the mapped area.
Every map has the following elements:
- Map area or data frame: the part of the map illustrating the map area itself;
- Legend: a guide to the different symbols used on the map, such as lines representing roads and streams;
- Scale: this defines the relationship of distances on the map to real distances in the area that the map represents;
- North arrow: indicates the direction of geographic north; and
- Title: a name that generally describes the location of the map area.
The topographic and geological maps produced by Natural Resources Canada (NRCan, which includes the Geological Survey of Canada, GSC), and the United States Geological Survey (USGS) are oriented with north at the top of the map. Therefore if you move towards the top of the map you are moving in a northerly direction, and if you are moving towards the bottom of the map, you are moving towards the south. Any movement to the right or left will be towards the east or west, respectively.
Coordinate Systems
There are two coordinate systems commonly used to define positions on Earth’s surface: geographic (latitude and longitude, abbreviated lat/long) and universal transverse mercator (UTM). Latitude and longitude coordinates are used on NRCan and USGS maps; they are simpler to interpret and plot on a map than UTM coordinates. UTM coordinates are more accurate and are used in navigation and field work. Often geologists will report coordinates of important features using both coordinate systems.
Geographic Coordinates: Latitude & Longitude
The longitude is an imaginary line that circles the globe and is oriented so that it passes through the north and south geographic poles (Figure 6.2, top label). Starting with the 0° longitude line (zero degrees, known as the Prime Meridian) that passes through the town of Greenwich, England, the lines of longitude increase to 180° in both directions east and west of the Prime Meridian, meeting at a line of 180° longitude that passes through the Pacific Ocean. Lines of longitude converge as you move north or south of the equator, eventually intersecting at the geographic poles.
It may help to visualize longitude lines if you think of an orange, which when peeled will show the sections of orange oriented like longitude lines that section the Earth into segments.
Lines of latitude are imaginary lines that circle the globe parallel to Earth’s equator (Figure 6.2, lower label). The 0° latitude line is the Earth’s e