Topographic maps are detailed, quantitative representations of real-world terrain in two-dimensional map form. They use the concept of contour lines to indicate the shape of the landscape and allow users to visualize the presence and extent of key terrain features including hills, ridges, valleys and depressions. Other features including streets, rivers and vegetation are indicated by symbols.
If you’re interested in mapping, surveying, or orienteering, and want to learn more about common terrain features in a topographic map, then this article is for you. We tell you more about the key characteristics of topographic maps, the most common terrain features shown, and how you can use them.
What is a topographic map?
A topographic map is a kind of map characterized by large-scale detail, elevation contour lines and representation of varying terrain.
Topographic maps use contour lines to express 3D reality in two dimensions, allowing people to visualize rising and falling landscapes, and identify features such as mountains or canyons by height and shape. They show geographic positions and elevations for natural and man-made land features in a specified area, at a specific scale.
Beyond contours, topographic maps include symbols representing important features such as streets, buildings, waterways and forest areas. The specific symbols used may be varied in order to improve the readability and visual appearance of a map. A key to symbols is normally provided with each individual map and should be consulted before reading.
What are contour lines?
Contour lines are the primary feature of topographical mapping and are used to quantitatively represent the formation of the Earth’s surface in relation to a reference surface, usually sea level.
Contour lines are drawn to correspond with the real world landscape, indicating the presence and extent of mountains, valleys, depression, flat plain areas etc.. They show the overall shape of the physical terrain, including hills, slopes, and depressions, by tracing a constant line of elevation on the map. You can think of them as imaginary lines connecting map points with the same elevation on the land’s surface.
Index contours are prominent and bold-colored contour lines which have the elevation marked on them for quick reference when map reading.
What is a map scale?
Map scale is generally noted as a ratio at the bottom of the map sheet. This ratio tells you how big to think – one unit of map measurement is equivalent to the specified number of real-world units.
The most common scale for US topographical maps is 1:24,000, which means that 1 inch of map represents 24,000 inches (2000 feet) of physical land area. The scale of a topographic map is important and gives an indication of how detailed the map will be.
What are grid lines?
Topographic and other maps have horizontal (west to east) and vertical (south to north) grid lines to help you locate precise points on the map and in reality. When reading the map, you should first read right on the vertical grid line before reading upward on the vertical line.
What are the common terrain features in a topographic map?
Terrain features will be identified in a similar way on all standard topographical maps. Understanding these features is key to being able to navigate between locations or pinpoint a precise site using a topographic map.
Major Terrain Features
There are five major terrain features in topographic mapping:
– Hill
– Ridge
– Valley
– Saddle
– Depression
Minor Terrain Features
Alongside the major terrain features are three minor terrain features:
– Draw
– Spur
– Cliff
Supplementary Terrain Features
In addition, there are two further supplementary terrain features, Cut and Fill.
How do you read terrain features in a topographic map?
When reading a topographic map, first look at the scale and consult the key for symbols. This will tell you the real-life size of the terrain shown on the map, and allow you to visualize individual map symbols as real features.
Contour lines can then give an immediate broad view of the landscape and can be deciphered as individual terrain features. For example, contour lines which are spread far apart indicate a gentle slope, while a steep slope is indicated by tightly packed contour lines. If contour lines are stacked directly on top of one other, this shows a cliff. An area which is completely flat will have no contour lines.
Knowing the contour interval may be important, as this shows the amount of elevation change occurring between each separate contour line. Contour intervals will vary between maps according to scale and terrain (e.g. 40-feet intervals are often used on 1:24,000 scale maps but small scale maps may use 50 or 100-foot intervals). Glancing at index contours can give you a quick view of exact elevation levels.
Here are some of the ways in which you can easily recognise some key terrain features:
Peak: The peaks of mountains or hills are evident from concentric contour lines. Peaks may also be marked in some other way, including by name or with elevation height.
Valley: Valleys are shown with contour lines which form a “V“or ”U“ shape on the map, pointing uphill towards higher elevations
Ridge: Like valleys, ridges are also shown by “V“ or ”U“ shaped clusters of contour lines, but this time pointing down towards lower elevations.
Saddle: Saddles are the lower spots which sit between between two hill or mountain peaks. They are shown with counter lines in an hourglass-shape.
What are topographic maps used for?
Topographic maps contain information that makes them extremely useful in a range of different fields today. From Earth sciences, geography and resource management, through city planning and civil engineering, to leisure activities such as orienteering, hiking and camping, topographic maps have a vital role to play.
When was topographic mapping invented?
Topographic mapping appears to have originated in the late 18th century with the development of contour lines by French engineer J.L. Dupain-Triel in 1791. A 1774 survey of the Scottish Schiehallion peak by British mathematician Charles Hutton marked the first known use of contour lines in practice. Use of topographic mapping spread widely amongst governments and professional surveyors in the 19th century.
A final word…
We hope this article has given you a good initial understanding of the common terrain features you will find in topographic maps. Topographic mapping is a useful and fun way of viewing the Earth and its terrain features and you may enjoy practicing what you’ve learned by reading some topographic maps for your area
