Topography and topographic maps

Topography is the physical shape of the land, particularly as it relates to elevation. Topographic maps are two-dimensional graphical representations of the three-dimensional topography that also provide a detailed and accurate inventory of what exists on the land surface, such as geographic and cultural features.

Topographic maps are distinguished from other maps in their representation of elevation as contour lines. Contour lines are drawn to match the shape of physical features and successive contour lines represent ascending or descending elevations. This allows a user to quickly discern the shape of any landform, determine its elevation, and estimate the rate of elevation changes. For example, a round hill would appear as a series of concentric closed loops that become successively smaller with increased elevation. The closer the contour lines are to one another, the steeper the slope.

In addition to contoured elevations, topographic maps show many other features of the land, including names of natural features such as mountains, valleys, plains, lakes , and rivers . They identify the amount of vegetative cover and include constructed features like minor and major roads, transmission lines, and buildings. Topographic maps also show political boundaries, survey markers, and different map coordinate systems such as latitude and longitude .

The value of topographic maps is in their accuracy and consistency. Topographic maps are based on a rigorous geodesic base, which defines the shape of Earth over a given land area . This ensures that all included features will be shown in the exact position. All features on the maps conform to a consistent set of map symbols, allowing comparison of topographic maps from anywhere in the country.

The high accuracy and range of information of topographic maps makes them useful to professional and recreational map users alike. Topographic maps are used for outdoor activities like hiking, camping, and fishing and in professional fields such as engineering, energy exploration, natural resource conservation, environmental management, public works design, commercial and residential planning.

To meet the needs of various users, the United States Geological Survey produces topographic maps at different scales for the entire United States. The scale is the ratio of a unit of measurement on the ground to that on a map. For example, if one inch on a topographic map equals one mile (or 63,360 inches) on the ground, the scale of the map is 1:63,360. The most common scale for topographic maps is 1:24,000, where one map inch equals 2000 feet on the ground. This size map is called a 7.5-minute quadrangle because it covers 7.5 minutes of latitude by 7.5 minutes of longitude . Maps at this scale are very detailed. A map with a larger ratio, such as 1:100,000 will cover more area but show less detail.

See also Cartography; Relief