Geologic and topographic maps
Geologic and topographic maps are essential tools for understanding and managing Earth's resources and features. Topographic maps illustrate the Earth's surface through contour lines and symbols, highlighting both natural formations like mountains and rivers, as well as human-made structures. They are created using advanced techniques, including aerial photography and GPS technology, ensuring high levels of accuracy in depicting terrain elevation and shape. Conversely, geologic maps detail the distribution and characteristics of rock formations beneath the surface, contributing invaluable data for fields such as oil and gas exploration, environmental studies, and construction planning. These maps are often prepared from extensive field observations and can integrate satellite imagery for enhanced detail.
Both types of maps serve practical applications, including infrastructure development, environmental protection, and resource management. They play a crucial role in planning projects such as roads, dams, and urban developments, as well as in recreational pursuits like hiking and camping. Additionally, geologic maps provide critical insights into natural hazards, aiding in the identification of areas susceptible to landslides or earthquakes. Overall, the combination of topographic and geologic mapping enables informed decision-making for sustainable development and environmental stewardship.
Geologic and topographic maps
One of the earliest known geological maps is a geologic map of England constructed by William Smith in 1815. Topographic and geologic maps are basic tools used for the management and development of the earth's resources. Topographic maps represent on paper the earth's surface and its various landforms. Geologic maps show the distribution of the rocks that underlie the landforms and provide a view of the earth's surface to a depth of several thousand feet.
Topographic Maps
A topographic map is a line-and-symbol representation of natural and selected human-made features of a part of the earth's surface, plotted to a definite scale. Topographic maps portray the shape and elevation of the terrain by contour lines, or lines of equal elevation. The physical and cultural characteristics of the terrain are recorded on the map. Topographic maps thus show the locations and shapes of mountains, valleys, prairies, rivers, and the principal works of humans.
In the past, topographic maps were constructed by labor-intensive field methods, which involved detailed field measurements made with telescopic-type instruments. These data were translated in the field to actual distances and plotted by hand on field sheets for eventual office compilation and printing. More recently, however, most maps have been prepared using adjacent pairs of aerial photographs. Highly accurate, these photographs are further checked for accuracy by reference to global positioning satellites. With the advent of the global positioning system (GPS) network, location in respect to longitude and latitude or to Universal Metric Coordinates and elevations may be readily determined without extensive baselines and triangulation networks. In some instances, laser-beam surveys provide extremely precise control in areas that are subject to earthquakes, such as California, to monitor stress buildup. Complex stereoscopic plotting instruments are used by a trained observer to delineate contour lines and various features on a base map. Field verification of place names and features is required before the map is printed. The maps are then compiled on a stable base (a type of plastic that does not change dimensions during temperature and humidity fluctuations). Such a base helps to ensure the map's accuracy by preventing distortions. Modern photographic and photochemical techniques are used to prepare the map for printing.
All maps must meet accuracy standards established by the government. Special standardized symbols, each with its own meaning, are used to convey a wide variety of information. Also, colors are frequently used to show the more common features. Generally, blue indicates water bodies, brown indicates contour lines, red indicates map features with special emphasis (chiefly land boundaries), pink indicates built-up urban areas, and purple indicates revisions based on new photographic information since the original map was made.
Geologic Maps
Geologic maps are a representation of the distribution of mappable units (formations). These maps provide the data for an accurate compilation of the rock units at the surface or in the subsurface. A geologist makes hundreds of observations each day in the field. Many of these observations are recorded in a notebook or on the field sheet that eventually becomes a geologic map. Some geologic maps are prepared from aerial photographs or from remotely sensed images created through satellites. New detailed geologic mapping frequently reveals information that may require reevaluation of previously mapped areas. Large-scale (1:24,000) geologic maps require detailed examination of the area being mapped. Field investigation describes outcrops as close as a spacing of several hundred feet using a topographic map as a base. Outcrop descriptions include determinations of fossils, rock type, and mineralogy, along with descriptions of rock properties, such as color, thickness, and type of bedding units, and attitudes of the rocks. Some studies are supplemented by geophysical surveys. Drilling (cores or cuttings) is integral to many studies, including oil and gas exploration, mining, and engineering. Samples are examined later in the laboratory. Some of the laboratory work includes microscopic study of thin slices of rock to establish mineral relations, binocular study and identification of fossils, or detailed chemical analyses of the whole rock or of separated minerals, and radiometric age dating.
Compilations of geologic maps require regular reviews of field notes and observations, laboratory data, and information from the scientific literature. Data are transferred to a topographic map database and hard copies are stored in plastic to ensure stability during temperature or humidity variations. Finalized contacts are drawn that divide rocks of one unit from those of another. The degree of certainty of the contacts is shown by a standard set of line symbols. The orientation of the various rock units is indicated by uniform symbols. When a geologic map is complete, it is prepared for publication by conventional drafting methods or by digitization and computer plotting methods. Orientations of surfaces are given as “strike” and “dip,” and orientations of linear features are given as “trend” and “plunge,” giving the directions of maximum slope and their angles from the horizontal.
Geologic and topographic maps are added to large computer databases known as geographic information systems (GISs). Through computer manipulation of layers of information such as geology and topography, informed decisions can be reached by combining these with other data layers. Individual geologic maps are issued in a numbered series by the US Geological Survey and related agencies, and a large number of these maps are available online.
Map Scales and Series
Map scale defines the relationships between measurements of features shown on the map and measurements of features on the earth's surface. These comparisons are numerically expressed as a ratio—for example, 1:24,000, 1:125,000, 1:250,000, 1:500,000, and 1:1,000,000 scale. Large-scale maps (1:24,000 or larger) are used when highly detailed information is required. Examples include proposed projects (roads, large construction projects, and so on) in highly developed or populated areas. Intermediate-scale maps are quite useful in land-and-water management planning projects and in resource management. The 1:100,000 scale (metric) maps have become popular for a growing number of applications, particularly environmental protection and planning. Small-scale maps (1:250,000 to 1:1,000,000) cover very large areas. They are useful mainly in regional planning.
The topographic map series of the National Mapping Program includes quadrangles and other map series published by the US Geological Survey. A map series is a family of maps conforming to the same specification or having common characteristics, such as scale. Adjacent maps of the sample quadrangle series can be combined to form a single large map manually, photographically, or by computer methods. Geologic maps are prepared using existing topographic and/or planimetric base maps (maps showing boundaries but no indications of relief). Thus, the scales of geologic maps generally correspond to those of the common topographic and/or planimetric maps. In special cases, such as a major engineering project (for example, a dam or nuclear power plant), preparation of a site-specific large-scale topographic map may include detailed geologic mapping.
Practical Applications
Topographic and geologic maps help to reveal the structure and resources of the surrounding environment. These maps are basic tools for resource management and planning and for major construction projects. They are used in the planning of roads, railroads, airports, dams, pipelines, industrial and nuclear plants, and basic construction. Both types of maps are also used in environmental protection and management, water quality and quantity studies, flood control, soil conservation, and reforestation planning. In addition, topographic maps are widely used in recreational activities such as hunting, fishing, boating, rock climbing, camping, and orienteering.
Geologic maps provide baseline data for the identification and orderly development of the earth materials required for modern civilization. Examples include sand and gravel, crushed stone and aggregate, clay, metal deposits, and hydrocarbon fields. Geologic maps are also used extensively in environmental monitoring and protection, in local regional planning, and in scientific studies. They help to identify areas prone to landslides or earthquakes, groundwater recharge areas, and potential sand and gravel resources. They are therefore used in land-use and planning studies to determine technically suitable and environmentally safe locations for subsurface solid, hazardous, or low-level and high-level nuclear waste repositories and excavations, waste disposal, water resources investigations, and military applications.
Principal Terms
contour lines: on a topographic map, lines of equal elevation that portray the shape and elevation of the terrain
geologic map: a representation of the distribution of mappable units (formations)
map scale: the scale that defines the relationship between measurements of features shown on the map and the actual features on the earth's surface
topographic map: a line-and-symbol representation of natural and selected human-made features of a part of the earth's surface, plotted to a definite scale
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