Maps Glossary

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Gaging Station

A gaging station is a location along a river or stream where measurements are taken to monitor the water’s level and flow rate.  These stations play a crucial role in predicting floods and droughts, as well as managing water resources.  To measure the water level, a staff gauge or pressure transducer is used. Flow rate is determined by measuring the velocity of the water and the cross-sectional area of the stream.  Gaging stations are often operated by the US Geological Survey (USGS), which has over 8,000 stations across the country.

Gasoline Survey

A gasoline survey is a method used to gather information about the quality of gasoline sold by gas stations.  The survey involves taking samples of gasoline from different stations and testing them for various properties, such as octane rating and ethanol content. The results of the survey can be used to ensure that gas stations are meeting regulatory standards and to identify potential problems with fuel quality.  Gasoline surveys are often conducted by state agencies or private organizations.


Geocaching is an outdoor treasure-hunting game where players use GPS-enabled devices to search for hidden containers, called geocaches. Geocaches can be found all over the world, and they come in all shapes and sizes.  Some are easy to find, while others are more challenging and require solving puzzles or deciphering clues.  Geocaching is a great way to explore new places and connect with other people who share a love for adventure. There are over three million active geocaches worldwide, and the game is enjoyed by people of all ages and backgrounds.


Geodesy is the science of measuring and understanding the Earth’s shape, orientation, and gravity field.  It involves the use of advanced technologies, such as GPS and satellite imagery, to collect data about the Earth’s surface and interior.  Geodesy plays an important role in many areas of science and engineering, including mapping, navigation, and geology. It is also used to study the effects of climate change on the Earth’s surface and to monitor tectonic activity and volcanic eruptions.  Geodesy has been practiced for thousands of years, and it continues to evolve as new technologies and techniques are developed.

Geodetic Datum

A geodetic datum is a reference system used to measure and represent the shape and orientation of the Earth.  It is a set of parameters that define the size and position of the Earth in space, as well as the orientation of its axis of rotation.  Geodetic datums are used in many applications, such as surveying, navigation, and map-making.  The most commonly used geodetic datum in the United States is the North American Datum of 1983 (NAD83), which is based on a network of survey points across North America.

Geographic Information System (GIS)

A Geographic Information System (GIS) is a computer-based tool used to store, analyze, and visualize geographic data.  GIS technology allows users to view and understand spatial relationships between different features on a map, such as rivers, roads, and buildings. It is widely used in many fields, including environmental management, urban planning, and emergency response.  GIS software can be used to create maps, perform spatial analysis, and generate reports based on the data collected.  GIS is an essential tool for modern-day geographers and is continually evolving as new technologies become available.


The geoid is the surface of the Earth’s gravity field, which is a theoretical surface that would coincide with mean sea level if the Earth’s surface were completely covered by water.  The geoid is used as a reference surface for measuring elevations on the Earth’s surface.  It is an irregular shape, and its height above or below the Earth’s surface varies by several meters.  The geoid is used in conjunction with other reference surfaces, such as ellipsoids, to create accurate maps and survey measurements.

Geologic Map

A geologic map is a type of map that shows the distribution and age of rocks and sediments on the Earth’s surface.  Geologic maps are created by geologists who study the structure and composition of the Earth’s crust.  They use a combination of field observations, laboratory analysis, and remote sensing data to create maps that show the location of different rock types, faults, and other geologic features.  Geologic maps are essential for understanding the Earth’s history, predicting natural hazards, and locating mineral resources.  They are used by many industries, including mining, construction, and environmental management.


Geology is the scientific study of the Earth’s structure, composition, and processes.  Geologists use a variety of methods, including field observations, laboratory analysis, and computer modeling, to understand the Earth’s history and its present-day features.  They study rocks, minerals, and fossils to determine how the Earth has changed over time and to predict future geological events, such as earthquakes and volcanic eruptions.  Geology is an interdisciplinary field that encompasses many sub-disciplines, including petrology, mineralogy, and structural geology.

Geomorphic Map

A geomorphic map is a type of map that shows the landforms and surface features of a region. It is created by analyzing aerial photographs, satellite images, and other types of remote sensing data.  Geomorphic maps are used by geologists and other earth scientists to understand the processes that shape the Earth’s surface, such as erosion, weathering, and tectonic activity.  They are also used in land-use planning, environmental management, and resource exploration.


Geomorphology is the study of the processes that shape the Earth’s surface and the landforms that result from these processes.  It is an interdisciplinary field that combines elements of geology, geography, and environmental science.  Geomorphologists study the effects of natural forces, such as water, wind, and ice, as well as human activities, such as mining and construction, on the Earth’s surface.  They use a variety of methods, including field observations, laboratory analysis, and computer modeling, to understand how the Earth’s surface has changed over time and to predict future changes.


Geotextile is a type of fabric that is used in geotechnical and environmental engineering applications.  It is made from synthetic materials, such as polyester or polypropylene, and is designed to have specific properties, such as high tensile strength, durability, and permeability.  Geotextiles are used in a wide range of applications, including erosion control, soil stabilization, and drainage.  They can be woven, non-woven, or knitted, and can be used in conjunction with other geosynthetic materials, such as geogrids and geocells, to enhance their performance.  Geotextiles are an essential component of many civil engineering projects and are used in both residential and commercial applications.


A geyser is a type of hydrothermal feature that is found in areas with volcanic activity, such as Yellowstone National Park.  It is a natural hot spring that periodically erupts, sending water and steam into the air. Geysers are formed when groundwater is heated by magma and forced to the surface through a narrow vent.  They are characterized by their intermittent eruptions, which can occur anywhere from minutes to months apart. Some of the most famous geysers in the world include Old Faithful in Yellowstone and Strokkur in Iceland.

Glacial Erratic

A glacial erratic is a large boulder that was transported by a glacier and deposited in a different location.  It is called an erratic because it is not native to the area in which it was deposited. Glacial erratics can range in size from small rocks to massive boulders, and can be found in a variety of environments, including valleys, hillsides, and coastal areas.  They are a visible reminder of the power of glaciers and the forces that shape the Earth’s surface.


A glacier is a large mass of ice that forms over many years from the accumulation of snow and ice. It is a natural phenomenon that occurs in areas with high snowfall and low temperatures, such as mountain ranges and polar regions.  Glaciers can range in size from small cirque glaciers to massive ice sheets that cover entire continents. They are a major contributor to sea level rise and can have a significant impact on local and global climate.  Glaciers also play an important role in shaping the landscape, eroding mountains and carving valleys.


A glen is a narrow valley that is usually created by a stream or river. It is a common feature in areas with rugged terrain, such as mountain ranges and highland regions. Glens are characterized by their steep sides, which are often covered in vegetation, and their flowing water.  They can be a popular destination for hiking and other outdoor activities, as they offer stunning views and a unique environment. Glens are also important habitats for a variety of wildlife, including birds, fish, and mammals.

Gleyed Soil

Gleyed soil is a type of soil that is characterized by its gray or bluish-gray color and its poor drainage.  It is formed in areas with high water tables or poor drainage, where the soil is constantly saturated with water.  Gleyed soil is often found in wetlands, floodplains, and other low-lying areas.  The soil’s color is a result of the lack of oxygen in the soil, which causes iron and other minerals to remain in a reduced state.

Global Positioning System (GPS)

The Global Positioning System, or GPS, is a satellite-based navigation system that allows users to determine their precise location and track their movement.  It was developed by the United States government for military use, but is now widely used for civilian purposes, such as navigation, surveying, and mapping.  The GPS system consists of a network of satellites in orbit around the Earth, which transmit signals to GPS receivers on the ground.  These receivers use the signals to calculate the user’s precise location, speed, and direction of travel.


Gneiss is a type of metamorphic rock that is formed from the alteration of other rocks under high pressure and temperature.  It is characterized by its banded appearance, with alternating layers of light and dark minerals.  Gneiss is commonly found in mountainous regions and is often used as a building material due to its durability and attractive appearance.  The rock’s composition and appearance can vary widely depending on the original rock type and the conditions under which it was formed.


A graben is a type of geological feature that is formed by the downward displacement of a block of land between two parallel faults.  It is characterized by its steep, inward-facing walls and flat floor.  Grabens can range in size from small trenches to large rift valleys, and can be found in a variety of environments, including deserts, mountain ranges, and oceanic crust.  They are often associated with tectonic activity and can be an important source of geologic information about the Earth’s crust.  Grabens can also be a popular destination for outdoor recreation, such as hiking and rock climbing.


Gradient refers to the degree of steepness of a slope or change in elevation over a distance.  It is commonly expressed as a percentage or a ratio of vertical change to horizontal distance.  A steeper gradient indicates a more rapid change in elevation over a given distance, while a flatter gradient indicates a more gradual change.

Gradient Line

The gradient line, also known as the hydraulic gradient, is a line that represents the elevation of a water surface in a hydraulic system.  It is defined as the sum of the pressure head and the elevation head at a given point.  The gradient line is an important concept in hydraulic engineering, as it is used to calculate the flow rate and pressure of water in a system.

Grading Plan

A grading plan is a detailed blueprint that outlines the grading and excavation work that is required for a construction project.  It typically includes information on the location and extent of earthwork, the location of drainage features such as culverts and swales, and the proposed finished grade elevations.  Grading plans are an essential part of the construction process, as they ensure that the site is properly prepared for the construction of buildings and infrastructure.


Granite is a type of igneous rock that is composed mainly of quartz, feldspar, and mica.  It is a very hard and durable rock that is commonly used for building and construction purposes.  Due to its resistance to weathering and erosion, granite is often used for monuments, statues, and other outdoor structures. It is also used as a decorative stone in homes and buildings.


Gravel is a type of sedimentary rock that consists of small, rounded stones or fragments of rock.  It is commonly used as a construction material for roads, driveways, and walkways, as well as for landscaping and decorative purposes.  Gravel is available in a range of sizes, from small pebbles to large boulders, and can be sorted by color, shape, and texture.

Great Circle

A great circle is a circle on the surface of a sphere that is the largest possible circle that can be drawn between two points.  It is an important concept in navigation and is used to calculate the shortest distance between two points on the surface of the earth.  The equator is an example of a great circle, as it is the largest possible circle that can be drawn around the earth’s circumference.

Greenfield Site

A greenfield site is an undeveloped piece of land that has not been previously used for industrial or commercial purposes.  It is often used to refer to land that is being considered for new construction or development projects.  Greenfield sites can be desirable for development because they are typically free from existing infrastructure and can be designed and built from scratch.  However, they can also present environmental and social challenges, as they may be located in areas that are important for wildlife or have cultural significance.


A grid is a network of parallel lines that intersect to form a series of squares or rectangles.  It is commonly used in cartography and mapping to create a reference system for locating points on a map.  Grids can be oriented in different directions and can be scaled to different sizes to suit the needs of a particular map.

Grid System

A grid system is a framework of lines that form a grid and are used to reference and locate points on a map or plan.  The lines of a grid system are typically numbered or lettered to make it easier to reference specific locations.  Grid systems can be used for a variety of applications, such as mapping, surveying, and urban planning.

Ground Survey

A ground survey is a survey that is conducted on the ground to gather data about the physical characteristics of an area.  Ground surveys are commonly used for a variety of applications, including construction, land use planning, and environmental monitoring.  They may involve measuring distances, angles, elevations, and other physical features of the land, as well as collecting samples of soil, vegetation, and other materials for analysis.


Groundwater is water that is found underground in the pores and crevices of soil and rock.  It is an important source of water for drinking, irrigation, and industrial use. Groundwater is replenished by rain and snowmelt that percolates into the ground and is stored in underground aquifers.  The quality and quantity of groundwater can be affected by a variety of factors, including land use, climate, and geology.

Groundwater Contour Map

A groundwater contour map is a topographical map that shows the elevation and distribution of groundwater in an area.  The map is created by measuring the elevation of the water table at various points and then connecting the points of equal elevation with contour lines.  Groundwater contour maps can be used to help manage and protect groundwater resources, to locate wells and other water sources, and to identify areas that may be prone to flooding or other water-related hazards.

Groundwater Elevation

Groundwater elevation refers to the height of the water table in an aquifer relative to a reference point, such as sea level or the surface of the earth. Groundwater elevation can vary depending on a number of factors, including precipitation, evaporation, pumping of groundwater, and the permeability of the soil and rock.  Knowledge of groundwater elevation is important for a variety of applications, including well drilling, groundwater management, and environmental monitoring.


A groyne is a man-made structure that extends from the shoreline into the water to prevent erosion and promote the accumulation of sediment. Groynes are typically made of wood, concrete, or rock and are placed perpendicular to the shoreline.  They work by trapping sediment that is carried by currents along the shoreline and slowing down the movement of water, which reduces erosion. Groynes can also be used to create beaches and recreational areas.


A gully is a channel or ravine that is eroded by flowing water. Gullying can occur naturally or as a result of human activities such as logging or construction.  Gullies can range in size from small channels to large, deep ravines and can pose a hazard to infrastructure, agriculture, and natural ecosystems.  Gully erosion can be prevented or mitigated by the use of erosion control measures such as terracing, reforestation, and the use of vegetation and cover crops.

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