Datums are the basis for all geodetic survey work

A geodetic datum is an abstract coordinate system with a reference surface (such as sea level) that serves to provide known locations to begin surveys and create maps. In this way, datums act similar to starting points when you give someone directions. For instance, when you want to tell someone how to get to your house, you give them a starting point that they know, like a crossroads or a building address.

Geodesists and surveyors use datums to create starting or reference points for floodplain maps, property boundaries, construction surveys, levee design, or other work requiring accurate coordinates that are consistent with one another.

There are two main datums in the United States. Horizontal datums measure positions (latitude and longitude) on the surface of the Earth, while vertical datums are used to measure land elevations and water depths.

The horizontal datum can be accessed and used through a collection of specific points on the Earth whose latitude and longitude have been accurately determined by NOAA's National Geodetic Survey. One application of the horizontal datum is monitoring the movement of the Earth's crust. This type of monitoring is often used in places like the San Andreas Fault in California where many earthquakes occur.

The vertical datum is similarly "realized" through a collection of specific points on the Earth with known heights either above or below a nationally defined reference surface (e.g., mean sea level). Geodetic vertical datums are generally used to express land elevations. However, water level datums are a slightly different vertical datum, and are used as a reference level to which bathymetric soundings are referenced for nautical charts. Conversion between these two can be done through geodetic surveys at tide gauges.

For more information:
Learn About Datums and Transformations

NOS Education: The Elements of Geodesy: Datums

National Geodetic Survey: Frequently Asked Questions

Introduction to Geodetic and Tidal Vertical Datums

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While we often think of the earth as a sphere, our planet is actually very bumpy and irregular.

The radius at the equator is larger than at the poles due to the long-term effects of the earth's rotation.  And, at a smaller scale, there is topography—mountains have more mass than a valley and thus the pull of gravity is regionally stronger near mountains.

All of these large and small variations to the size, shape, and mass distribution of the earth cause slight variations in the acceleration of gravity (or the "strength" of gravity's pull). These variations determine the shape of the planet's liquid environment.

If one were to remove the tides and currents from the ocean, it would settle onto a smoothly undulating shape (rising where gravity is high, sinking where gravity is low).

This irregular shape is called "the geoid," a surface which defines zero elevation. Using complex math and gravity readings on land, surveyors extend this
imaginary line through the continents. This model
is used to measure surface elevations with a high
degree of accuracy.

For more information:
National Geodetic Survey
National Spatial Reference System
NGS Geoid information
Diving Deeper Podcast, What is geodesy?

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Geodesists assign coordinates to points all over the Earth. Using the Global Positioning System (GPS), geodesists can accurately define the coordinates of points on the surface of the Earth in a consistent manner. This set of accurately measured points is called the National Spatial Reference System (NSRS), which allows different kinds of maps to be consistent with one another.

Developers, local officials, city planners, and many others use the National Spatial Reference System to determine land boundaries for development or conservation efforts. Government agencies also rely on the NSRS to update maps of the U.S. shoreline.

Geodesy is also critical to the transportation industry. Surveyors use the National Spatial Reference System as one of their tools to develop nautical charts. Mariners use these nautical charts and GPS to accurately position their ships. This technology, accurate down to a centimeter scale, allows mariners and commercial vessels to assess where the bottoms of their ships are relative to the bottom of the ocean. With this information, a ship can hold extra cargo, sinking deeper into the water, and still safely navigate through a channel. The ability to move more cargo at a time is direct economic benefit to the shipping industries and ultimately to the consumer.

In addition, geodetic data, specifically datum information, and water level data are critical for agency officials to properly plan, design, and engineer coastal restoration projects. These data provide baseline information that assist in the construction of phases of marsh restoration projects, for example.

Geodesy is the science of measuring and monitoring the size and shape of the Earth, including its gravity field, and determining the location of points on the Earth’s surface.

For more information:
National Geodetic Survey
him him him himNational Spatial Reference System
Diving Deeper Podcast, Episode 9 (May 20, 2009) -- What is geodesy?
Geodesy Tutorial, NOS Education

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Geodesists assign coordinates to points all over the Earth. Using the Global Positioning System (GPS), geodesists can accurately define the coordinates of points on the surface of the Earth in a consistent manner. This set of accurately measured points is called the National Spatial Reference System (NSRS), which allows different kinds of maps to be consistent with one another.

Developers, local officials, city planners, and many others use the National Spatial Reference System to determine land boundaries for development or conservation efforts. Government agencies also rely on the NSRS to update maps of the U.S. shoreline.

Geodesy is also critical to the transportation industry. Surveyors use the National Spatial Reference System as one of their tools to develop nautical charts. Mariners use these nautical charts and GPS to accurately position their ships. This technology, accurate down to a centimeter scale, allows mariners and commercial vessels to assess where the bottoms of their ships are relative to the bottom of the ocean. With this information, a ship can hold extra cargo, sinking deeper into the water, and still safely navigate through a channel. The ability to move more cargo at a time is direct economic benefit to the shipping industries and ultimately to the consumer.

In addition, geodetic data, specifically datum information, and water level data are critical for agency officials to properly plan, design, and engineer coastal restoration projects. These data provide baseline information that assist in the construction of phases of marsh restoration projects, for example.

Geodesy is the science of measuring and monitoring the size and shape of the Earth, including its gravity field, and determining the location of points on the Earth’s surface.

For more information:
National Geodetic Survey
National Spatial Reference System
Diving Deeper Podcast, Episode 9 (May 20, 2009) -- What is geodesy?
Geodesy Tutorial, NOS Education

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NOAA’s National Geodetic Survey (NGS) defines and maintains the NSRS. The NSRS includes a network of permanently marked points; a consistent, accurate, and up-to-date national shoreline; a network of Continuously Operating Reference Stations (CORS) which supports three-dimensional positioning activities; and a set of accurate models describing dynamic, geophysical processes that affect spatial measurements.

For over 200 years, NGS and its predecessor agencies have collaborated with surveyors in both the public and private sectors to place hundreds of thousands of survey marks throughout the United States, determining positional information for each mark. Each survey mark is published with accurate horizontal and/or vertical information such as latitude, longitude, and/or height. Typically, a mark is a brass, bronze, or aluminum disk, but it might also be a deeply driven rod or prominent object like a water tower or church spire. Increasingly, the continuously operating global positioning system receivers of CORS stations are used as reference stations as well.

This collection of points (over 1,500,000 of them) forms a network that is used to accurately position other points of interest. Surveyors and others use the NSRS throughout the country to ensure that their positional coordinates are compatible with those determined by others. In this way, when they create maps; mark off property boundaries; and plan, design, and build roads, bridges, and other structures, everything matches up.

For more information:
National Geodetic Survey
The National Spatial Reference System: Fundamental Data for Land Surveys, Nautical Charts, and the Nation's Infrastructure

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Many organizations use geodesy to map the U.S. shoreline, determine land boundaries, and improve transportation and navigation safety. To measure points on the Earth’s surface, geodesists assign coordinates (similar to a unique address) to points all over the Earth. In the past, geodesists determined the coordinates of points by using Earth-based surveying tools to measure the distances between points. Today, geodesists use space-based tools like the Global Positioning System (GPS) to measure points on the Earth’s surface.

Geodesists must accurately define the coordinates of points on the surface of the Earth in a consistent manner. A set of accurately measured points is the basis for the National Spatial Reference System, which allows different kinds of maps to be consistent with one another.

To measure the Earth, geodesists build simple mathematical models of the Earth which capture the largest, most obvious features. Geodesists have adopted the ellipsoid as the most basic model of the Earth. Because the ellipsoid is based on a very simple mathematical model, it can be completely smooth and does not include any mountains or valleys. When additional detail of the Earth is needed, geodesists use the geoid. A geoid has a shape very similar to global mean sea level, but this exists over the whole globe, not just over the oceans. For more information:
National Geodetic Survey
National Spatial Reference System
Diving Deeper Podcast, Episode 9 (May 20, 2009) - What is geodesy?
Geodesy Tutorial, NOS Education

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