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1. Home
2. Education
3. Geodesy
4. Datums

**What is a datum?** Global Positioning Tutorial  *One way to think about a datum is as a set of spatial information that acts as a foundation for other data, just like concrete acts as a foundation for the structure of a building, including all the furnishings and decorations inside.Without a datum there would be nothing to securely support other spatial information, such as digital elevations, land use, or population. There are two types of datums, horizontal and vertical, that make up the National Spatial Reference System (NSRS). This series of illustrations demonstrates how geodesists help others construct spatial information into a map, like designing or decorating the interior of a building, by providing a solid foundation.*  Datums (sets of data) are the basis for all geodetic survey work. They  act as reference points in the same way that starting points do 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 road or a building. Geodesists and surveyors use  datums as starting or reference points when they create maps, mark off  property boundaries, and plan, design and build roads, bridges, and other structures. Another way to think about a datum  is as a set of information that acts as a foundation for other data.  For example, when a skyscraper is about to be built, the construction  team must first pour the foundation. Without this element, the  skyscraper would be unstable and unsafe. This is the same concept as a  datum. While a datum is a mathematical and geometric concept, it acts  like the concrete foundation of a skyscraper. Once the foundation is  set, the construction workers can build on top of it, creating the  building's structure. After the building is complete, offices or  apartments can be created inside the building. If the structure is an  apartment building, its tenants can bring in furniture and decorate as  they please. Although the foundation of the building probably isn't the  first thing on the minds of the tenants, without it, the building would  not be a safe place to live. In geodesy, two main datums create the foundation for navigation and  transportation in the United States. These datums — called the  horizontal and vertical datums — make up the National Spatial Reference System (NSRS). Geodesists, surveyors, and people interested  in precise positioning use the NSRS as their foundation for reference.
  Geodesy **Lessons** Welcome What is Geodesy? The History of Geodesy The Geoid What is a Datum? The Horizontal Datum The Vertical Datum Gravity The National Spatial Reference System The Global Positioning System CORS and GIS References
  More **Information** Roadmap to Resources Subject Review Is the Earth round? (Ocean Facts)  Geodesy Education Heard of geodesy? These videos from NOAA's National Geodetic Survey offer a deep dive into the science of knowing where you – and everything else is in the world!
  The National Science Teaching Association (NSTA) includes this resource in its database. NSTA provides educators and students access to Web-based, educationally appropriate science content that has been formally evaluated by master teachers.
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  **Last updated:** 02/26/21 **Author:** NOAA How to cite this article Contact Us


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