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1. Home
2. Education
3. Geodesy
4. The Horizontal Datum

**The Horizontal Datum** Global Positioning Tutorial  At its most basic level of definition, the horizontal datum is a  collection of specific points on the Earth that have been identified  according to their precise northerly or southerly location (latitude)  and easterly or westerly location (longitude). To create the horizontal datum, or  network of horizontal positions, surveyors marked each of the positions  they had identified, typically with a brass, bronze, or aluminum disk  or monument. These markers were placed so that surveyors could see one  marked position from another. To maximize the line-of-sight between  markers, they were usually set on mountaintops or at high elevations.  When markers were set on flat land, towers were built above them to  aid surveyors in locating them. To "connect" the horizontal markers into a unified network, or datum,  surveyors have used a variety of methods, including triangulation. As  technology has improved, surveyors now rely almost exclusively on the  Global Positioning System (GPS) to identify locations on the Earth and  incorporate them into existing datums.  Close up view of brass monument*Brass markers permanently affixed in concrete or surrounding bedrock indicate accurate geodetic reference  positions within the National Geodetic Survey's horizontal and/or vertical datums.*  *When positioning markers are located on flat ground, large towers such as this were often erected above them allowing geodesists to observe the monument's accurate position over large distances. Learn more and view a larger version of this image.*
 In 1927 the U.S. Coast and Geodetic Survey, the predecessor of the  National Geodetic Survey, "connected" all of the existing horizontal  markers together and created the North American Datum of 1927 (NAD  27). This datum was used extensively during the next 60 years as the  primary reference for horizontal positioning. In 1983, NAD 27 was  adjusted to remove inaccuracies and to correct distortions. The new  datum, called NAD 83, is the most commonly used horizontal positioning  datum today in the United States. 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. Extending  from northern California south to San Bernardino, the San Andreas Fault  is where two plates of the Earth's crust meet. The fault is  approximately 800 miles long and extends 10 miles below the Earth's  surface. By looking at the movement of markers in the horizontal  datum, geodesists can determine just how much the surface of the Earth  has moved after an earthquake.
  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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