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In the context of GIS (Geographic Information Systems), a datum is a set of parameters that define the coordinate system and reference frame used to represent the location of features on the earth’s surface. A datum includes information such as the shape of the earth, the origin and orientation of the coordinate system, and the scale and precision of the measurements. Datums are used to ensure that different maps and data sets can be accurately aligned and compared with one another. Some common datums include WGS84, NAD83, and NAD27.

What is a horizontal datum?

A horizontal datum is a specific type of datum that defines the reference frame for measuring horizontal (x,y) positions on the earth’s surface. It establishes a coordinate system and an origin point and establishes the scale and orientation of the system. A horizontal datum is used to specify the location of a point on the earth’s surface in terms of x, and y coordinates.

A horizontal datum is also known as a geographic coordinate system or a geodetic datum. It is used in GIS and surveying to locate points on the earth’s surface and to ensure that different maps and data sets can be accurately aligned and compared with one another. Common horizontal datums include WGS84, NAD83, and NAD27.

WGS84 (World Geodetic System 84) is the most widely used horizontal datum in GIS and GPS systems. It is based on the GRS80 ellipsoid and is used in most applications by default.

What is a vertical datum?

A vertical datum is a specific type of datum that defines the reference frame for measuring vertical (z) positions on the earth’s surface. It establishes a vertical coordinate system and an origin point and establishes the scale and orientation of the system. A vertical datum is used to specify the elevation of a point on the earth’s surface in terms of z coordinate.

A vertical datum is used to measure elevations or depths relative to a specific reference point or surface. It is used in surveying, engineering, and mapping to specify the elevation of points on the earth’s surface. Common vertical datums include the North American Vertical Datum of 1988 (NAVD 88), the European Vertical Reference System (EVRS), and the Mean Sea Level (MSL).

NAVD 88 is the standard vertical datum for the United States, Canada, and Mexico, it is based on a combination of geodetic survey data and leveling data. MSL is an average sea level over a long period of time, it is used as a reference point for measuring elevation and depth in coastal areas.

It’s important to note that when working with GIS data, it’s very common to have a horizontal datum and a vertical datum specified separately, this is because the earth’s shape and gravity field are not uniform, so different datums are used to accurately describe the earth’s shape and gravity field.

The U.S. National Geodetic Survey (NGS) is an office of the National Oceanic and Atmospheric Administration (NOAA) within the U.S. Department of Commerce. It is responsible for providing the framework of horizontal and vertical control that enables accurate positioning by all users, including mapping, charting, surveying, and navigation.

The NGS’s mission is to define, maintain, and provide access to the National Spatial Reference System (NSRS), which is a consistent coordinate system that defines latitude, longitude, height, scale, gravity, orientation, and shoreline throughout the United States. The NSRS provides the foundation for transportation, communication, and other infrastructure, as well as safe and efficient commerce, land-use planning, resource management, and emergency response.

The NGS provides a range of services and products, including:

• Surveying and mapping to establish and maintain the NSRS, including geodetic control points, datums, and reference frames.
• Data and software tools to access the NSRS and other geodetic information.
• Technical assistance and training for surveyors, engineers, and other professionals who use the NSRS.
• Outreach and education to promote understanding of the NSRS and its importance.

The NGS also maintains the National Spatial Reference System (NSRS) which is a consistent coordinate system that defines latitude, longitude, height, scale, gravity, orientation, and shoreline throughout the United States. The NSRS provides the foundation for transportation, communication, and other infrastructure, as well as safe and efficient commerce, land-use planning, resource management, and emergency response.