What is datum in surveying?

Datum in surveying is a reference point or surface that is used to define the location of points on the Earth's surface. Geoids, ellipsoids, and local datums are all different types of datums that are used in surveying.

A datum in surveying is a reference point or surface used as a starting point for measuring positions on the Earth's surface. It is used to define the location of points on the Earth in a consistent and standardized manner. Several different types of datums are used in surveying, including geoids, ellipsoids, and local datums.

Geoids and Ellipsoids

A geoid is a surface of equal gravitational potential that approximates the shape of the Earth's sea level.  The geoid is not a perfect sphere as the Earth's gravity field varies across the planet.  The geoid is used as a reference for measuring heights. An ellipsoid is a mathematical shape that closely approximates the shape of the Earth and is used as a reference for measuring positions. Both geoids and ellipsoids are used in surveying and are closely related, but they are not the same thing.

Geoids and Local Datum

A local datum is a reference point or surface specific to a particular area and is used to define the location of points in that area. Local datums are based on the geoid and take into account the specific shape of the Earth's surface in a particular area. This is important because the Earth's shape can vary significantly from place to place, and a datum that is accurate in one area may not be accurate in another.

Horizontal and Vertical Datums

A horizontal datum is a reference point or surface used to define the position of points on the Earth's surface in a horizontal plane. This is typically based on an ellipsoid or a geoid. A vertical datum is a reference point or surface used to define the height of points on the Earth's surface. This is typically based on a geoid or a mean sea level.

NAD 83 and Local Datum

The North American Datum 1983 (NAD 83) is a datum that is used throughout North America to define the position of points on the Earth's surface. It is based on the GRS 1980 ellipsoid and is a horizontal datum. NAD 83 is widely used in surveying and mapping in North America, but it is not the only datum in use. Local datums are also used in specific areas to take into account the specific shape of the Earth's surface in that area.

Datum in surveying is a reference point or surface that is used to define the location of points on the Earth's surface. Geoids, ellipsoids, and local datums are all different types of datums that are used in surveying. Horizontal and vertical datums are also used to define positions and heights, respectively. NAD 83 is a widely used datum in North America, but in specific areas, local datums are used to take into account the specific shape of the Earth's surface.