Establishment of Datums

What is a datum?

A datum is a reference system that is used to measure distances, elevations, and other geospatial data. Datums are important because they allow us to compare geospatial data from different sources and to create maps and charts that are accurate and consistent.

Types of datums

There are two main types of datums: horizontal datums and vertical datums.

Horizontal datums

are used to measure distances on a flat surface, such as the Earth’s surface. Horizontal datums are typically based on a geodetic reference frame, which is a global network of survey markers. The most common geodetic reference frame is the World Geodetic System 1984 (WGS 84). WGS 84 is used by GPS receivers to determine their positions.

Vertical datums

are used to measure elevations. Vertical datums are typically based on a mean sea level datum. Mean sea level is the average height of the sea’s surface over a period of time, typically 19 years. The most common vertical datum in the United States is the North American Vertical Datum of 1988 (NAVD 88). NAVD 88 is used to measure elevations for a variety of purposes, including mapping, surveying, and construction.

Establishment of datums

Datums are established by government agencies, such as the National Geodetic Survey in the United States. These agencies use a variety of geodetic and surveying techniques to establish datums.

To establish a horizontal datum, geodetic surveyors measure the positions of survey markers using a variety of techniques, such as triangulation, trilateration, and GPS. Once the positions of the survey markers have been measured, the surveyors use this data to calculate the geodetic reference frame.

To establish a vertical datum, geodetic surveyors measure sea levels at a network of tide gauge stations. Once the sea levels have been measured, the surveyors use this data to calculate the mean sea level datum.

Maintenance of datums

Once a datum has been established, it is important to maintain it. This means regularly resurveying the geodetic reference markers and measuring sea levels. It is also important to update datums as new geodetic and surveying technologies are developed.

Applications of datums

Datums are used in a wide variety of applications, including:

Mapping:

Datums are used to create maps and charts that are accurate and consistent. For example, datums are used to create nautical charts, which are used by mariners to navigate the seas.

Surveying:

Datums are used by surveyors to measure distances and elevations. For example, datums are used to survey construction sites and to create boundary maps.

GPS:

Datums are used by GPS receivers to determine their positions. For example, datums are used by hikers to find their way on trails and by drivers to navigate to their destinations.

Other examples of datums

In addition to the horizontal and vertical datums mentioned above, there are a number of other datums that are used for specific purposes. For example, the Global Navigation Satellite System (GNSS) datum is used to coordinate the different GNSS systems, such as GPS, GLONASS, and Galileo. The International Earth Rotation and Reference Systems Service (IERS) provides a variety of datums for use in different applications, such as geodesy, astronomy, and oceanography.

Conclusion

Datums are essential for a wide variety of applications. They allow us to measure distances, elevations, and other geospatial data accurately and consistently. Datums are established and maintained by government agencies, and they are used by a variety of people and organizations for a variety of purposes.