What are Geodetic Datums?

Summary

Datums are crucial reference points used in mapping and surveying to measure distances, elevations, and positions. They can be horizontal, for measuring surface distances, or vertical, for elevation and water depth. Technological advancements like GPS have led to the creation of geometric datums, improving accuracy by combining latitude, longitude, and height. Consistent datum usage is essential in various applications, including flood plain mapping, property boundaries, and infrastructure projects. Understanding the correct datum is vital, especially when combining data from different eras or sources, to ensure accurate and reliable measurements.

Takeaways

• 😀 A datum is a reference point, set of points, or surface from which survey measurements are based, similar to a starting point when giving directions.
• 😀 Horizontal datums are used to measure distances and directions across the Earth's surface, with reference lines at the equator and Greenwich Meridian.
• 😀 Vertical datums are used to measure elevation and water depth, commonly using sea level as the basic reference plane.
• 😀 The use of tide gauges for vertical datums was abandoned in 1988, with one tide gauge becoming the standard for defining the vertical datum.
• 😀 GPS and other space-age techniques have led to the creation of geometric datums that combine latitude/longitude, height, and time information.
• 😀 The North American datum of 1983 (NAD 83) is the country's horizontal or geometric datum, providing latitude, longitude, and height information.
• 😀 The North American vertical datum of 1988 (NAVD 88) is the nation's vertical datum, providing elevation information relative to a reference surface defined by mean sea level.
• 😀 US territories have their own separate vertical datums, and older datums are still in use, requiring transformation to a common datum when combining data.
• 😀 Consistent datums are crucial when combining data from different sources or eras to ensure accuracy, especially in fields like flood plain mapping and land surveying.
• 😀 It is essential to know which datum is being used in applications like flood plain mapping, designing levees or dams, and planning for coastal inundation scenarios.

Q & A

What does the term 'datum' refer to in mapping and surveying?

A datum is a standard reference point, set of points, or surface from which survey measurements are based. It provides a starting point, like a known location, to align measurements and ensure accuracy.

Why are datums important in surveying and mapping?

Datums are important because they provide a common reference for measurements, ensuring consistency when mapping properties, designing infrastructure, or analyzing geographical data across different regions.

How does a datum relate to giving directions?

Just like a starting point in directions, such as a crossroads or a landmark, a datum serves as a reference point for surveying. This makes it easier to align data when working with adjoining properties or areas.

What is the difference between horizontal and vertical datums?

Horizontal datums are used to measure distances and directions across the Earth's surface, while vertical datums are used to measure elevations and water depths, usually referencing sea level.

How were vertical datums traditionally defined in North America?

Vertical datums in North America were traditionally based on multiple tide gauges along the coast to define sea level. However, this approach was abandoned in 1988 in favor of using a single tide gauge.

How has GPS technology impacted the use of datums?

GPS technology has revolutionized datums by providing a more accurate and unified reference system that combines latitude, longitude, height, and time information, moving beyond separate horizontal and vertical datums.

What is the North American Datum of 1983 (NAD 83)?

NAD 83 is the horizontal or geometric datum for the United States, providing reference for latitude, longitude, and some height information.

What is the North American Vertical Datum of 1988 (NAVD 88)?

NAVD 88 is the vertical datum used in the United States, which provides elevation information relative to a reference surface defined by mean sea level.

Why is it important to know which datum is used in a map or survey?

Knowing the datum is essential because using different datums can lead to discrepancies in measurements, such as significant differences in elevation or location, which can affect decisions related to flood plains, construction, and property boundaries.

What are some practical applications of datums in the real world?

Datums are used in creating flood plain maps, marking property boundaries, designing infrastructure like levees and dams, building roads, and planning for natural events like coastal inundation. Consistent use of the correct datum ensures accuracy and reliability in these tasks.