Geoid & Ellipsoid in English #earth #remotesensing #geography #geoid

Summary

This video explores the concepts of the geoid and ellipsoid models, explaining the fundamentals of coordinate systems and reference surfaces. It discusses how latitude, longitude, and altitude work within a horizontal and vertical coordinate system, with emphasis on the Earth's shape as an oblate ellipsoid. The video also introduces the geoid model, an equipotential surface used to measure height more accurately, and highlights NASA’s GRACE satellite, which helps refine this model by measuring Earth’s varying gravity. The content provides valuable insights into understanding Earth's true shape and height measurement techniques.

Takeaways

• 😀 The horizontal coordinate system uses latitude and longitude to locate positions on Earth's surface.
• 😀 Altitude is part of the vertical coordinate system, which is concerned with measuring height above or below a reference surface.
• 😀 The Earth's shape is an oblate ellipsoid, meaning the equatorial radius is greater than the polar radius due to Earth's rotation.
• 😀 The central meridian is an imaginary reference line used for measuring longitude in the Earth's coordinate system.
• 😀 The orthometric height is the height of a location measured from a local reference surface, typically mean sea level.
• 😀 A single reference surface can't measure the orthometric height of all places accurately, hence the need for global reference surfaces.
• 😀 GPS systems use the ellipsoid model as a global reference surface to measure heights, though this may sometimes yield negative height values.
• 😀 The geoid model represents the Earth's equipotential surface, which is influenced by gravity variations across the Earth's surface.
• 😀 The geoid model helps to achieve more accurate height measurements compared to the ellipsoidal model.
• 😀 NASA's GRACE satellite measures the varying gravity of the Earth, allowing scientists to create an accurate geoid model.
• 😀 Understanding the geoid and ellipsoid models is crucial for accurate navigation, geodesy, and height measurements on Earth.

Q & A

What is the primary coordinate system used to locate places on Earth's surface?

The primary coordinate system used to locate places on Earth's surface is latitude and longitude, known as the horizontal coordinate system in cartography.

What does the vertical coordinate system help to measure?

The vertical coordinate system helps to measure altitude, or the height above a reference point such as mean sea level.

How is Earth’s shape described in relation to its rotation?

The Earth is described as an oblate ellipsoid, meaning it is slightly flattened at the poles and bulging at the equator due to the centrifugal force from its rotation.

What is the difference between a global and a local reference surface?

A global reference surface is a broad model used worldwide, like the ellipsoidal surface in GPS. A local reference surface is specific to a region, such as mean sea level, which is used to measure the height of landmarks.

What is orthometric height and how is it measured?

Orthometric height refers to the height of a place above the local reference surface, such as mean sea level. It is used to measure the height of landmarks like Mount Everest.

Why is the ellipsoidal surface not always accurate for measuring height?

The ellipsoidal surface, while useful, does not account for variations in Earth's gravity and mass distribution, which is why it can sometimes lead to inaccurate measurements, including negative heights.

What is the geoid model and why is it important?

The geoid is an equipotential surface that accounts for Earth's varying gravitational field. It provides a more accurate model for measuring height compared to the ellipsoidal surface, as it reflects real variations in gravity.

How does Earth's gravity change across its surface?

Earth's gravity changes due to variations in density and mass distribution across the surface, both horizontally and vertically, which causes local differences in gravity readings.

What role does the GRACE satellite play in geoid measurements?

Launched by NASA in 2002, the GRACE satellite measures Earth's varying gravity, allowing scientists to refine and develop a more accurate geoid model based on these measurements.

Why is it important to study both global and local reference surfaces in geodesy?

Studying both global and local reference surfaces is important because global models provide a broad, universal understanding of Earth's shape, while local models allow for more precise measurements of specific regions, such as the height of landmarks.