Geoid: there are infinitely many level surfaces (equipotential surfaces) and the one which coincides with MSL is called a geoid. But what is the quasigeoid?
The quasigeoid is a mathematical model that approximates the geoid, which is the surface of equal gravitational potential on the Earth. The geoid is an equipotential surface that represents the mean sea level (MSL) of the Earth's oceans and extends continuously under the continents. It is used as a reference surface for defining heights and depths on the Earth's surface, particularly in navigation and surveying.
However, the geoid is not a perfect reference surface because it is difficult to measure directly. Instead, measurements of gravity, which is directly related to the geoid, are used to construct an approximate reference surface called the quasigeoid. The quasigeoid is a continuous surface that approximates the geoid with an accuracy of a few centimeters or less.
To compute the quasigeoid, gravity measurements are collected at points on the Earth's surface and processed to produce a gravity anomaly, which is the difference between the measured gravity and the theoretical gravity predicted by a model of the Earth's shape and rotation. The gravity anomaly is then used to calculate the geoid height, which is the difference between the quasigeoid and the shape and rotation model.
The resulting quasigeoid is a more accurate reference surface for measuring heights than simple models such as the ellipsoid or the mean sea level, which do not take into account the irregularities of the Earth's gravity field. The quasigeoid is also useful for studying the Earth's interior structure and dynamics, as the gravity field is influenced by the distribution of mass within the Earth. Here's an example of how the quasigeoid is used in practice:
Suppose we want to determine the elevation of a point on the Earth's surface, such as the top of a mountain. We first need to establish a reference surface, which is typically the geoid or a model that approximates it, such as the quasigeoid.
We can obtain gravity measurements at the point of interest using a gravimeter, which is a device that measures the gravitational acceleration at a specific location. The gravity measurements will include the effects of the Earth's gravitational field, as well as any local anomalies caused by the presence of nearby mass.
We can then use the gravity measurements to calculate the geoid height at the point of interest, which is the difference between the quasigeoid and the ellipsoid or another reference surface. The quasigeoid is determined by combining the gravity measurements with a model of the Earth's shape and rotation.
Once we have the geoid height, we can add it to the height of the reference surface (e.g., the ellipsoid) to obtain the elevation of the point above the reference surface. This gives us a more accurate estimate of the point's height above sea level than using a simple reference surface such as the ellipsoid.
The quasigeoid is also used in geodetic surveys to establish precise control networks for mapping and navigation. By using the quasigeoid as a reference surface, surveyors can accurately measure the heights and depths of features on the Earth's surface, such as buildings, rivers, and coastlines. The resulting data can be used for a variety of applications, such as urban planning, construction, and disaster response.
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