Hope you have already observed oceans or lakes, even rivers and Mountains out there! The figure of the Earth has a true surface which is uneven and irregular such as partly land and partly water. However for geophysical purposes, the Earth's shape is represented by a smooth closed surface, which is called " Figure of the Earth ", since there is an association between it and gravity.
The Figure of the Earth can be defined as the shape of an equipotential surface of gravity in particular the one that coincides with Mean Sea Level. While the term An Oblate Ellipsoid (Spheroid) can be used to refer to as the best approximate mathematical to the figure.
Let's see in brief regarding this topic as I have described to you here below.
What is Geodesy?
Geodesy: Is the science of precise determination of the Earth's dimensions such that its polar and equatorial radii.
There is no doubt that as the term Geodesy described, you may notice the relationship between it and Geodetic survey. Geodetic survey determines the curvature of the shape of the Earth, such that any two points on the Earth are taken as curved.
Core subfields of Geodesy
- Geometric (Mathematical) Geodesy: It involves the determination and calculation of geometrical properties of the Earth such as Radii (Polar and equatorial).
- Physical (Geophysical) Geodesy: It deals with association between figure (shape) of the Earth and gravity.
- Satellite Geodesy: It deals with spacecraft such as satellites, missile in relation to the gravity effect from the Earth. It can be used to track their trajectories (path).
International Reference Ellipsoid: It is a standardized reference figure of the Earth, used to define the best fitting of oblate ellipsoid data from artificial satellites.
Figure 1: Ellipse of rotation and Sphere shape of the Earth (Reynolds, 1991)
In comparisons of dimensions by International Reference Ellipsoid, refer to the figure 1 above.
According to Geodetic Reference System (GRS80)
Reference Ellipsoid has Polar radius (Rp) = 6356.752 Km and Equatorial radius (Re) = 6378.137 Km
The Radius of Equivalent sphere, R = (Re2Rp)1/3 = 6371.000 Km.
Polar Flattening, f = (Re - Rp)/Re
f ~ 3.35287 × 10-3 (1/298.252)
The main cause of Polar Flattening is the deforming effect of the Centrifugal acceleration.
Equatorial acceleration Ratio, m : Is the ratio of Centrifugal acceleration to equatorial gravitational acceleration.
m =w2a/GEa-2 = w2a3/ GE
Where m ~ 1/288.901, GE - Geocentric gravitational constant = 3.98 × 1014
Normal Gravity, the direction of gravity at a point is defined as perpendicular to the equipotential surface through a point.
Geoid: Is the physical equipotential surface of the Earth. It is approximated by the International Reference Ellipsoid.
Figure: Simple geoid representation. (Reynolds,1991)
Geoid Undulation (Warp): Is the displacement between Geoid and Ellipsoid. It represents the local elevation of Geoid above Ellipsoid. Since Geoid reflect the true distribution of mass inside the Earth, it differs from the theoretical Ellipsoid by a small amount.
If the elevation caused by mass above the Ellipsoid it is Positive Undulation and if it is caused by mass below the Ellipsoid it is Negative Undulation.
Figure: Simple geoid Undulation (Reynolds,1991)
Causes: The mass within the Ellipsoid causes downward Gravitational attraction towards the centre of the Earth but Hill or mountain whose centre of gravity is outside the Ellipsoid causes upward attraction.
Geodesy is a very wide scientific discipline, as described in above three core subfields, but we have to be specific that our niche is physical geodesy that has a direct relationship with the Earth's gravity and geophysics.
Thanks for your time!
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