EDUCATOR’S PAGE
Geodesy: Journey to the Ends of the Earth
Michael J.Urban, MEM-1910
Will you fall off the edge of the world if you walked far enough in any direction? Of course not – such a notion is totally absurd, yet some people must have believed in a flat Earth (and, apparently, some still do). Ancient seafaring cultures were obviously aware of at least some degree of curvature to the Earth, since ships appeared and disappeared along the horizon from mast to hull, or vice versa, respectively, when returning to and departing port. And, the shadow of our planet against the moon provided ample verification of Earth as a globe to the early Greeks. With satellites and a space station circling about our planet today, incontrovertible evidence is so abundant, and readily available, that most of us simply take for granted that the Earth is but one of many orbs amongst the stars. But, what is the best description for the shape of the Earth?
The physical characteristics of the Earth – including its size, shape, and topographic generalities – along with understand- ing maps and mapping, are critical components to any program of study in geology. Maps and depictions of the Earth, or its figure, can vary in precision depending on the requirements of the analysis. In many instances approximating the planet as a smooth ellipsoid is sufficient. In other cases, those neces- sitating the most refined measurements, the many nuances of Earth’s true shape must be taken into account. Geodesy is the branch of science dealing with measuring the earth or large areas of the earth for making maps, determining the size and shape of the earth, and correlating gravitational, geological, and magnetic surveys of the earth (as defined by the United States Geological Survey).
Spherical Earth?
To a crude approximation the Earth is a sphere, however the Earth is not a perfectly spherical object. The Earth is described as a flattened sphere (ellipsoid), or oblate spheroid, because its pole-to-pole diameter differs from its equatorial diameter (due to its rotation). Upon close inspection we find the Earth is covered by tiny bumps and cracks, the presence of which cause the planet to deviate from the precise shape of even an oblate spheroid. So, referring to Earth as a sphere, oblate spheroid, or ellipsoid, is also a generalization. Earth is a lumpy, irregular object, complicating mapping endeavors. See Figure 1.
The topography, or change in elevation of Earth’s specific surface features (e.g., Mount Everest, Marianas Trench, Grand Canyon, etc.), is measured relative to mean sea level or zero elevation. Sea level is the chosen baseline for the geoid and the National Geodesic Survey (2001) defines the geoid to be
Figure 1. Exaggerated illus- tration of a sphere (dark solid) and ellipsoid (dashed line) superimposed on the lumpy Earth (light solid).
the equipotential surface of the Earth’s gravity field which best fits…global mean sea level. The concept of the geoid is abstract and a rather complex idea for some students to understand; therefore, some elaboration is warranted here.
What is the Geoid?
The idea behind mapping the Earth is to compare all sur- face feature heights to some known, or defined, baseline or reference elevation (i.e., zero elevation). Using space-based satellites, the height of features on the Earth are known accurately relative to the center of the planet, but this is not practically useful. In practice, sea level was initially selected as a good baseline for elevation determinations because water covers 70% of the planet, and as a liquid, presents a relatively level surface regardless of the underlying topography. Crustal features of the planet could be measured both above and below mean sea level.
As the precision of measurements increased with advances in technology, the elevation of sea level was found to depend on gravity and varied with anomalies in the gravitational field; concentrations of mass exhibiting stronger gravity, lead to a higher sea level, and conversely, mass deficits, with their weaker gravity, produce a lower sea level. In fact, gravitational differences cause some permanent differences in sea level elevations compared to the general ellipsoid shape (National Ocean Service, 2014). This explains why gravity is included in the definition of the geoid. The geoid is an approximation of a sea level surface where every point on that surface would have the same gravitational acceleration. Because the mass of the planet is unevenly distributed, the surface of the geoid is not smooth or regular, like the surface of an ellipsoid would be, and instead undulates up and down with topography and mass1. See Figure 2.
The geoid, and surface elevations relative to it, constantly evolve as measurements continue to be refined (and as the dynamic surface of the Earth changes through time). Since a gravitational field is present and measurable over the entire
1. It is worth noting that gravity is always directed downward perpendicular to the geoid surface, resulting in two important points: a ball would not roll down any apparent “hills” on the equipotential surface, and geodetic instruments align to the direction of gravity – always perpendicular to the geoid (National Ocean Service, 1983).
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