Physical geodesy

Earth has a pull.

This pull is called gravity. This pull is not the same everywhere. Scientists study how this pull works.

They look at the shape of the sea. The sea is not flat. It bumps up and dips down. This happens because gravity pulls harder in some spots. Scientists use this to find height. They can see how high things are above the sea. It is a way to map our world.

Earth has a pull called gravity. But this pull is not the same everywhere.

Scientists study this with physical geodesy. This is the study of Earth's gravity. They also study the geopotential. This is a field of gravity power. Because gravity is uneven, the sea is not flat. It has a bumpy shape. We call this shape the geoid. The geoid is like a mathematical sea level. It exists under the land, too.

In some spots, the geoid sticks out. It can rise up by 100 meters. In other spots, it dives down. This happens because gravity pulls more or less. The gap between the geoid and a smooth shape is called undulation.

Scientists use tools to measure this. They use a plumb line. A plumb line is a weight on a string. It shows the local vertical direction. This direction is always straight down. By using these tools, they can find height. They can see how high a point is above sea level.

Earth has a pull called gravity. But this pull is not the same everywhere.

Physical geodesy is the study of these gravity properties. Scientists study the geopotential, which is a field of gravity power. This work helps us understand the shape of our world. It is very important for measuring height and location.

Scientists use many tools to see how gravity works. They use instruments called theodolites to find angles. These tools use gravity to find the local vertical direction. A plumb line shows this straight down path. They also use leveling instruments to find differences in potential. This helps them turn measurements into height in meters. This way, they know how high a point is.

Gravity is measured in units like metres per second squared. Scientists also use a special unit called the GPU. This stands for geopotential unit. One GPU equals 10 metres per second squared times metres. Moving one meter vertically changes your potential by about 1 GPU. This makes it easier to measure height above sea level. It is a very helpful way to track changes.

Because gravity is uneven, the sea is not smooth. The shape of the sea is called the geoid.

The geoid is like a mathematical sea level. It exists under the land as well. In some places, the geoid sticks out quite far. West of Ireland, it rises by 100 meters. Near Sri Lanka, it dives down by nearly 100 meters.

Scientists look for gravity anomalies to learn more. An anomaly is a difference between true gravity and normal gravity. They use these numbers to study the Earth's crust. They can also use a Bouguer reduction to fix data. This helps account for the pull of hills and land. These steps make the maps of our Earth more accurate. It is a big job for geophysicists.

Physical geodesy is a specialized branch of science focused on Earth's gravity. It studies the physical properties of the gravity field and the geopotential. The geopotential is a field of gravity potential that scientists use for many applications. Understanding these properties is vital for geodesy, which is the science of measuring Earth's shape. By studying how gravity pulls at different points, we can better map our planet.

To measure these properties, scientists use specific instruments and procedures. Traditional tools include theodolites, which are used to find vertical and horizontal angles. These instruments rely on the local gravity field to find the vertical axis. This axis follows the local plumb line, which is the direction gravity pulls straight down. Scientists also use leveling instruments to find geopotential differences between two points. They then convert these differences into metric units to express the height of a location.

Measuring these values requires specific scientific units. Gravity is commonly measured in meters per second squared (m·s⁻²). Potential is expressed as gravity multiplied by distance, which results in m²·s⁻². A more convenient unit is the geopotential unit, or GPU. One GPU equals 10 m²·s⁻². In our environment, moving one meter vertically changes your potential by approximately 1 GPU. This makes the GPU a useful tool for measuring height above sea level.

Because Earth's gravity is irregular, the sea surface is not a smooth shape. The equilibrium figure of sea water is called the geoid. The geoid represents the mathematical mean sea level. This surface is not just on the oceans; it also exists under the land. If water could move freely through tunnels under the continents, it would follow this geoid shape. The geoid is technically an equipotential surface of the true geopotential.

The geoid does not match a perfect, smooth mathematical model called a reference ellipsoid. The distance between the geoid and this ellipsoid is called the undulation of the geoid. This undulation can be quite large in different parts of the world. For example, west of Ireland, the geoid rises as much as 100 meters above the ellipsoid. Near Sri Lanka, the geoid dives nearly 100 meters below it. These variations show how uneven the Earth's gravity field truly is.

Scientists also study gravity anomalies to understand the Earth's structure. A gravity anomaly is the difference between true, observed gravity and calculated, normal gravity. One type is the free-air anomaly. In geophysics, researchers often use a process called Bouguer reduction. This reduction removes the effect of the attraction from local topography. If the land is a flat, horizontal plate, they use the Bouguer plate formula. If the terrain is not flat, they must also apply a terrain correction.

These measurements are essential for creating accurate vertical datums. A vertical datum is a reference surface used to measure height. Because of dynamic sea surface topography, different coastlines may use slightly different "near-geoids." These can differ by several decimeters or even over one meter. By mastering physical geodesy, scientists can create precise maps of the entire planet. This work connects gravity, math, and geography to define our world.