Understanding the Effect of Non-Cardinal Directions on True Bearing: Geodetic Considerations

In navigation, true bearing is the direction in which an object or location is located, measured in degrees clockwise from true north. However, on a globe, the true bearing of an object can change as the observer moves in a non-cardinal direction. This phenomenon is commonly referred to as “geodetic azimuth variation”.

Geodetic azimuth is the angle between a reference direction, such as true north, and the direction to a point on the Earth’s surface, measured in a horizontal plane. The variation in geodetic azimuth occurs because the Earth is not a perfect sphere, but rather an oblate spheroid with irregularities in its shape and gravity field. As a result, the shortest distance between two points on the Earth’s surface, known as the geodesic, is not always a straight line in a two-dimensional plane.

In this article, we will explore the proper term to describe the observation that your true bearing changes as you move in a non-cardinal direction on a globe. We will also discuss the factors that contribute to this effect and how it is relevant to geodesy and earth science.

The Geodetic Effect

The variation in geodetic azimuth is caused by the curvature of the Earth’s surface and the fact that the shortest distance between two points on the surface is not a straight line. To understand this effect, consider an observer standing at the equator and looking due north. The observer’s true bearing is 0 degrees, or due north. However, if the observer travels due east or west and then looks due north again, the true bearing will no longer be 0 degrees, but rather a small angle to the east or west of 0 degrees. This is because the observer has followed a curve on the Earth’s surface, rather than a straight line, and his direction has changed accordingly.

The amount of variation in geodetic azimuth depends on the distance traveled and the latitude of the observer. At the equator, the variation is zero for any distance traveled due east or west, since the geodesic is a straight line along the equator. However, as latitude increases, the geodesic curves toward the poles, and the variation in geodetic azimuth becomes more significant for the same distance traveled. At the poles, any movement in a non-cardinal direction is along a meridian, and the variation is infinite.

Terminology

Geodetic azimuth variation is a mouthful, and there are several other terms used to describe this effect. One common term is “geodetic bearing change,” which is simply the change in geodetic azimuth between two points on the Earth’s surface as the observer moves in a non-cardinal direction. Another term is “geodetic deflection,” which refers to the deviation of a line from a straight line due to the curvature of the Earth. This can also contribute to the variation of the geodetic azimuth, as the direction of the geodesic changes with the curvature of the Earth’s surface.

Geodesy and Earth Science Applications

The variation of geodetic azimuth is an important consideration in geodesy, the science of measuring and modeling the shape and orientation of the Earth. Geodetic measurements, such as GPS readings, rely on accurate knowledge of the Earth’s shape and gravity field to determine position and orientation. The variation of the geodetic azimuth can introduce errors into these measurements, especially at long distances and high latitudes.
The effect is also relevant in Earth science, particularly in the study of plate tectonics and the movement of the Earth’s crust. The motion of plates is often described in terms of their direction and velocity relative to a fixed reference frame, such as a point on the Earth’s surface. The variation of the geodetic azimuth can affect the accuracy of these measurements, especially over long periods of time.

Conclusion

The variation of the geodetic azimuth is an important phenomenon that affects the true bearing on a globe when an observer moves in a non-cardinal direction. This effect is caused by the curvature of the Earth’s surface and the deviation of the geodesic from a straight line. There are several terms used to describe this effect, including geodetic bearing change and geodetic deflection. The variation of the geodetic azimuth is relevant to geodesy and earth science, particularly in the measurement of position and bearing, and in the study of plate tectonics.

FAQs

1. What is true bearing?

True bearing is the direction in which an object or location lies, measured in degrees clockwise from true north.

2. Why does the true bearing change if you move in a non-cardinal direction on a globe?

The true bearing changes because the shortest distance between two points on the Earth’s surface, known as the geodesic, does not always follow a straight line in a two-dimensional plane, due to the curvature of the Earth’s surface.

3. What is the variation of the geodetic azimuth?

The variation of the geodetic azimuth refers to the change in the geodetic azimuth between two points on the Earth’s surface when an observer moves in a non-cardinal direction.

4. What is geodetic deflection?

Geodetic deflection refers to the deviation of a line from a straight line due to the Earth’s curvature. This can contribute to the variation of the geodetic azimuth.

5. How does the magnitude of the variation of the geodetic azimuth depend on latitude?

The magnitude of the variation of the geodetic azimuth increases as the latitude increases, as the geodesic curves towards the poles. At the poles, any movement in a non-cardinal direction is along a meridian, and the variation is infinite.

6. Why is the variation of the geodetic azimuth important in geodesy?

The variation of the geodetic azimuth can introduce errors into geodetic measurements, such as GPS readings, which rely on accurate knowledge of the Earth’s shape and gravity field to determine location and direction.

7. How is the variation of the geodetic azimuth relevant to the study of plate tectonics?

The variation of the geodetic azimuth can affect the accuracy of measurements of plate movement, particularly over long time periods, as the movement of plates is often described in terms of their direction and speed relative to a fixed reference frame, such as a point on the Earth’s surface.