We have talked about a number of important “big picture” concepts for our planet, but we have only touched on how we know these things. For example, you know that we learn about Earth’s interior with seismic waves, and about the geological time scale through careful study of rocks and fossils. But there are many other ways that scientists study our planet. Before you read on, try the following discussion.

You’ve probably come up with quite a few different ways by which scientists study the Earth. The full list is very long, and new ways of studying Earth are frequently developed. Here, we’ll discuss just a few of the most important methods of study — methods that we’ll refer to again later as we study Earth in more detail.

Geological Studies

When we speak about “geological” features of the Earth, we generally mean features that we can see on the surface landscape (including the ocean floor) and features of Earth’s interior. We’ve already discussed how we learn about Earth’s interior, including the important role of seismic waves. For the surface landscape, our primary tools are careful study of rocks and fossils, which we’ll discuss further in Chapter 5.

In recent decades, one more tool has become particularly important to studying Earth’s geology: the Global Positioning System , or GPS. You are probably familiar with GPS, because it is what your phone or car uses to navigate from one place to another. GPS is also used by ships and airplanes.

The Global Positioning System gets its name from the fact that it allows anyone on Earth to determine their position — that is latitude, longitude, and altitude — very precisely. In most cases, GPS is precise to within a few meters. For example, a car navigation system will know what road you are on, because there is probably not another road within a few meters of you, though it will not know exactly which lane you are in.

For geology, the importance of GPS arises from the fact that scientists have figured out ways to use it to get even more precise positions — in some cases, positions precise to only about a millimeter. This means that scientists can use GPS to track subtle movements of Earth’s surface, such as those that occur during earthquakes or volcanic eruptions. Scientists even use GPS to track the movement of Earth’s tectonic plates, despite the fact that these plates typically move at a pace that wouldn’t be noticeable to our eyes for thousands or millions of years.

The details of how GPS works are beyond the scope of what we’ll discuss here, but in case you are curious, Figure 4.25 provides a visual summary. The “system” in the Global Positioning System is a set of at least 24 satellites (there are 31 as of 2020) orbiting Earth. Their orbits are designed so that from any point on Earth, it is possible to “see” — or more accurately, receive a radio signal from — at least 3 GPS satellites at a time. This allows a GPS receiver to calculate a 3-dimensional position on Earth (latitude, longitude, altitude). Note that the satellites orbit at an altitude of a little over 20,000 kilometers, which makes them medium-altitude satellites.

Local Measurements (“Ground Truth” Data)

Besides learning about geological features, Earth scientists also want to learn about the atmosphere, oceans, weather, distribution of life, and much more. There are two basic ways to do this:

• We can make direct, local observations or measurements; that is, measuring something where you are located. These types of direct observations or measurements are sometimes called ground truth data , because they represent facts (“truth”) observed or measured on the ground where the observer is standing.
• We can make observations or measurements of things that are at some distance from our location through what is called
  remote sensing ; in this context, “remote” means that the thing we are studying is not in the same place that we are located, but instead is “remote” from us.

Let’s first consider local (ground truth) data. There are many different kinds of local data that scientists may collect. In fact, these data are often collected even by non-scientist volunteers, including school children. For example, weather data — such as temperature, pressure, humidity, rainfall, wind speed and direction, and more — are collected by both scientists and nonscientists around the world (Figure 4.26). This type of “citizen science” (science done by nonscientists) enables scientists to have a lot more data than they might have otherwise, and more data almost always leads to more understanding.

Remote Sensing

Try this question before you continue. Click to make sure you have found and understand the answer.

• In what sense is sonar a form of “remote” sensing?
  
  Open

  Sonar allows us to detect or map objects located at some distance from us, such as mapping the seafloor from a ship on the surface. In other words, the objects we are studying are “remote” from us.

Sonar is considered a form of what is known as active remote sensing, because we must actively send out the sound waves that then bounce back to provide us with information. Two other forms of active remote sensing that you have probably heard of and that are commonly used are:

• Radar (originally an abbreviation for “radio detection and ranging”) works similarly to sonar, except instead of bouncing sound waves of distant (“remote”) objects, it bounces radio waves off them.
• Lidar (originally an abbreviation for “light detection and ranging”) works like radar except using laser light rather than radio waves.

For learning about the Earth, radar and lidar are usually used from aircraft, balloons, drones, or satellites.

Some forms of remote sensing are considered passive, since we simply observe light or sound from objects at a distant. These include taking photographs of Earth’s surface or studying the spectra of light from the surface. In fact, while sonar, radar, and lidar are all very useful, most remote sensing is passive.