Hobbies And Interests
Military Intelligence
At any instant, an orbiting telescope can collect information about a wider area than an entire team of scouts could collect in a day. The problem lies in interpreting the images. How do you tell the difference between a double-decker bus, a military van and a missile-toting truck? There are little hints in the pictures, but they are just that: little hints. So someone needs to identify the reality on the ground. Then the ground truth gets compared to the aerial image, so that any time the same set of "little hints" is seen --- called the "signature" --- the object can be identified from the aerial image.
Other Aerial Images
In today's world, aerial or satellite images are used to identify wildfires, find illegal building activities and identify vegetation health and type --- among dozens of other applications. Take, for example, identifying the health of a corn crop. Satellites gather information at different wavelengths in the visible and infrared spectral regions, then people try to connect those measurements to the reality in the fields. So a high reflectance at an infrared wavelength, with a low reflectance in the green might correspond to corn that needs water --- but the only way of knowing that for certain is to measure in the field: find the ground truth.
Assuming Intention
Obtaining ground truth is even harder than that when trying to measure intention. For example, surveillance satellites often are in a "10:30 a.m." orbit. That means they pass over areas of interest when it's 10:30 in the morning so that they can take advantage of the morning shadows to help identify objects. What if, every Saturday morning, a group of people is seen collecting at the edge of a field? They could be there for a weekly hunting trip, they could be on a military training exercise or they could be playing in a disc golf league. The only way to be absolutely certain is to send someone there to find the ground truth.
Fact and Theory
You could believe, for example, that high school students are not doing as well in science because they have bad dietary habits and spend too much time texting their friends. It might even "make sense"; that is, you could look at growing sales of junk food and statistics that show increased cell phone text traffic right after school lets out. But just because something makes sense doesn't mean it's true. "Ground truth" has come to mean making the kinds of measurements that would validate a theory. If ground truth shows, for example, that some students subsist on potato chips and have their thumbs glued to their cell phone keyboards yet still perform well in science, that would substantially weaken your theory.