Remotely Sensed Images from Above

Where Aerial Photography Came From, Color Infrared Photos, Orthophotos, Oblique Photos, Visual Image Interpretation, and Photogrammetric Measurements

Whenever you take a picture of something with a digital camera, the image is captured and stored as a file on a memory card. Taking a picture at a birthday party or on vacation is actually a form of remote sensing—acquiring data without being in direct contact with the subject. For instance, you don’t shove the camera right on top of the birthday cake to take a picture of it, you stand back a distance to do it. In essence, the camera is “sensing” (taking the picture) while you’re some distance (“remotely”) away from the target. In the same way, your eyes function as “remote sensing” devices—when you’re looking at this book, you don’t place your eyeball directly on the page. Instead, your eyes are “sensing” the data, and they’re “remotely” located about a foot away from the page. Both a camera and your eyes are performing “remote sensing” as they’re acquiring information or data (in this case, visual information) from a distance away without actually making contact with the item.

However, when it comes to remote sensing in geospatial technology, things are a little more specific. In remote sensing, the data being acquired is information about the light energy being reflected off of a target. In this case, your eyes can still function like remote sensing devices, as what you’re actually seeing is light being reflected off objects around you processed by your eyes. A digital camera does the same thing by capturing the reflection of light from whatever you’re taking a picture of. In 1826, a French inventor named Joseph Niepce took the first photograph, capturing an image of the courtyard of his home. Photography has come a long way since those days (when it took 8 hours to expose the film) to the point where digital cameras are now commonplace, and it’s difficult to find a cell phone without a built-in camera. In many ways, you can think of a camera as the original device for remote sensing—it captures data from the reflection of visible light nearly instantaneously from a distance away from the target.

People’s fascination with the possibilities of remote observation led quickly to the development of aerial photography. Photographers have been capturing images of the ground from the sky for over 150 years. Initially they used balloons and kites to achieve the necessary height, but developing technologies of flight created new opportunities—airplanes, rockets, and, most recently, satellites, have enabled us to capture images of Earth’s surface from greater and greater distances. When we’re dealing with remote sensing in geospatial technologies, we’re concerned with acquiring information from a platform on some type of aircraft or spacecraft. Currently, these types of imagery are obtained in both the public and private sector.

Whether for military reconnaissance, surveillance, studies of the landscape, planning purposes, or just for the good old purpose of being able to say, “Hey, I can see my house,” people have been acquiring images of Earth from above for a long time. Today’s aerial photographs are used for all manner of applications—for instance, aerial imagery serves as a base source for the creation and continuous updating of geospatial data and maps (such as USGS topographic maps, and maps of road networks). Some of the extremely crisp high-resolution imagery you see on Google Earth is taken from aerial photography sources. In this chapter, we’ll focus on how aerial images are captured and analyzed, and we’ll learn to get used to looking at Earth from above.