9.3 How Can You Interpret Objects in an Aerial Image?

Understanding what you’re looking at when you’re viewing Earth from the sky instead of from ground level takes a completely new skill set. Objects that you’re used to looking at head-on have a whole different appearance from directly above. For example, Figure 9.11 shows two different views of the Stratosphere Hotel and Casino in Las Vegas, Nevada—one showing an overhead aerial view and the other an oblique photograph showing details of the tower and its buildings. Obviously, the complex looks completely different from the oblique angle than from directly above. If you look closely, you can see where areas in the two photos match up, such as the tower on the right-hand side of the image and the remainder of the hotel and casino on the left. However, if you’re only given the aerial image and asked what building this is (of all possible structures in the world), it’ll be a whole different ballgame.

FIGURE 9.11 The Stratosphere Hotel and Casino in Las Vegas, Nevada: (a) an overhead aerial view of the building and (b) an oblique image showing the tower and the hotel layout.(Sources: (a) DigitalGlobe/Getty Images (b) Ethan Miller/AFP/Getty Images/Newscom)

From the aerial image alone, you’ll have to search for clues to determine what’s really being shown. For instance, based on the size of the complex (roughly a city block) you’ll probably guess that it’s some sort of hotel, resort, entertainment attraction, or museum. By looking at its location amidst other large buildings and multiple-lane roads, you may guess that the building’s in a big city. On closer examination, you might make out a different colored object (the light blue) among the main complex and determine that it’s a swimming pool, which narrows the chances further on the building being some sort of hotel or resort. Although the height of the tower on the right-hand side of the image isn’t really discernible (since the image was taken looking down onto the tower itself), the huge shadow cast by the tower (stretching upward) is visible. Based on the length of the shadow (and the shape of the shadow being cast), you may determine that the object casting it is a tower (or a similar very large, very tall object). Putting all of these clues together (city-block-sized hotel/resort in a big city with a massive tower in front of it), you should be able (with the help of some outside information, like a few travel books and maybe a quick Web search) to determine pretty quickly that you’re looking at the Stratosphere Hotel in Las Vegas.

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visual image interpretation the process of examining information to identify objects in an aerial (or other remotely sensed) image

pattern the arrangement of objects in an image: an element of image interpretation

When you try to interpret features in an aerial photo or a satellite image (for instance, objects in developed areas or physical features in a natural landscape), you act like a detective searching for clues in the image to figure out what you’re really looking at. As in the Stratosphere Hotel example, clues like the size and shape of objects, or the shadows they cast, help to determine what you’re really looking at. Visual image interpretation is the process of identifying objects and features in an aerial (or other remotely sensed) image based on a number of distinct elements:

site and association the information referring the location of objects and their related attributes in an image. Used as elements of image interpretation

size the physical dimensions (length, width, and area on the ground) of objects. An element of image interpretation

shadow the dark shapes in an image caused by a light source shining on an object. An element of image interpretation

shape the distinctive form of an object. An element of image interpretation

texture repeated shadings or colors in an image. An element of image interpretation

tone the grayscale levels (from black to white), or range of intensity of a particular color discerned as a characteristic of particular features present in an image. An element of image interpretation

These same elements can be used to identify items in satellite images as well. Figure 9.12 is an overhead view of an area containing several objects that all add up to represent one thing. We’ll apply these elements to this image to determine what is being shown.

FIGURE 9.12 An overhead view of an area containing several different objects.(Source: Space Imaging Europe/Photo Researchers, Inc.)

Bringing all of these elements of interpretation together, we can find three pyramids of different sizes (although still very large), in a definite fixed arrangement, in sand near a busy city. All of these clues combine to identify the Great Pyramids at Giza, just outside Cairo, Egypt. Aerial images often contain clues that help to identify the objects they contain, or at least to narrow down the options to a handful. Some brief research among collateral material (like groupings of large desert pyramids in that particular arrangement) should nail the identification.

Visual image interpretation skills enable the viewer to discern vital information about the objects in a remotely sensed image. The elements of interpretation can be used to identify (for instance) the specific type of fighter jet on a runway—based on the size and shape of its features, wingspan, engines, and armaments. A forestry expert examining a stand of trees in a remotely sensed image might use visual image interpretation to identify the species of trees in the stand based on the features of the trees.

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