Aerial photographic and satellite image interpretation
Photographic interpretation is “the act of examining photographic images for the purpose of identifying objects and judging their significance” (Colwell, 1997). This mainly refers to its usage in military aerial reconnaissance using photographs taken from reconnaissance aircraft.
Principles of image interpretation have been developed empirically for more than 150 years. The most basic of these principles are the elements of image interpretation. They are: location, size, shape, shadow, tone/color, texture, pattern, height/depth and site/situation/association. These are routinely used when interpreting an aerial photo or analyzing a photo-like image. A well-trained image interpreter uses many of these elements during his or her analysis without really thinking about them. However, a beginner may not only have to force himself or herself to consciously evaluate an unknown object with respect to these elements, but also analyze its significance in relation to the other objects or phenomena in the photo or image.
Elements of Interpretation
The following are elements of aerial photographic and satellite image interpretation.
- There are two primary methods to obtain precise location in the form of coordinates. 1) survey in the field using traditional surveying techniques or global positioning system instruments, or 2) collect remotely sensed data of the object, rectify the image and then extract the desired coordinate information. Most scientists who choose option 1 now use relatively inexpensive GPS instruments in the field to obtain the desired location of an object. If option 2 is chosen, most aircraft used to collect the remotely sensed data have a GPS receiver. This allows the aircraft to obtain exact latitude/longitude coordinates each time a photo is taken.
- The size of an object is one of the most distinguishing characteristics and one of the more important elements of interpretation. Most commonly, length, width and perimeter are measured. To be able to do this successfully, it is necessary to know the scale of the photo. Measuring the size of an unknown object allows the interpreter to rule out possible alternatives. It has proved to be helpful to measure the size of a few well-known objects to give a comparison to the unknown-object. For example, field dimensions of major sports like soccer, football, and baseball are standard throughout the world. If objects like this are visible in the image, it is possible to determine the size of the unknown object by simply comparing the two.
- There is an infinite number of uniquely shaped natural and man-made objects in the world. A few examples of shape are the triangular shape of modern jet aircraft and the shape of a common single-family dwelling. Humans have modified the landscape in very interesting ways that has given shape to many objects, but nature also shapes the landscape in its own ways. In general, straight, recti-linear features in the environment are of human origin. Nature produces more subtle shapes.
- Virtually all remotely sensed data is collected within 2 hours of solar noon to avoid extended shadows in the image or photo. This is because shadows can obscure other objects that could otherwise be identified. On the other hand, the shadow cast by an object may be key to the identity of another object. Take for example the Washington Monument in Washington D.C. While viewing this from above it can be difficult to discern the shape of the monument, but with a shadow cast, this process becomes much easier. It is good practice to orient the photos so that the shadows are falling towards the interpreter. A pseudoscopic illusion can be produced if the shadow is oriented away from the observer. This happens when low points appear high and high points appear low.
- Real world materials like vegetation, water and bare soil reflect different proportions of energy in the blue, green, red, and infrared portions of the electro-magnetic spectrum. An interpreter can document the amount of energy reflected from each at specific wavelengths to create a spectral signature. These signatures can help to understand why certain objects appear as they do on black and white or color imagery. These shades of gray are referred to as tone. The darker an object appears the less amount of light it reflects.
Color imagery is often preferred because, as opposed to shades of gray, humans can detect thousands of different colors. Color aids in the process of photo interpretation.
- This is defined as the “characteristic placement and arrangement of repetitions of tone or color in an image.” Adjectives often used to describe texture are smooth (uniform, homogeneous), intermediate, and rough (coarse, heterogeneous). It is important to remember that texture is a product of scale. On a large scale depiction, objects could appear to have an intermediate texture. But, as the scale becomes smaller, the texture could appear to be more uniform, or smooth. A few examples of texture could be the “smoothness” of a paved road, or the “coarseness” a pine forest.
- Pattern is the spatial arrangement of objects in the landscape. The objects may be arranged randomly or systematically. They can be natural, as with a drainage pattern of a river, or man-made, as with the squares formed from the United States Public Land Survey System. Typical adjectives used in describing pattern are: random, systematic, circular, oval, linear, rectangular, and curvilinear to name a few.
Height and Depth
- Height and depth, also known as “elevation” and “bathymetry”, is one of the most diagnostic elements of image interpretation. This is because any object, such as a building or electric pole that rises above the local landscape will exhibit some sort of radial relief. Also, objects that exhibit this relief will cast a shadow that can also provide information as to its height or elevation. A good example of this would be buildings of any major city.
- Site has unique physical characteristics which might include elevation, slope, and type of surface cover (e.g., grass, forest, water, bare soil). Site can also have socioeconomic characteristics such as the value of land or the closeness to water. Situation refers to how the objects in the photo or image are organized and “situated” in respect to each other. Most power plants have materials and building associated in a fairly predictable manner. Association refers to the fact that when you find a certain activity within a photo or image, you usually encounter related or “associated” features or activities. Site, situation, and association are rarely used independent of each other when analyzing an image. An example of this would be a large shopping mall. Usually there are multiple large buildings, massive parking lots, and it is usually located near a major road or intersection.
- Jensen, John R. Remote Sensing of the Environment, Prentice Hall, 2000
- Colwell, R.N., Manual of Photographic Interpretation, American Society for Photogrammetry & Remote Sensing, 1997