Interpreting satellite images

Information is extracted from images by digital image processing (with computers) or visual interpretation (by eye). We use the same clues every day to recognise things.

Elements of visual interpretation. Image characteristics: tone or colour, texture, size, shape, shadow and pattern. Terrain characteristics: location and association.

• Tone or colour: how much energy the object reflects. Smooth, dry surfaces reflect more than rough, moist ones. On a standard False Colour Composite (FCC), green light is shown as blue, red light as green, and near infrared as red. Healthy leaves reflect strongly in infrared, so healthy vegetation looks red (evergreen red to magenta; cropped land bright red); clear water dark blue to black; turbid water light blue; dense built-up areas dark blue to bluish green; fallow land and sand light blue to white.
• Texture: fine variations of tone. Dense housing gives a fine texture, scattered housing a coarse one; sugarcane or millet look coarse beside the fine texture of rice or wheat.
• Size: tells apart, say, factories from houses, or a city stadium from brick kilns on the edge of town.
• Shape: some outlines are unmistakable, like Parliament House in Delhi. A railway curves gently, while roads can bend sharply.
• Shadow: helps identify tall, thin objects such as the Qutub Minar, minarets, water tanks and power lines; but it can also hide what lies in it. It is more useful on large-scale aerial photographs than on satellite images.
• Pattern: repeated arrangements, such as a planned residential colony of identical plots, or orchards and plantations with evenly spaced trees.
• Association: objects and their surroundings, such as a school near housing with a playground inside its grounds, or industries along a highway at a city's edge.

Example (new): on an FCC, a bright red rectangle with an even, fine texture beside a village, crossed by thin straight lines, is most likely a field of a growing crop with its bunds, not a forest, which would look darker red and rougher.