Visual Interpretation

Visual Interpretation - 1 GIS 4035C Remote Sensing of the...

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1 GIS 4035C Remote Sensing of the Environment Dr. Charles Roberts Visual Interpretation of Aerial Photography and Satellite Imagery Visual analysis is analysis done visually as opposed to quantitative analysis which is numerical, often digital. Much remote sensing that is done falls under the category of visual interpretation. Visual interpretation is both a science and an art and is critical to any work that you do in remote sensing beyond this course. What we will do today is explore the fundamentals of visual interpretation of aerial photographs. There are nine key characteristics used by analysts in aerial photograph interpretation: 1. TONE Tone is contrast, related to reflectance characteristics. Incident radiation upon a surface is either reflected, transmitted or absorbed. High reflectance = Light tones High absorption = Dark tones Tone is also partially a function of viewing angle with both the angle of the sensor and the angle of incident radiation taken into consideration. The same surface can have different tones from different angles. Tone is very important for interpreting black and white and color infrared photography. With color infrared, the tone changes in vegetation, for example, there is a distinct difference between deciduous and coniferous trees. 2. COLOR or HUE : Color is an important clue in the natural environment. In the last class, we strictly discussed image analysis using color. In normal color photography, color decreases with distance due to Rayleigh scattering. In this case, there is a bias towards blue and green and less red. Also, distant landscapes lack sharp color distinctions. With black and white imagery, hue distinctions are lost. In these we are actually looking at tone changes. For example, a bright green lawn and a concrete surface will look the same. With black and white, we can't use the rules that apply to color much, so we will stick mainly to the other 7 clues. 3. SIZE : relates to scale. Size is the element that you use to distinguish a telephone pole from a fencepost, a shed from a house, a sheep from a cow, a clump of grass from a tree. 4. SHAPE : Identifies an object, and places it in a class of objects. While we are familiar with the shape of objects on the ground, we now have to get used to the notion of looking at things from the air; there is a kind of fear that first causes you to be a bit reluctant to interpret imagery, because you are seeing new shapes of common things.
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