A Brief Introduction to Image Interpretation

 

What we sense with our eyes are spatial objects that are observed as a result of visible wavelengths of the electromagnetic spectrum being reflected off them. Various wavelengths cast a variety of colors and brightnesses adding a sense of three dimensions to these objects. Tone is the lightness or darkness of objects represented in an image. This applies to black and white, color, and false color images. Contrast is the variation in brightness from the lightest to the darkest objects. A maximum contrast occurs between two objects when one is expressed as white while the other is black, or in the case of color images, when there is maximum variance in brightness between hues (colors). Texture is the apparent smoothness or roughness which is exhibited in an image by a contrast in the distribution of brightness values. Smooth objects have many brightness values that are similar or the same value whether dark or light; while coarse textured objects have more variable (contrast) brightness values. Spatial objects that are expressed with similar or like brightness values are generally of a homogeneous feature class. Large variations in brightness values can denote different feature classes; or display a pattern within a course textured field of brightness values which may be of a similar class such as row crops, an orchard, or other objects created within a human settlement. Tone and texture are closely associated and based upon the distribution of brightness values. Shape defines the geometry of objects, while size represents length, area, or volume. Shadow reveals the position of the sun, and can be useful in determining symmetry or asymmetry, size, and shape. This can be particularly effective in identifying buildings and other structures that only show a top view of the structure. Shadows also illuminate smaller surface features casting a linear or area dimension that may not be noticeable when viewed from an airborne or space borne platform. Pattern suggests something is influencing the spatial distribution of surface features, although a lack of pattern can also be significant in that it may suggest an anomaly that is not a function of a normal condition or process. Trees aligned in a linear pattern may suggest the underlying geology has some control or influence on the distribution of surface features. On the other hand, specific vegetation species that are not indigenous to an area may suggest something more interesting is a contributing factor. Density refers to the number of objects taking up an area or region which may be an indication of vegetation health or vigor. Clustering of objects can generally indicate something is controlling the manner in which they have assembled.