Tuesday, August 16, 2016

Interpretation of remote sensing data

The basic principles of image interpretation are:
  1. Location
  2. Size
  3. Shape
  4. Shadow
  5. Tone and Colour
  6. Texture
  7. Pattern
  8. Height and Depth
  9. Situation and Association
Location refers to the geographic location and is an important tool that helps to identify the type of vegetation. This is because any type of vegetation is specific in its requirement of soil, climate and other factors that are typical to a certain location.

Size of objects on images is important with reference to the image scale. Length, width and perimeter are commonly measured. Measuring the size of an unknown object helps the interpreter to rule out possible alternatives. For example, the dimensions of standard objects are known and this makes it possible to determine the size of an unknown object by comparison.

Shape refers to the general form, configuration or outline of individual objects. In case of stereoscopic images, the objects height also defines the shape.

Shadows may either aid or hinder in interpretation. Extended shadows can make it difficult to understand other objects that can be identified easily.  A shadow cast by an object may be a key to the identity of another object. It is always recommended that the photos are oriented so that the shadow falls towards the interpreter otherwise a pseudoscopic illusion is produced leading to low points appearing high and vice versa.

Tone and Colour of all matter refers to different proportions of energy reflected in the blue, green, red and infra-red portions of the electromagnetic spectrum. This can be used as a spectral signature to identify the type of matter. Different shades of a colour are called as tone. The darker an object appears, the less light it reflects. Colour imagery is preferred as humans can detect thousands of colours. Colour help in the process of photo interpretation.

Texture is the frequency of tonal change on an image. It determines the overall smoothness or coarseness of image features. It is defined as the characteristic placement and arrangement of repetitions of tone or colour in an image. As the scale of an image is reduced, the texture of any given object or area becomes progressively finer and ultimately disappears. An interpreter can distinguish between features of similar reflectances based on their textural differences. For example: the contrasting textures of two tree species.

Pattern refers to the spatial arrangement of objects. Objects may be arranged systematically or randomly. A few other patterns are: Circular, Linear, Oval, Rectangular and Curvilinear to name a few. The repetition of a few general forms is characteristic of natural and constructed objects thus forming a pattern that helps the image interpreter in recognizing objects. For example: the spatial arrangement of trees in an orchard versus the random distribution of trees in a forest.

Height and depth is also known as elevation and baythymetry. It is one of the most important important diagnostic element of image interpretation. Any object that rises above local landscape will show some radial relief. This casts a shadow that provides information regarding its height.

Situation and Association Situation refers to the manner in which the objects in the image are organized and situated with respect to each other. Association refers to the fact of finding a particular activity in an image. Location, Situation and Association are normally interrelated to each other in an image. As an example, consider a commercial complex. It has several large buildings, huge parking areas and is usually located near a major road.