Use of Transects for Remote Sensing Applications: Counting Density of Objects (Especially for GATE-Geospatial 2022)

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Users of remotely sensed imagery frequently extract brightness values between points on an image. A transect is a straight line between any two user-specified points within an image. Transects are used in many digital image processing applications. The pixels that lie on the transect can be measured and displayed to compare spectral or spatial differences.

  • A transect is a path along which one counts and records occurrences of the species of study (e. g. plants) . It requires an observer to move along a fixed path and to count occurrences along the path and, at the same time (in some procedures) , obtain the distance of the object from the path. This results in an estimate of the area covered and an estimate of the way in which detectability increases from probability 0 (far from the path) towards 1 (near the path) . Using the raw count and this probability function, one can arrive at an estimate of the actual density of objects.
  • Transects being used to measure the changes around the boundary of a grassland fire near Backhouse Tarn, Tasmania.
  • The estimation of the abundance of populations (such as terrestrial mammal species) can be achieved using several different types of transect methods, such as strip transects, line transects, belt transects, point transects and curved line transects.
  • Image analysts may use several transects in a single image to determine environmental trends or patterns. When several transects are used, the analyst must make sure that the endpoints of each transect fall on the same scan line to ensure equal transect distance. This often requires rotating the image to accommodate geometric integrity.
  • If the analyst extracts transect where the endpoints do not fall on the same scan line (or column) , the hypotenuse of stair-stepped pixels must be considered instead of the simple horizontal pixel distance (Jensen, 1996) .

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