 The PDF is a histogram of collected distances and describes the probability that an object at distance "y" will be detected by an observer on the center line, with detections on the line itself ("y" = 0) assumed to be certain ("P" = 1).By preference, g("y") is a robust function that can represent data with unclear or weakly defined distribution characteristics, as is frequently the case in field data.Several types of functions are commonly used, depending on the general shape of the detection data's PDF