View source for File:6.2.3-fig109.png

Two approaches of area estimation when line sampling is used: 

#either the entire line is used for line intercept sampling, or
#only the end points are observed, so that the observation unit is actually a cluster of two points at a defined distance away from each other.

Reference:

Kleinn C. 1991. Der Fehler von Flächenschätzungen mit Punkterastern und linienförmigen Stichprobenelementen. Dissertation. Mitteilungen der Abteilung für Forstliche Biometrie 91‑1, Universität Freiburg, 128 S.</ref>).]]