Geoscience Reference
In-Depth Information
depends on the assumptions made. If these assumptions are not correct,
then an estimate might be quite different from the true value.
4.2 Basic Procedures in Line Transect Sampling
Two types of data are recorded in line transect sampling, as shown in
Figure 4.1. These are either (1) the perpendicular distances from the transect
line
x
or (2) the sighting distances
r
and angles θ. However, studies based on
sighting distances and angles have been found to be subject to biases and are
only discussed briefly here.
The usual assumptions made with line transect sampling are the following:
1. All objects on the transect line are detected.
2. Objects do not move in response to the observer before the detection
is recorded.
3. Objects are only counted once.
4. Objects are recorded at the point of initial detection.
5. Distances are measured without errors.
6. Transect lines are randomly located in the study area.
A further assumption sometimes made for the estimation of standard errors
is that
7. Sightings are independent events, and the number of objects detected
follows a Poisson distribution.
P
x
r
θ
O
FIGURE 4.1
Graphical representation of statistics collected in line transect sampling. The letters O and
P represent the positions of the observer and detected object, respectively. The perpendicu-
lar distance from the point P to the transect line is labeled x, the sighting distance from the
observer to the point is labeled r, and the sighting angle is denoted by θ.
