Systematic samples are often carried out along a transect. A transect is simply a line along which the sampling is done. This technique is often used along a known environmental gradient. E.g. from the cliff face to the low water mark on a rock shore.
The simplest way to survey along a transect, is to sample points at regular intervals along the transect line. This is known as a point transect. A more detailed survey would be to place quadrats at regular intervals along the transect line. This is known as a ladder transect.