Final answer:
The process of establishing intermediate points with the help of a theodolite on a straight line whose ends are intervisible is called Triangulation.
Step-by-step explanation:
Triangulation allows surveyors to measure distances to inaccessible objects, such as a tree across a river, by using a baseline and observing the angles from two different vantage points.
The apparent change in direction of the remote object due to a change in vantage point of the observer is known as parallax.
By applying principles such as the Pythagorean theorem and understanding azimuths, surveyors can accurately determine the properties of the triangle made by these angles and calculate the distance to the object of interest.
Triangulation relies not only on angular measurements but also on the concept of linear perspective, where orthogonal lines or orthogonals are used to represent the illusion of depth as they converge at a vanishing point on the horizon line. This principle is analogous to how railroad tracks appear to converge in the distance.