Final answer:
The artist used orthogonal lines in their drawing to create an illusion of depth through linear perspective.
Step-by-step explanation:
The artist has used orthogonal lines in their drawing. These are the lines that converge at the vanishing point to create an illusion of depth, a technique that is central to linear perspective. Orthogonals are primarily seen in images of buildings and streets, or any scenes that require a convincing three-dimensional effect on a two-dimensional surface. If the painting in question is similar to Albert Bierstadt's The Rocky Mountains, Lander's Peak, then atmospheric perspective may also have been employed. This technique relies on objects appearing smaller, grayer, and less distinct the further away they are due to the 'haze' caused by elements such as water vapor in the air. This creates the illusion of depth in landscape paintings.