The station point is the fixed point in object space from which a perspective image or view is observed, captured or constructed. In visual perspective it represents the position of the eye or camera relative to the spatial scene or object being viewed. In graphical linear perspective it establishes the viewpoint from which the perspective construction is made.
The Perspective Research Centre uses Station Point as the preferred term for this geometrical and visual position. Other terms that have been used for the same or closely related concept include eye point, point of view, viewpoint, centre of projection, camera point, observer’s point and point of sight. The variety of historical terms can cause confusion because some have also been applied to points located on the picture plane.
The Station Point as the Viewpoint
Perspective concerns the changing appearance of spatial reality when it is observed from a particular viewpoint or series of viewpoints. The station point therefore provides one of the fundamental spatial references of a perspective system: it specifies where the observer or imaging system is located.
Changing the station point changes the relationship between observer and object. The object may consequently be seen from a different direction, distance or aspect, producing a different perspective view. A spatial scene does not therefore possess only one perspective appearance. Many different perspective views can be obtained from different station points.
In a conventional monocular perspective system, the station point can also be regarded as the point through which the relevant projection or visual rays pass. For this reason it has historically been described by terms such as the centre of projection, apex of the visual cone or apex of the visual pyramid.
Station Point in Visual Perspective
In Visual Perspective Type 2, the station point corresponds to the position of the observer’s eye from which the spatial scene or object is viewed. Visual rays from different points of the scene converge towards the eye, establishing the visual angles through which the spatial arrangement is seen.
The resulting appearance depends not simply upon the objects themselves, but upon their spatial relationships to the station point. Their direction, distance and angular relationship to the observer contribute to changes in apparent size, shape, position, aspect and foreshortening.
Human vision is normally dynamic rather than permanently fixed. The observer can move, turn the head or eyes, or look from successive positions. A single station point therefore represents one particular viewing condition within the much larger range of possible visual relationships between observer and spatial reality.
Station Point in Linear Perspective
Classical linear perspective normally constructs a spatial image from a fixed station point. The artist selects a viewpoint and represents the spatial scene as it would appear from that location.
This principle is closely associated with the traditional perspective window. The observer looks at a spatial scene from a fixed eye position through an actual or imaginary transparent plane. The picture plane intersects the visual pyramid or rays connecting the scene with the eye, creating the perspective image upon that plane.
Fixing the station point helps stabilise the geometrical relationship between the observer, the object, the viewing direction and the picture plane during construction. If the observer continually changes position while constructing the same conventional linear-perspective image, the various parts of the drawing may derive from different viewpoints rather than from one unified projection system.
The fixed station point is therefore an important characteristic of conventional one-point, two-point and three-point linear-perspective constructions, although the number of vanishing points should not be confused with the number of station points.
Station Point and Picture Plane
The station point is located in object space; it does not lie on the picture plane.
In the standard arrangement, a perpendicular from the station point meets the picture plane at the principal point, also called the centre of vision in this context. The two positions must therefore be distinguished:
- Station Point: the eye, camera or viewpoint located in object space.
- Principal Point / Centre of Vision: the corresponding point located on the picture plane.
This distinction is important because older and modern perspective terminology has sometimes used expressions such as point of sight, eye point or centre of vision for different parts of the system. A point representing the observer’s eye in object space is not the same thing as a point produced where a visual or projection direction intersects the picture plane.
Station Point and Viewing Distance
The distance between the station point and the picture plane is also significant. A perspective image geometrically constructed from a particular station point has a corresponding intended viewing position.
If the completed picture is viewed from the same station point, at the same distance and relationship to the picture plane from which the original projection was established, its visual angles most closely correspond to those of the original view.
A viewer may nevertheless look at the finished image from a different position. This changes the optical relationship between the viewer and the physical picture surface and can introduce locally produced visual distortions. In ordinary viewing conditions these effects may be small enough that the perspective image continues to appear convincing even when the observer is not positioned at its geometrically correct station point.
Station Point and the Perspective View
The station point should not be understood merely as a construction mark used in perspective drawing. It expresses a more fundamental relationship between observer and spatial reality.
A perspective view is always associated with some viewing condition. Where the observer is located influences what can be seen, the aspect from which it is seen, its apparent scale and shape, and the spatial relationships presented in the resulting image or view.
For this reason, the concept applies beyond traditional drawing. A camera can occupy a station point; a virtual camera can have a modelled station point; and digital or optical systems may employ fixed, moving, multiple or changing viewpoints according to the type of perspective being produced.
Fixed, Moving and Multiple Station Points
A traditional linear-perspective image commonly employs a single fixed station point. This produces the familiar stationary viewpoint associated with a unified perspective construction.
Perspective systems are not limited to this arrangement. A station point may move through space, or a system may combine information obtained from more than one viewpoint. Modern imaging and computational systems can also model a virtual viewpoint whose position changes in real time.
The distinction is important because a single fixed viewpoint, a roaming viewpoint and multiple viewpoints describe different relationships between the observer or imaging system and the spatial reality being represented. They may consequently produce different perspective forms and viewing experiences.
A Second Meaning in Plan Construction
The term station point also has a second, more specialised use in graphical perspective construction. In a plan, it may refer to the ground-plane projection of the eye point rather than directly to the eye position itself.
The context must therefore be established whenever the term is used. In its primary and preferred meaning, the station point is the actual or represented eye/camera position in object space. In certain construction diagrams, a station point marked on the ground plan may instead represent the orthographic projection of that eye position onto the ground plane.
Why the Station Point Matters
The station point establishes the location from which spatial reality is viewed or represented. It is therefore fundamental to understanding the geometrical and visual organisation of many forms of perspective.
It helps distinguish the viewing position in object space from related elements situated on the picture plane, and clarifies why changing the observer’s position can change the perspective appearance of the same spatial scene.
Understanding the station point consequently provides a foundation for studying the picture plane, centre of vision, principal point, line of sight, field of view, visual angle, perspective projection and the various forms of linear, visual and optical perspective.