Written By: ST
Parallax (meaning 'alternation') is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.
Due to foreshortening, nearby objects show a larger parallax than farther objects when observed from different positions, so parallax can be used to determine distances.
To measure large distances, such as the distance of a planet or a star from Earth, astronomers use the principle of parallax. Here, the term parallax is the semi-angle of inclination between two sight-lines to the star, as observed when Earth is on opposite sides of the Sun in its orbit. These distances form the lowest rung of what is called "the cosmic distance ladder", the first in a succession of me
thods by which astronomers determine the distances to celestial objects, serving as a basis for other distance measurements in astronomy forming the higher rungs of the ladder.
Parallax also affects optical instruments such as rifle-scopes, binoculars, microscopes, and twin-lens reflex cameras that view objects from slightly different angles. Many animals, including humans, have two eyes with overlapping visual fields that use parallax to gain depth perception; this process is known as stereopsis. In computer vision the effect is used for computer stereo vision, and there is a device called a parallax rangefinder that uses it to find range, and in some variations also altitude to a target.
A simple everyday example of parallax can be seen in the dashboard of motor vehicles that use a needle-style speedometer gauge. When viewed from directly in front, the speed may show exactly 60; but when viewed from the passenger seat the needle may appear to show a slightly different speed, due to the angle of viewing.
A few examples of parallax in daily life scenarios:
· A simplified illustration: The parallax of an object against a distant background due to a shift in perspective. When viewed from "Viewpoint A", the object appears to be in front of the blue square. When the viewpoint is changed to "Viewpoint B", the object appears to have moved in front of the red square.
· Hold up a finger and focus on something in the distant background. While looking at the background alternate opening your left and right eye one at a time. You will find that your finger jumps back and forth (or splits into two). This is the parallax effect in action.
· While exploring some rocky cliffs, you notice a lighthouse in the distance that you can't quite reach. It would be nice to know how far away that lighthouse is, but there is no way to make a direct measurement over the uneven terrain. Luckily you have your trusty compass and some knowledge about parallax and triangles!
For the first vantage point, you start with the lighthouse directly north of you. You then pace out 120 120 120 steps (about 100 100 100 meters) west. From this new spot, you measure the angle between due north and the lighthouse as 12 12 12 degrees.
Given that, determine the average distance to the lighthouse.
Aerial picture pairs, when viewed through a stereo viewer, offer a pronounced stereo effect of landscape and buildings. High buildings appear to 'keel over' in the direction away from the centre of the photograph. Measurements of this parallax are used to deduce the height of the buildings, provided that flying height and baseline distances are known. This is a key component to the process of photogrammetry.
Parallax is an apparent displacement or difference of orientation of an object viewed at two different locations during vertical aerial photography. The objects at a higher height lie closer to the camera and appear relatively larger than similar objects at a lower elevation. The tops of the objects are always displaced relative to their bases. Parallax can be measured by the angle of inclination between those two lines. Nearby objects have larger parallax than distant objects when observed from different positions. This difference in parallax gives a three-dimensional effect when stereo pairs are viewed stereoscopically.
Parallax is an apparent shift in the position of an object due to shift in the position of the observer camera (in aerial surveying). We experience this phenomenon when a moving body (a shift in position when compared to a static object considering camera as eye). This depends on the distance between the observer and the object. Nearer object move faster than that of the far distanced object, similar is the case of an aerial camera exposed to overlapping photographs which is caused by the movement of the aircraft is termed as stereoscopic parallax.
The above figure determine the images of object at points A and B; is considered as the part of the overlapping area within the two successive imagery vertical aerial images taken from the camera at the position in the space. Camera focus on point parallel to the flight line In the side image they appear as a’ and b’. Point A movement is greater because it occurs at higher position than that of the point B. The stereoscopic parallax at A and B are parallax a(Pa) and parallax b(Pb).
Pa = Xa-Xb’
Considered as the x direction parallax, similarly, to the x, y is also calculated to the relative position of the object in the imagery. As explained in the second figure Xa and Xa’ are the measuring photo coordinates of object A and the left and right respectively. Above coordinates, are based on the flight direction of points X and X’ for measuring parallax. It can be along the flight line for every imagery which is captured during the data acquisition of a stereoscopic pair.
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