4.12.2. Measurements by Parallax Bar

The change in position of an image from one photo to the next due to aircraft’s motion is called stereoscopic parallax, x-parallax, or simply parallax. The parallax is directly related to the elevation of the point, and is greater for high points than for low points.

Parallax refers to the apparent shift in position of an object when viewed from different angles or perspectives. In the context of aerial photography, parallax occurs when the aircraft moves, causing the image of the same land point to appear in different positions in successive photographs. This effect is important because it helps measure the height of terrain features. High points create a larger parallax than low points, meaning they will appear to move more between two images, revealing their elevation above the reference level.

Imagine watching a car drive past you while standing on a sidewalk. If you shift your position just a little to the left or right, the car appears to move against the background. The larger the distance between your view and the car's position, the more apparent the change is. This is similar to how we measure terrain heights from different aerial views.

The parallax bar is a device used to measure the difference of parallax between any two points on the stereo-photographs, more precisely. The parallax bar consists of a graduated metallic rod (in mm) attached with two bars, as shown in Figure 4.19.

A parallax bar is a specialized instrument designed for precise measurements in photogrammetry. It consists of a main scaled rod that measures in millimeters and has two bars attached to it. One bar is fixed, while the other is adjustable using a micrometer drum, allowing for fine-tuning and very accurate readings of parallax. The parallax bar enables researchers to take measurements between selected points on stereo-pair photographs, which helps in calculating the elevations of those points through triangulation methods.

Think of the parallax bar like a ruler with a sliding marker. When you are trying to measure a distance on a drawing or model, you can slide the marker to align it perfectly with your point of interest. This process ensures you get a precise measurement of the distance, similar to how the parallax bar aligns markers with features in aerial images.

The points/objects on stereo-pair are selected whose parallax bar readings are to be taken for the computation of heights. The oriented stereo-pair is now viewed in stereo mode, and the floating marks of graticules are kept at a point/object.

To use the parallax bar effectively, first identify the objects whose heights you want to measure from the stereo-pour photographs. Once identified, the stereo-pair images are viewed through the stereoscope to create a 3D effect. The floating marks on the graticules (thin glass plates) are then placed over the corresponding points on the left and right images. The goal is to adjust their alignment so that they appear to fuse into one mark, indicating the precise height of the object above the datum.

Imagine playing a game where you have to line up two images perfectly to create an illusion—like a magic trick. In this case, the images are the aerial photographs, and the floating marks act as your magic wands that you adjust until the images blend seamlessly. This gives you the precise height measurement you need for each object.

The absolute parallax of a point on a stereo-pair is determined as the algebraic difference between two distances which are measured from the corresponding principal points, parallel to the direction of flight (air base). It is also called x-parallax.

Absolute parallax, or x-parallax, is mathematically represented by the difference in horizontal distance between the same point observed in two different aerial images. These distances are measured from the principal point of each photograph, along the line of flight. This algebraic difference provides information about the elevation of the terrain relative to a known reference point, which is crucial for calculating the height of various objects in the images.

Think of absolute parallax like tracking two points on a moving car as you stand on the sidewalk. If you measure the distances from your own viewpoint (the principal point) to each of the two points (the front bumper and the back bumper of the car) in two snapshots from different angles, you can determine how far the car has moved and how high it is above the ground compared to where you are standing.

The elevation of unknown point B can be determined using parallax bar measurements based on the known height of point A. Since, it is difficult to measure the coordinates of all the objects on the photos, so these coordinates can be replaced by the parallax bar readings (P and P ) at points a and b.

Once the parallax readings from the bar are obtained, estimations of heights at different points can be made using these measurements. The elevation at one known point serves as a reference. The relationship derived from the parallax readings allows for the computation of other heights based on the parallax difference and known measurements. This method helps in bridging gaps where direct measurements are challenging.

This is like using a known building height to estimate the heights of nearby trees. If you know the height of the building, you can then apply similar calculations for the trees, using their positions and distances to guess their heights. The building's known height acts as a benchmark for making additional calculations.

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