4.12.1. Orienting a stereo-pair of photographs

The first step in creating a stereo-model is that the stereo-pair must be properly oriented under the stereoscope. The process of orientation is called base lining, which is performed as below:

To create a 3D model from two photographs taken from slightly different angles, these images must first be correctly aligned or oriented. This initial process is called 'base lining.' The following steps detail how to achieve the proper orientation.

Think of this as setting up two projectors to show a movie from different angles. To get a clear image on the screen, you must align both projectors so that the images overlap perfectly. This alignment is crucial for balanced viewing.

On both the photographs, their respective principal point and conjugate principal point are marked, as shown in Figure 4.18a. Principal point and conjugate principal point are joined by a straight line and the line extended on each photo. This line represents the flight line.

The principal point is where the camera was pointed when the photograph was taken, while the conjugate principal point is the corresponding point on the other photograph. By marking these points and connecting them with a straight line, you are effectively identifying the direction in which the camera moved during aerial photography, which is crucial for aligning the images.

Imagine drawing a straight line between two points where you took a photo of the same house from different angles. That line helps you track the route you took, similar to how the flight line helps align the images.

Under a mirror stereoscope, two photographs are to be kept apart in the direction of flight line on a flat surface with overlap region inwards.

Once the key points are marked, the photographs must be placed under a special device called a mirror stereoscope. They should be arranged so that the flight line aligns in the direction of the photos, with the area that overlaps placed towards the center. This positioning is important to ensure that the viewer sees the areas that match in both photographs.

It’s like setting up two pictures on a table facing you at a slight angle so you can see where they overlap in the middle. You wouldn't want them placed at odd angles where overlapping areas don’t align.

The stereo-pair is aligned in such a way that the line drawn on both the photos lie in a straight line, as shown in Figure 4.18a.

Aligning the pair of photos means adjusting them until the lines you drew earlier appear straight and continuous across both images. This is similar to making sure two pieces of a jigsaw puzzle fit together correctly.

It’s like matching the edges of two puzzle pieces together. You twist and turn them until their edges align perfectly, creating a seamless picture.

Stereo-pair is seen through the stereoscope so that left lens is over the left photograph and the right lens is over the right photograph. The line joining the centre of the lens should almost be matching with the direction of flight line.

After aligning, you look through the stereoscope, where each lens magnifies one photograph. You want to ensure that the lenses correspond to the photographs correctly, enhancing the areas where they overlap to form a 3D image.

This can be compared to looking through binoculars. Each lens focuses on a different view, and when they are positioned correctly, your brain merges the views into one cohesive image.

The distance between the photographs may be adjusted inward or outward till the two images are fused in the brain and a 3D model of the overlap region is created.

Adjusting the distance between the photographs is necessary for achieving the right level of depth. By moving the images closer or further apart, you can facilitate your brain’s ability to merge them into a single 3D visual.

It’s similar to adjusting the focus on a camera; if the pictures are too close or too far apart, you can’t get a clear, focused view. The right adjustment allows for clear visualization.

In the beginning, it might appear a bit difficult to see the two image fusing together to create a stereo-vision, but with a little more practice and concentration, it will appear to be easy.

Achieving the perfect stereo-vision can be challenging at first. It requires practice to train your brain to perceive the combined images as one coherent 3D structure. Persistence pays off, and eventually, it becomes easier and more natural.

This can be compared to learning to ride a bicycle. Initially, it may seem difficult to balance, but with practice, it becomes second nature.

Once the perfect 3D model is created and the lines drawn on the photographs fall in a line, the photographs are said to be properly oriented (or base lining is completed).

Once the images are aligned and a clear stereoscopic effect is achieved, you can proceed to select key features or points in the overlapping area. These points are crucial for further measurements, as they represent locations where height needs to be determined.

Imagine you are ready to mark spots on a map for which you need to measure distances. Once your map is perfectly oriented, you can accurately select locations to calculate measurements.

Use the parallax bar for taking the measurements of these points. The difference between two parallax bar readings will provide parallax differences between the two points.

To measure the heights of selected points accurately, a tool called a parallax bar is employed. By measuring the difference in readings taken from both images, you derive the parallax difference, which is directly related to the height of those points.

It's like using a ruler to measure how much higher a mountain peak is compared to a valley. The more you measure, the clearer the differences in height become.

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