4.8.1. Exposure Station

Exposure station: Location of an aircraft in the air at the time of taking a photograph is called exposure station (e.g., 1,2, 3, etc., in Figure 4.7).

An exposure station refers to the specific spot in the air where an aircraft is positioned when it captures a photograph. This can be thought of as a reference point for where the camera was located during the photo-taking process. These stations are essential in photogrammetry because they help in determining where the image was captured relative to the ground features.

Imagine you are taking a family photo with friends at a park. The exact spot where you stand to take the picture can be thought of as your own 'exposure station'. Just like that, the aircraft's exposure station is critical because it tells us from where the aerial image was taken, which helps in understanding the view of the area.

Air-base or Camera base: The distance between two successive exposure stations along a flight line is called the air-base or camera base (e.g., 1-2, 2-3 etc., in Figure 4.7).

The air-base or camera base is defined as the distance between two consecutive exposure stations during a flight. This distance is crucial as it determines how much overlap there is between the photographs taken. The greater the overlap, the more information can be gathered, which is important for creating accurate maps and models.

Think of a photographer following a long train track while taking pictures of it. If they take a photo every few feet, that distance between each shot is similar to the air-base. The closer the photos are to each other (less distance), the more continuous the view of the train track will be, with less chance of missing parts of it.

Perspective centre: The point of origin or termination of bundles of perspective light rays is called the perspective centre (Figure 4.8).

The perspective centre is the theoretical point from which all the light that creates the image through the camera lens originates. In simpler terms, it is where the virtual 'eye' of the camera sits. This point is significant because it influences how the image is projected onto the film or digital sensor, affecting the depth and angle at which objects appear in the photograph.

Imagine you are looking through a telescope. The point inside the telescope where the light converges and you get a clear view is akin to the perspective centre. Just as that point determines what you see through the telescope, the perspective centre determines how ground features are captured in a photograph.

Perspective (Central) projection: All the projecting rays by a camera lens pass through the perspective centre to form the perspective projection (Figure 4.8). An aerial photograph is based on a perspective (central) projection. Due to relief variations of the ground objects, an aerial photograph differs geometrically from the map of corresponding area.

Perspective projection is the method by which images are formed in photography. In this context, when the camera takes a shot, all the light rays pass through the perspective centre, which produces a realistic view of the area. However, due to changes in elevation or relief of the land, photographs can appear distorted when compared to a flat map, which has a uniform scale.

Think of looking at a mountain landscape. The closer you stand to the mountain, the more pronounced it appears compared to the valley. If you were to take a picture from afar, the mountain and valley might seem more compressed together. This effect is like perspective projection in photography, which captures the depth and height differences in a more realistic manner than a flat map can show.

Flight line: The flying path an aircraft takes when taking the photographs is called flight line. It represents x-axis for photographic measurements (e.g., line 1-6 in Figure 4.7), while y-axis is perpendicular to it passing through the principal point of the photograph.

A flight line is the trajectory along which an aircraft moves to capture aerial photographs. This is crucial for mapping because it defines the area that will be photographed and establishes the coordinates for the captured images. Effectively, the x-axis in this scenario represents the direction of the aircraft's travel, while the y-axis is used to denote the perpendicular distance to the principal point of the photograph.

Imagine taking a road trip where you drive straight down a highway (the flight line). Each exit you take represents a point along that line where you might stop to look at a view (the photographs), and the further you travel along that highway, the more of the scenery you can capture. The path you take defines what you capture in your journey.

Flight strip: Each flight line during photography of an area will cover some area on the ground in the form of a corridor, called a flight strip. Figure 4.7 shows two flight strips.

The flight strip is an area on the ground that corresponds to a particular flight line and is typically shaped like a corridor. This is significant because it helps to organize the captured images in a way that ensures all necessary landscape features are photographed. The width of this strip ensures that adjacent strips overlap sufficiently to cover the entire area.

Consider a farmer planting rows of crops in a field. Each row aligns with a flight strip. If the rows are spaced correctly, they will overlap slightly, ensuring that the farmer doesn't miss any section of the field. In aerial photography, just like planting rows, the photographer needs to ensure enough area is covered in each flight strip to have complete coverage without gaps.

Strip of photographs: The number of photographs covered during a flight strip is called strip of photographs (e.g., 6 photographs in one strip in Figure 4.7).

A strip of photographs refers to the total number of images captured along a single flight strip during the photo-taking process. This is essential for photogrammetry as it helps in assembling a complete and coherent image of the area covered. The number of photographs ensures there is sufficient data for creating accurate models and maps.

Think of a scrapbook where you collect souvenirs from a trip. Each page in the scrapbook is like a strip of photographs showing different parts or events of your journey. If you are careful and take enough pictures on each page, it tells a complete story. Similarly, in aerial photography, taking enough photos in a strip helps create a full picture of the area.

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