4.8. Technical Terms in Aerial Photogrammetry

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 is the precise location where a photograph is captured from an aircraft. Each time the aircraft takes a photo, it has a specific geographic spot in the air designated as that moment's exposure station. This term is critical for understanding the placement of photographs in aerial mapping.

Imagine you are taking a group photo at a family gathering. Each time you snap a picture, you stand in a specific spot. Likewise, in aerial photogrammetry, the aircraft's position over the land serves as the spot for capturing images.

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 refers to the horizontal distance between two consecutive points where photographs are taken along the same flight path. This measurement helps in planning the overlap between images for accurate mapping.

Think of the air-base like steps in a dance routine where each step is an exposure station. The distance from one step to the next determines how closely the dancers need to coordinate to keep the routine smooth.

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

The perspective centre is a geometric concept that helps define how images are formed through a camera lens. It is the point from which light rays diverge, which aids in understanding how photographs capture depth and 3D perception.

Imagine the perspective centre like a flashlight beam. Where the light originates is your perspective centre, and as the light spreads out, it creates shadows and depth, similar to how a camera captures depth in a photograph.

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 process through which a camera captures the three-dimensional world onto a two-dimensional photograph. In this method, all rays from the subject target the perspective centre, influencing how the final image is shaped based on the terrain below.

Consider looking through a window at a city. The buildings appear smaller and more distorted as they get further away, just like in perspective projection. It captures the depth and dimensional characteristics of the scene, similar to how aerial photos present the real-world layout.

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 path taken by an aircraft while it captures images from the sky. Understanding the flight line is essential for organizing and preparing aerial photographs for accurate mapping, as they define the systematic approach to flying over the area being surveyed.

Think of a flight line like the lanes on a highway. Just as cars follow marked lanes to ensure a smooth journey, aircraft follow designated flight lines to ensure they capture overlapping images necessary for accurate mapping.

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.

A flight strip is the area on the ground that is photographed during a flight along a specific line. The strip width is dependent on the camera's field of view and helps to ensure that there is adequate coverage of the area being surveyed.

Imagine spraying paint along a fence. Each pass with the spray can covers a strip of the fence, similar to how a flight strip covers a section of land. The more passes you make (flight lines), the more thoroughly you paint (capture images of the area).

The total number of photographs are obtained by multiplying the number of photographs in each strip by the number of flight lines) (e.g., 6x2= 12 photographs in Figure 4.7).

Calculating the total number of photographs involves understanding both the number of images taken during a single flight strip and how many flight lines are completed. This helps in quantifying the data collected and planning for analysis.

Think of total photographs like sheets of stickers you want to create. If each sheet has a certain number of stickers (photographs per strip) and you have several sheets (flight lines), to find out how many stickers you have in total, you would multiply the stickers per sheet by the number of sheets.

The height at which the aircraft flies (altitude of aircraft above mean sea level) when the photographs were taken is known as the flying height (Figure 4.8). It has a direct relationship with the photographic scale; higher the flying height, smaller the scale.

Flying height is a crucial aspect of aerial photography, impacting how much detail appears in the photographs. The higher the aircraft flies, the broader the view; however, this also decreases the detail within the images (the scale).

Think of how a bird views an area from a great height compared to being at ground level. At higher altitudes, you see a larger area, but details blur and become indistinct, similar to how aerial photos reflect detail based on flying height.

The common region (expressed as a percentage) between two photographs is called overlap. The overlap between two successive aerial photographs in the same flight line is called longitudinal overlap or forward overlap or end lap, and the overlap between photographs in adjacent parallel flight lines is called the lateral overlap or side lap (Figure 4.7). The amount of end lap is kept a minimum of 60%, which is useful to generate 3D view of the common area between the photographs. The lateral overlap or sidelap is kept between 25-40%, which is used to create a mosaic of the area.

Overlap is vital in aerial photography as it ensures that there are common areas between pictures that can be used for creating 3D views and mosaics. Adequate overlap provides continuity and ensures accurate data collection.

Picture reading a book with pages that overlap slightly. The overlaps help provide a coherent story without missing pieces. In the same way, the overlap in photographs creates a complete view of the area with no gaps.

The process of seamlessly joining a series of overlapping air photos together to form one large image, is called a mosaic. It is created to view and analyse the large area.

Creating a mosaic from aerial photographs involves stitching together multiple images to form a broader and more comprehensive view of the landscape. This ensures continuity and minimal loss of information about the surveyed area.

Imagine piecing together a jigsaw puzzle. Each piece represents a photograph, and when joined, they form a complete picture of a landscape. Similarly, mosaics provide a unified representation from many snapshots.

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