4.3. Types of Aerial Photographs

There are two main types of images in photogrammetry. Photographs are taken with either aerial camera mounted in an aircraft or with terrestrial camera mounted on a tripod on the ground. Aerial camera in aircraft is usually pointing vertically down towards the ground, while in terrestrial photogrammetry it is pointing near horizontal.

In photogrammetry, the two primary types of photographs differentiate based on their capturing method. Aerial photographs are taken from an aircraft, typically directed vertically toward the ground, making them ideal for large-scale mapping. On the other hand, terrestrial photographs are captured from ground-level using a tripod or handheld camera, focusing on smaller areas or specific objects. This distinction is essential since the orientation of the camera influences the type of data collected.

Imagine you're taking a group photo at a concert. If you use a drone, you capture the entire crowd (aerial photograph), but if you're standing in front of the stage, you can only focus on your friends (terrestrial photograph). This difference illustrates how perspective changes the scope of what's documented.

An aerial photograph taken with the camera axis held in a vertical or nearly vertical position is classified as vertical photograph. A tilt of camera axis up to ±3º from vertical line is acceptable. When the geometry of a vertical photograph is considered, the photograph is assumed to be taken with the optical axis truly vertical.

Vertical aerial photographs are crucial in mapping as they capture the ground from a directly overhead position. A slight tilt, up to 3 degrees, is permissible and doesn't significantly affect the data because such photographs maintain the geometry needed for accurate measurements. This type of photograph allows for a more straightforward creation of maps and the extraction of detailed measurements, as the relationship between the photograph and real-world coordinates is easier to derive.

Think of a vertical photograph like looking straight down at a pancake from above. If the pancake is perfectly flat, you can easily take its measurements. Even if the pan tilts a bit, you still have a good idea of the pancake's size and shape. However, if you look at it from an angle, it becomes harder to judge those exact measurements.

Sometimes due to unavoidable conditions (e.g., strong winds), the camera axis is unintentionally tilted more than 3˚ from the vertical. The resulting photograph is called tilted photograph. The tilted photographs may further be classified in two categories viz., low oblique and high oblique photographs.

Tilted photographs arise when the camera is not aligned vertically, often due to environmental conditions such as wind. These photographs are categorized as low oblique (15° to 30° tilt) or high oblique (more than 30° tilt). Both types provide different views of the landscape, but they aren't ideal for precise measurements since they distort distances and angles.

Imagine you're trying to take a picture of a tall building while standing close to it. If you hold your camera straight up, you get a clear photo of the top. But if you tilt the camera sideways, you capture more of the surrounding area, and the building might look smaller or distorted. Similarly, a tilted photograph can help understand the context of an area but complicates measurements.

Low oblique photographs: An aerial photograph taken with an intentional tilt of camera axis 15° to 30° from the vertical axis is called as low oblique photograph. This kind of photographs is often used in reconnaissance surveys of the area as measurements can’t be made directly on such photographs.

High oblique photographs: A photograph in which the apparent horizon appears is termed as high oblique photograph. The apparent horizon is the line in which the Earth appears to meet the sky as visible from the aerial exposure station. The high oblique photographs are obtained when the camera axis is intentionally inclined about 60° from the vertical axis. Such photographs are useful for mapping the international boundary and extracting the details of the territory on other side as well as military applications.

Low oblique photographs provide a dynamic view of an area trending downward from the camera, making them helpful for reconnaissance but not for precise measurements. High oblique photographs, showing the horizon, provide context and can capture details such as borders but are less accurate for surveying due to their angled perspective. These types of images help planners understand the situation of an area but do not replace the need for precise vertical photography in technical surveys.

Think of a low oblique photograph like looking at a landscape while leaning over the edge of a hill—you're able to see a lot, but not all the details. A high oblique shot is like standing on a skyscraper and looking out across the city; you get a broader view but can miss some finer details below.

Terrestrial or close-range photographs are taken when the camera, either hand-held or tripod mounted, is located on or near the ground. For a detailed mapping of the feature/object, the camera is set up close to the object and photographs are taken. The output may be photographs or non-topographic products, like 3D models, measurements, or point clouds.

Terrestrial or close-range photographs capture objects from ground level, allowing for intricate details and measurements that aerial photographs cannot provide. These are especially useful in various applications, including industrial and engineering projects, where precise dimensions or features are critical. The type of output can vary from simple photographs to complex 3D models that give a fuller understanding of the space and objects within it.

Imagine you're drawing a picture of a statue. If you stand far away, you’ll capture its general shape, but standing right next to it enables you to see the nuances, textures, and details in the sculpture. Similarly, close-range photographs give better detail for specific targets compared to aerial shots.

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