Chapter 6 Introduction To Aerial Photographs

We are familiar with photographs taken from a ground-level perspective, providing a horizontal view of objects similar to how we see them with our eyes (Figure 6.1 shows a terrestrial photograph). In contrast, an aerial photograph is taken from an elevated position in the air, such as from an aircraft or helicopter, using a specialized precision camera.

A photograph taken from the ground, showing a horizontal view of the town of Mussorrie.

Aerial photographs provide a "bird's eye view" or aerial perspective (Figure 6.2 shows an aerial view). They capture a different viewpoint of the Earth's surface features, looking down from above. These photographs are valuable tools in geography and other fields for studying the Earth's surface, especially for topographical mapping and interpretation.

An aerial view (bird's eye view) of Tehri town in Uttarakhand, showing features from above.

Glossary terms introduced in the text:

• Aerial Camera: A camera designed for use in aircraft.
• Aerial Film: High-sensitivity, stable film used in aerial cameras.
• Aerial Photography: Art, science, and technology of taking aerial photographs.
• Aerial Photograph: A photo taken from an airborne platform.
• Fiducial Marks: Index marks on a camera that appear on the film negative.
• Forward Overlap: Common area on successive photos in flight direction.
• Image Interpretation: Identifying objects in images and assessing their significance.
• Nadir Point: Point on the ground directly below the camera lens center.
• Principal Point: Point on the photo plane directly below the camera lens center.
• Principal Distance: Perpendicular distance from camera lens to photo plane (focal length).
• Perspective Centre: The point (camera lens) where light rays converge.
• Photogrammetry: Science of making measurements from photographs.

Uses Of Aerial Photographs

Aerial photographs serve two primary uses, which have led to the development of related scientific disciplines:

• Topographical Mapping and Interpretation: Aerial photographs are a key source of data for creating and updating topographical maps and for interpreting the features visible in the images.

These two uses correspond to the fields of Photogrammetry and Image Interpretation:

• Photogrammetry: This is the science and technology of making accurate and reliable measurements from photographs, especially aerial photographs. Principles of photogrammetry allow for precise determination of distances, areas, elevations, and volumes from overlapping photographs. This makes aerial photographs valuable data sources for creating and updating topographical maps, digital elevation models, and other geospatial products.
• Image Interpretation: This is the art and science of identifying objects, features, and patterns visible in photographs and images, and judging their relative significance. Image interpretation focuses on obtaining qualitative and quantitative information about land use/land cover, geological features, soil types, vegetation types, and changes over time by visually analyzing the characteristics of the images. Skilled interpreters can deduce significant information about the terrain and human activities from aerial photographs.

Advantages Of Aerial Photography

Aerial photographs offer several distinct advantages compared to observations made from the ground level:

• Improved Vantage Point: Provides a comprehensive bird's eye view over large areas. This allows for seeing features and their spatial relationships (how they are arranged relative to each other) in a way that is impossible from the ground.
• Time Freezing Ability: An aerial photograph captures a snapshot of the Earth's surface at a specific moment in time. This makes the photograph a historical record that can be used to study conditions and changes over time by comparing photographs taken at different dates.
• Broadened Sensitivity: The film used in aerial cameras can be sensitive to wavelengths of light beyond the visible spectrum (which is what human eyes perceive, 0.4 to 0.7 μm). Aerial film sensitivity typically ranges from 0.3 to 0.9 μm, including parts of the ultraviolet and near-infrared spectrum. Using filters or different film types allows capturing information that is not visible to the naked eye.
• Three-Dimensional Perspective: Aerial photographs are often taken with overlapping coverage between successive exposures in a flight line. When two overlapping photographs (called a stereo pair) are viewed simultaneously through a stereoscope, the overlapping area appears in three dimensions (stereoscopic effect). This provides a realistic perception of relief and topography.

Aerial photography in India began in 1920 and has been systematically developed by the Survey of India. Today, aerial photography is conducted for the entire country under the supervision of the Directorate of Air Survey, Survey of India, by authorized flying agencies including the Indian Air Force, Air Survey Company, and the National Remote Sensing Agency. Aerial photographs are available for educational purposes through designated channels.

Types Of Aerial Photographs

Aerial photographs can be classified based on various factors, including the angle of the camera axis, the scale of the photograph, the angular extent of coverage, and the type of film used. Here, we will look at classifications based on the position of the camera axis and the scale.

Types Of Aerial Photographs Based On The Position Of The Cameral Axis

This classification depends on the orientation of the camera's optical axis relative to the vertical at the moment of exposure:

• Vertical Photographs: Taken with the camera's optical axis held as vertically as possible (intended to be perpendicular to the ground plane). Ideally, the optical axis coincides with the vertical axis (the plumb line or direction of gravity) dropped from the camera lens center to the ground. Perfect verticality is difficult to achieve from a moving aircraft over a curved Earth. A photograph is considered truly vertical if the deviation of the optical axis from the vertical axis is within $\pm 3^\circ$ (Figure 6.3 and 6.4). Photographs with slightly more deviation but still close to vertical are called near-vertical photographs.

Diagram illustrating the ideal geometry of a vertical aerial photograph, where the camera axis is perpendicular to the ground.

An example of a vertical aerial photograph of Arneham, The Netherlands, showing features from directly above.

• Low Oblique Photographs: Taken with the camera axis intentionally tilted from the vertical axis by an angle of about $15^\circ$ to $30^\circ$ (Figures 6.5 and 6.6). The horizon does not appear in a low oblique photograph. These are often used for reconnaissance surveys as they cover a relatively larger area than vertical photographs from the same height, but the distortion is greater.

Diagram illustrating the geometry of a low oblique aerial photograph, where the camera axis is intentionally tilted, and the horizon is not visible.

An example of a low oblique aerial photograph of Arneham, The Netherlands, showing a tilted perspective without the horizon.

• High Oblique Photographs: Taken with the camera axis intentionally inclined significantly from the vertical axis, typically around $60^\circ$ (Figure 6.7). In high oblique photographs, the horizon is visible. Like low oblique photographs, they are useful for reconnaissance surveys, covering a very large area, but the distortion is the greatest, particularly in the foreground compared to the background.

Diagram illustrating the geometry of a high oblique aerial photograph, where the camera axis is tilted significantly, and the horizon is visible.

Table 6.1 summarizes the comparison between vertical and oblique photographs:

Types Of Aerial Photographs Based On Scale

Aerial photographs are also classified based on the ratio of distance on the photograph to the corresponding distance on the ground (scale). The scale depends on the focal length of the camera and the flying height.

• Large Scale Photographs: Have a scale of 1:15,000 or larger (e.g., 1:10,000, 1:5,000). They cover smaller areas but show features in greater detail (Figure 6.8 shows a 1:5,000 photo). Suitable for detailed mapping and analysis of small areas.

An example of a large scale aerial photograph (1:5000) showing detailed features of Arnehem.

• Medium Scale Photographs: Have a scale ranging between 1:15,000 and 1:30,000. These provide a balance between area coverage and detail (Figure 6.9 shows a 1:20,000 photo). Commonly used for topographical mapping at medium scales.

An example of a medium scale aerial photograph (1:20000) of Arnehem, showing a moderate level of detail over a wider area.

• Small Scale Photographs: Have a scale smaller than 1:30,000 (e.g., 1:40,000, 1:50,000, 1:100,000). They cover very large areas but show less detail (Figure 6.10 shows a 1:40,000 photo). Suitable for regional studies and mapping large geographical extents.

An example of a small scale aerial photograph (1:40000) of Arnehem, showing a broader coverage with less detail.

Geometry Of An Aerial Photograph

The geometry of an aerial photograph describes how points on the ground are projected onto the photographic film or sensor. Understanding this geometry is essential for making measurements and relating the photograph to a map. Aerial photographs are considered a type of central projection, while maps are typically treated as orthogonal projections.

Parallel Projection

In a parallel projection, the lines connecting points on the original object to their corresponding points on the projection plane are parallel to each other. They are not necessarily perpendicular to the plane. This type of projection does not account for the perspective effect where objects closer to the viewer appear larger.

Diagram showing parallel projection, where light rays projecting an object onto a plane are parallel to each other.

Orthogonal Projection

An orthogonal projection is a specific type of parallel projection where the projecting lines are perpendicular to the projection plane (Figure 6.12). Maps are considered to approximate orthogonal projections of the ground. In orthogonal projection, features are shown as if viewed from directly above, and their representation is not affected by their elevation difference on the ground. Distances, angles, and areas are accurately represented on the projection plane, independent of relief variations.

Diagram showing orthogonal projection, a type of parallel projection where rays are perpendicular to the projection plane, approximating map geometry.

Central Projection

In a central projection, all the lines connecting points on the original object to their corresponding points on the projection plane pass through a single common point called the perspective center (Figure 6.13). The image formed by a camera lens is a central projection, with the lens center acting as the perspective center.

Diagram showing central projection, where light rays projecting an object converge at a single point (perspective center).

An aerial photograph is a central projection. Light rays from the ground converge through the camera lens (the perspective center 'S' in Figure 6.14) and then diverge to form an image on the negative plane. This means that points on the photograph correspond to points on the ground along lines that all meet at the camera lens center.

Detailed diagram illustrating the geometry of a vertical aerial photograph, showing the camera lens (S), negative plane, positive plane, principal point (P), nadir point, and ground principal point (PG).

Key points in the geometry of a vertical aerial photograph (Figure 6.14):

• Principal Point (P): The point on the photo plane directly below the camera lens center (S) if a perpendicular is dropped from S to the photo plane.
• Nadir Point: The point on the photo plane where the plumb line (direction of gravity) from the camera lens center intersects the photo plane.
• Ground Principal Point (PG): The point on the ground plane directly below the principal point on the photograph.
• Ground Nadir Point: The point on the ground plane directly below the nadir point on the photograph.

In an *absolutely vertical* photograph, the camera axis is aligned with the plumb line, so the principal point and nadir point coincide. For an *oblique* or *tilted* photograph, the camera axis is tilted from the plumb line, and the principal and nadir points are separate. The angle between the camera axis and the plumb line is the tilt angle.

Other important geometric elements:

• Focal Length (f): The perpendicular distance from the camera lens center (S) to the photo negative plane (SP).
• Flying Height (H): The perpendicular distance from the camera lens center (S) to the ground plane (SPG), assuming a flat ground surface.

Difference Between A Map And An Aerial Photograph

Despite being representations of the Earth's surface, aerial photographs and maps have fundamental geometric differences, meaning a map cannot be directly traced from an aerial photograph without correction. The key differences (summarized in Table 6.2) arise from their underlying projections and perspectives:

Even vertical aerial photographs contain distortions caused by ground relief (tall features are displaced outwards from the center) and slight camera tilt. To be used as accurate map substitutes, aerial photographs need to be geometrically corrected to remove these distortions and transform them into a planimetric (map-like) view. Such corrected photographs are called orthophotos or orthophotographs. Orthophotos combine the geometric accuracy of a map with the visual detail of a photograph.

Scale Of Aerial Photograph

The concept of scale for an aerial photograph is similar to that of a map: it is the ratio of a distance measured on the photograph to the corresponding actual distance on the ground. Photo scale determines the level of detail visible and is crucial for making measurements of length, area, and potentially height from the photograph.

The scale of an aerial photograph ($S_p$) can be expressed as a ratio (e.g., 1:50,000) or in unit equivalents (e.g., 1 cm = 500 m). For vertical or near-vertical photographs of relatively flat terrain, the scale is considered approximately uniform across the photo. However, for oblique photographs or photographs of areas with significant relief, the scale varies from point to point.

There are different methods to compute the average scale of an aerial photograph, depending on the additional information available:

By Establishing Relationship Between Photo Distance And Ground Distance

If you can identify two distinct points on the aerial photograph and you know the actual ground distance between those same two points, you can calculate the photo scale directly using the ratio of the photo distance to the ground distance.

Answer:

By Establishing Relationship Between Photo Distance And Map Distance

If a reliable map of the same area is available, and you can identify two points on both the map and the aerial photograph, you can use the map's known scale to determine the photo scale. This method requires measuring the distance between the two points on both the map and the photograph and knowing the map's scale factor.

The relationship can be expressed as: $\frac{\text{Photo scale (}S_p\text{)}}{\text{Map scale (}S_m\text{)}} = \frac{\text{Photo distance (}D_p\text{)}}{\text{Map distance (}D_m\text{)}}$. Rearranging to find photo scale: $S_p = S_m \times \frac{D_p}{D_m}$.

If the map scale is given as an R.F. (e.g., $1:50,000$), the map scale factor ($msf$) is the denominator ($50,000$). The map scale ($S_m$) as a fraction is $1/msf$. So, $S_p = (1/msf) \times (D_p / D_m)$.

Answer:

By Establishing Relationship Between Focal Length (f) And Flying Height (H) Of The Aircraft

If information about ground distances or map scales is unavailable, but the focal length of the camera and the flying height of the aircraft above the ground are known, the photo scale can be calculated using the geometric relationship between these values (Figure 6.15 illustrates this relationship). This method is most accurate for truly vertical photographs of flat terrain.

Diagram showing the geometric relationship between the camera lens (S), photo plane, ground plane, focal length (f), flying height (H), photo distance (Dp), and ground distance (Dg) used to derive the aerial photograph scale formula.

From the geometry, similar triangles can be formed relating photo distance (Dp) to ground distance (Dg) and focal length (f) to flying height (H): $\frac{D_p}{D_g} = \frac{f}{H}$. The photo scale ($S_p$) is defined as $D_p : D_g$ or $D_p / D_g$.

Thus, the scale of an aerial photograph ($S_p$) can be calculated as the ratio of the focal length ($f$) to the flying height ($H$): $S_p = f : H$ or $S_p = f/H$. It is crucial that both $f$ and $H$ are expressed in the same units for this calculation.

Answer:

Marginal information printed on vertical aerial photographs often includes details like the flying height and focal length, along with photo specifications and fiducial marks (index marks on the camera that appear on the negative, used for locating the principal point).

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