Introduction To Aerial Photographs

Uses Of Aerial Photographs

Aerial photographs, images taken from aircraft or satellites, are invaluable tools in geography and various other fields. They provide a bird's-eye view of the Earth's surface, capturing details that are often missed or generalized on maps.

Key Uses:

• Mapping and Surveying: They form the basis for creating detailed maps, including topographical maps, land-use maps, and thematic maps. Photogrammetry, the science of making measurements from photographs, is used to derive accurate spatial data.
• Land Use and Land Cover Analysis: Used to identify and map different types of land cover (forests, water bodies, agricultural land, urban areas) and track changes over time.
• Resource Management: Aid in assessing and managing natural resources like forests (inventory, health), water bodies (extent, quality), agricultural crops (yield estimation), and mineral deposits.
• Urban and Regional Planning: Help in planning infrastructure, monitoring urban sprawl, identifying suitable sites for development, and managing urban growth.
• Environmental Monitoring: Used to study environmental changes such as deforestation, desertification, pollution, coastal erosion, and the impact of natural disasters (floods, landslides, earthquakes).
• Disaster Management: Provide critical information for assessing damage, planning rescue operations, and recovery efforts after disasters.
• Archaeology: Can reveal hidden archaeological sites, ancient settlements, and earthworks that are not visible from the ground.
• Geological Surveys: Help in identifying geological structures, faults, folds, and rock formations.
• Agriculture: Used for crop health monitoring, precision agriculture, soil analysis, and irrigation management.
• Forestry: Used for forest inventory, assessing tree health, detecting forest fires, and monitoring logging activities.
• Military and Intelligence: Critical for reconnaissance, surveillance, and strategic planning.

Advantages Of Aerial Photography

Aerial photography offers several significant advantages over traditional ground surveys and even some types of mapping, making it a powerful tool for spatial data acquisition and analysis.

Key Advantages:

• Comprehensive Overview: Provides a synoptic view of a large area, allowing for the identification of relationships between different features that might not be apparent from ground level.
• Detailed Information: Captures fine details of the Earth's surface, including smaller features that might be generalized or omitted on maps.
• Accuracy: When taken under controlled conditions and processed using photogrammetric techniques, aerial photographs can be highly accurate for measurements of distance, area, and elevation.
• Up-to-Date Information: Photographs can be taken as needed, providing current information about an area, which is crucial for monitoring dynamic changes (e.g., urban growth, disaster impact).
• Access to Inaccessible Areas: Allows for the study of remote or difficult-to-access terrain (e.g., dense forests, high mountains, disaster-stricken zones) without direct ground access.
• Cost-Effectiveness (in some cases): For large-area surveys, aerial photography can be more cost-effective than extensive ground surveys.
• Permanent Record: Provides a permanent visual record of the landscape at a specific point in time, useful for historical analysis and comparison.
• Cost-Effective Data Collection for Mapping: Serves as the primary source data for creating highly accurate maps.

Types Of Aerial Photographs

Aerial photographs are classified based on various criteria, primarily related to how they are taken and the scale at which they are captured.

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

This classification is based on the orientation of the camera relative to the Earth's surface at the moment of exposure.

1. Vertical Aerial Photographs (True Vertical):

• Description: Taken with the camera axis pointed directly downwards, perpendicular to the ground surface.
• Characteristics:
• Scale: Approximately uniform across the photograph, assuming flat terrain.
• Shape: Features appear in their natural shapes, as seen from directly above.
• Use: Ideal for mapping, measurement, and photogrammetric analysis.

2. Tilted Aerial Photographs:

• Description: Taken when the camera axis is tilted slightly from the vertical position.
• Characteristics:
• Scale Variation: Scale is uniform only along a line running through the principal point and parallel to the tilt axis. It varies across the photograph.
• Perspective: Features are seen from an oblique angle, giving a sense of depth.
• Use: Can be used for general reconnaissance and to provide a more intuitive view of the landscape.

3. High Oblique Aerial Photographs:

• Description: Taken with the camera axis tilted significantly, but the horizon is not visible in the photograph.
• Characteristics:
• Scale Variation: Significant variation in scale from foreground to background.
• Perspective: Provide a strong oblique view, showing landscape features in perspective.
• Use: Useful for reconnaissance, visualization, and descriptive purposes.

4. Low Oblique Aerial Photographs:

• Description: Taken with the camera axis tilted significantly, and the horizon IS visible in the photograph.
• Characteristics:
• Scale Variation: Very significant variation in scale across the photograph, with foreground features appearing much larger than background features.
• Perspective: Offers a wide oblique view, showing landscape features in perspective, including the horizon.
• Use: Primarily for visualization and identification of features from a familiar viewpoint.

Types Of Aerial Photographs Based On Scale

The scale of an aerial photograph is determined by the focal length of the camera and the altitude of the aircraft from which the photograph is taken.

1. Large-Scale Aerial Photographs:

• Scale: Typically larger than 1:10,000 (e.g., 1:5,000, 1:2,000).
• Characteristics: Taken from low altitudes. Show a great amount of detail, allowing for the identification of individual features like buildings, trees, and street furniture.
• Use: Detailed mapping, urban planning, cadastral surveys.

2. Medium-Scale Aerial Photographs:

• Scale: Typically between 1:10,000 and 1:30,000 (e.g., 1:15,000, 1:25,000).
• Characteristics: Provide a good balance between detail and area coverage. Useful for mapping regional features, land use studies, and general surveys.
• Use: General topographical mapping, regional planning, resource assessment.

3. Small-Scale Aerial Photographs:

• Scale: Typically smaller than 1:30,000 (e.g., 1:50,000, 1:100,000).
• Characteristics: Taken from high altitudes. Cover a large area but show less detail. Useful for broad overview studies, monitoring large-scale phenomena, and regional analysis.
• Use: Land cover mapping, monitoring large environmental changes, regional planning.

Geometry Of An Aerial Photograph

The geometry of an aerial photograph is different from that of a map due to the perspective nature of the photograph and the varying altitudes from which features are captured. Understanding this geometry is crucial for accurate measurements and mapping.

Parallel Projection

Description: In parallel projection, lines that are parallel in the object remain parallel in the projection. The direction of projection is the same for all points. This is often used conceptually in the construction of some map projections.

Relevance to Aerial Photos: While not strictly a parallel projection in its geometry, the idea of parallel rays is used in conceptualizing vertical photography where projection rays are parallel to the camera's optical axis.

Orthogonal Projection

Description: This is a type of parallel projection where the projection lines are perpendicular to the projection plane. It is used to create true-to-scale representations of objects.

Relevance to Aerial Photos: A true vertical aerial photograph, taken over flat terrain, approximates an orthogonal projection. The scale is relatively uniform, and features appear in their natural planimetric shape.

Central Projection

Description: In central projection, all projection lines pass through a single point called the center of projection (or the viewpoint). This is how perspective in 3D vision and photography works.

Relevance to Aerial Photos: All aerial photographs, especially oblique ones, are based on central projection. The camera lens acts as the center of projection. Features closer to the camera (foreground in oblique photos) appear larger and more detailed than those further away (background), leading to scale variations.

Consequences for Scale: Because of central projection, the scale of an aerial photograph is not uniform across the entire image, especially for oblique photographs or vertical photographs over uneven terrain. The scale is generally largest at the point closest to the center of projection and decreases as the distance from it increases.

Difference Between A Map And An Aerial Photograph

While both maps and aerial photographs represent geographical information, they differ significantly in their nature, purpose, and characteristics.

Complementary Tools: Maps and aerial photographs are often used together. Aerial photographs provide raw, detailed data that can be used to create or update maps, while maps provide a structured, scaled, and oriented framework for understanding the information in aerial photographs.

Scale Of Aerial Photograph

The scale of an aerial photograph relates the distance measured on the photograph to the corresponding distance on the ground. Unlike maps, the scale of an aerial photograph is often not uniform across the entire image due to the perspective nature of photography and variations in ground elevation.

By Establishing Relationship Between Photo Distance And Ground Distance

For Vertical Photographs over Flat Terrain:

In a perfectly vertical photograph taken over a flat area, the scale is approximately uniform. It can be calculated using the following formula:

Scale = Photo Distance / Ground Distance

Or, as a Representative Fraction (RF):

RF = $d/D$

where:

• $d$ is the distance between two points on the photograph.
• $D$ is the corresponding distance between the same two points on the ground.

To use this, you need to measure a known distance on the ground and then measure the corresponding distance on the photo. For example, if a 2 km stretch of road measures 4 cm on the photograph, the scale is: $4 \text{ cm} : 2 \text{ km}$ $4 \text{ cm} : 200,000 \text{ cm}$ RF = $4 : 200,000$ = $1 : 50,000$.

By Establishing Relationship Between Photo Distance And Map Distance

This method is used when you have an existing map of the area and an aerial photograph of the same area, and you want to find the scale of the photograph relative to the map's scale.

Formula:

Scale of Photo = (Scale of Map) * (Photo Distance / Map Distance)

Example: Suppose a map has a scale of 1:50,000. A specific feature measures 2 cm on the map and the corresponding feature measures 5 cm on an aerial photograph. You want to find the scale of the photograph.

• Scale of Map = 1:50,000
• Photo Distance = 5 cm
• Map Distance = 2 cm
• Scale of Photo = (1/50,000) * (5 cm / 2 cm)
• Scale of Photo = (1/50,000) * (2.5)
• Scale of Photo = 2.5 / 50,000
• Scale of Photo = 1 / (50,000 / 2.5)
• Scale of Photo = 1 / 20,000
• So, the scale of the aerial photograph is 1:20,000.

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

This is the most fundamental way to calculate the scale of a vertical aerial photograph, assuming flat terrain. The scale is directly proportional to the camera's focal length and inversely proportional to the aircraft's flying height above the ground.

Formula:

Scale (RF) = $f / H$

where:

• $f$ is the focal length of the camera lens (the distance from the lens to the film/sensor inside the camera). This is a constant for a given camera.
• $H$ is the flying height of the aircraft above the ground (Absolute Flying Height).

Example: If a photograph was taken from an aircraft flying at an altitude of 3,000 meters above the ground using a camera with a focal length of 150 mm.

• First, ensure units are consistent. Convert $H$ to millimeters:
• $H = 3,000 \text{ m} = 3,000 \times 1,000 \text{ mm} = 3,000,000 \text{ mm}$
• Calculate RF:
• RF = $f / H$ = $150 \text{ mm} / 3,000,000 \text{ mm}$
• RF = $150 / 3,000,000$
• RF = $1 / (3,000,000 / 150)$
• RF = $1 / 20,000$
• The scale of the aerial photograph is 1:20,000.

Note on Flying Height: It's crucial to use the height above ground level (AGL). If only height above mean sea level (AMSL) is given, and the terrain is not flat, adjustments are needed.