Photogrammetry, a critical branch of surveying, involves the creation of planimetric or topographic maps by analyzing multiple photographs of a surveyed area. It is divided into two primary branches: terrestrial photogrammetry, which uses ground-based photography, and aerial photogrammetry, which employs cameras mounted on aircraft. This article delves into the principles, processes, and applications of aerial photography in photogrammetry.
Key Terms in Aerial Photography
To understand aerial photography, it is essential to grasp the foundational terms used:
- Exposure Station: The exact position in space where the aircraft is located when the camera captures a photograph.
- Flying Height: The height of the exposure station above the datum plane.
- Altitude: The vertical distance between the aircraft and the ground.
- Tilt: The angle caused by the camera’s rotation around the line of flight.
- Swing (Tip): The angle of horizontal rotation perpendicular to the line of flight.
- Principal Point: The intersection of the camera’s optical axis with the center of the photograph.
- Isocenter: A point on the photograph determined by the bisector of the tilt angle.
- Nadir Point: The point where a plumb line from the camera intersects the photograph.
- Homologous Points: Pairs of corresponding points on the ground and the photograph, enabling precise mapping.
The Procedure of Aerial Photography
The process of aerial photography involves several critical steps to ensure accurate map production:
1. Establishing Control Points
Control points are ground locations with known positions, serving as reference markers for interpreting photographs. These points should be easily identifiable and appropriately placed to encompass the photographed area. A minimum of three to four control points is essential per photograph, depending on the scale and cartographic requirements.
2. Flight Planning and Photography
Proper flight planning is crucial to ensure comprehensive coverage of the survey area. This includes determining the flight height, the number of photographs required, the area covered per photo, and the overlap between successive images. Key factors influencing flight planning include:
- Survey area size
- Camera focal length
- Desired photograph scale
- Aircraft ground speed
Mathematical formulas are employed to calculate parameters such as altitude and the number of required photographs, ensuring systematic coverage and minimizing gaps or redundancies.
3. Photo Interpretation and Stereoscopy
Photo interpretation, aided by stereoscopy, allows for the creation of three-dimensional models of the surveyed area. Instruments like stereoscopes enhance interpretation by magnifying details and enabling stereoscopic vision. Common stereoscope types include:
- Lens stereoscope
- Mirror stereoscope
- Scanning mirror stereoscope
- Zoom stereoscope
Accurate interpretation relies on specific characteristics such as:
- Shape: The outline or profile of objects
- Size: Differentiating large and small objects, scaled appropriately
- Pattern: Logical arrangements for easy recognition
- Shadow: Useful for identifying object profiles
- Texture: Determined by shape, size, and tone
- Site: The location of objects relative to their surroundings
4. Parallax and Measurement of Parallax
Parallax, the apparent displacement of an object when observed from different angles, helps determine an object’s third dimension. This measurement is achieved using floating marks or a parallax bar, allowing for precise three-dimensional mapping.
5. Construction of Map and Cartography
After capturing and interpreting the photographs, the final step involves creating a map using one of several methods:
- Radial Line Method: A graphical approach that minimizes errors by focusing on the principal point of the photograph.
- Slotted Template Method: A mechanical technique using enlarged templates of the photographs for accurate plotting. Although costlier, this method is rapid and precise.
- Stereoscopic Method: High-precision maps are created using stereoplotters, which render a three-dimensional spatial model of the area. This method is widely adopted by large mapping organizations for its accuracy and reliability.
Conclusion
Aerial photography plays a pivotal role in modern photogrammetry, offering a systematic approach to map production through accurate data collection, interpretation, and visualization. With advancements in stereoscopic technologies and mapping techniques, aerial photogrammetry continues to evolve as a cornerstone of surveying, contributing to urban planning, environmental monitoring, and infrastructure development.