Class 11 Geography Notes Chapter 6 (Introduction To Aerial Photographs) – Practical Work in Geography Book
Detailed Notes with MCQs of Chapter 6: Introduction to Aerial Photographs from your Practical Work in Geography book. This chapter is quite important, not just for your Class 11 understanding, but also as it forms a base for questions in various government competitive exams related to geography, remote sensing, and surveying. Pay close attention to the concepts, types, and applications.
Chapter 6: Introduction To Aerial Photographs - Detailed Notes
1. What are Aerial Photographs?
- An aerial photograph is any photograph taken from an airborne platform, such as an aircraft, helicopter, balloon, rocket, or more recently, drones (UAVs).
- They provide a bird's-eye view of the Earth's surface.
- They capture a momentary record of the features on the ground at the time of exposure.
2. Importance and Advantages:
- Synoptic View: Offers a broad overview of a large area in a single image.
- Time Freezing Ability: Captures the conditions of an area at a specific point in time, useful for change detection.
- Accessibility: Allows viewing and mapping of remote or inaccessible areas.
- Spatial Information: Provides detailed spatial data about the distribution and relationship of features.
- Cost-Effective (for large areas): Can be more economical than ground surveys for mapping extensive regions.
3. Types of Aerial Photographs (Based on Camera Axis Orientation):
- a) Vertical Photographs:
- Taken with the camera axis directed as vertically as possible towards the ground (typically tilt < 3°).
- Covers a relatively small, often near-rectangular area.
- Scale is relatively uniform across the photograph (though distortions exist due to relief).
- Most commonly used for mapping, surveying, and photogrammetry.
- b) Oblique Photographs:
- Taken with the camera axis intentionally tilted away from the vertical.
- i) Low Oblique:
- Tilt is significant, but the horizon is not visible in the photograph.
- Covers a trapezoidal area.
- Scale varies significantly across the photo (decreases towards the far end).
- Useful for reconnaissance and providing familiar, perspective views.
- ii) High Oblique:
- Tilt is large enough that the horizon is visible in the photograph.
- Covers a very large area, also trapezoidal in shape.
- Scale variation is extreme.
- Mainly used for reconnaissance and non-mapping purposes due to severe distortions.
4. Geometry of a Vertical Aerial Photograph:
- Exposure Station (L): The point in the air space where the camera lens was located at the moment of exposure.
- Focal Length (f): The distance from the camera lens (exposure station) to the focal plane (the film or sensor).
- Flying Height (H): The elevation of the exposure station above mean sea level.
- Altitude (h): The elevation of the ground surface above mean sea level.
- Height above Ground (H-h): The vertical distance from the exposure station to the ground point being imaged.
- Principal Point (PP): The geometric centre of the photograph, located by joining the fiducial marks (marks on the edges/corners of the photo). It represents the point where the optical axis of the camera intersects the film/sensor.
- Nadir Point (N) / Plumb Point: The point on the ground vertically beneath the exposure station. On a truly vertical photograph, the Nadir Point coincides with the Principal Point. If there's tilt, they separate.
- Isocentre (I): The point on the photograph that lies halfway between the Principal Point and the Nadir Point (relevant when tilt is present).
- Tilt: The angle between the camera axis and a true vertical line.
- Swing: The direction of tilt relative to the flight line.
5. Scale of a Vertical Aerial Photograph:
- Definition: The ratio between a distance measured on the photograph and the corresponding distance on the ground.
- Formula 1 (Using Focal Length and Flying Height):
- Scale (S) = Focal Length (f) / Flying Height above Ground (H-h)
S = f / (H - h)- Important: Ensure all units are consistent (e.g., convert focal length in mm to meters if H and h are in meters).
- Scale varies across the photo if the terrain elevation (h) changes. Higher ground appears larger (larger scale), lower ground appears smaller (smaller scale).
- Formula 2 (Using Photo Distance and Ground Distance):
- Scale (S) = Photo Distance (PD) / Ground Distance (GD)
S = PD / GD- Useful if you can identify two points on the photo whose actual ground distance is known (e.g., from a map).
- Formula 3 (Using Photo Distance and Map Distance):
- Scale (S) = (Photo Distance / Map Distance) x Map Scale
- Useful when comparing the photo to a map of a known scale.
- Representation: Scale is usually expressed as a Representative Fraction (RF), e.g., 1:10,000 or 1/10,000.
6. Overlap in Aerial Photographs:
- Aerial photographs are usually taken sequentially along a flight line with significant overlap between adjacent photos.
- End Lap (Forward Overlap): Overlap between successive photos along the same flight line. Typically 60% (±5%). This ensures complete coverage and allows for stereoscopic (3D) viewing.
- Side Lap (Lateral Overlap): Overlap between photos in adjacent parallel flight lines. Typically 30% (±5%). This ensures no gaps between flight strips and allows tying strips together.
7. Stereoscopic Vision (Stereovision):
- The ability to perceive depth and view objects in three dimensions (3D).
- Achieved by viewing the same object from two slightly different perspectives (like our two eyes).
- In aerial photography, the overlap area between two adjacent vertical photographs (a 'stereo pair') provides these two perspectives.
- Stereoscope: An optical instrument used to view a stereo pair of photographs, forcing each eye to look at only one photo, creating the illusion of a 3D model of the terrain.
- Pocket Stereoscope: Simple, portable, lenses mounted on legs. Views only a small part of the overlap area at once.
- Mirror Stereoscope: Uses mirrors and prisms, allowing the photos to be placed further apart. Views the entire overlap area at once, often with magnification options.
8. Elements of Aerial Photo Interpretation:
- The process of identifying objects and judging their significance from aerial photographs. Key elements include:
- Shape: The general form or outline of objects (e.g., rectangular buildings, circular tanks, meandering rivers).
- Size: The relative or absolute dimensions of objects (e.g., comparing building sizes, road widths). Scale is crucial here.
- Pattern: The spatial arrangement of objects (e.g., grid pattern of streets, radial drainage pattern, orchards).
- Tone/Colour: The relative brightness or colour of objects. Depends on reflectance properties (e.g., water bodies are dark, dry sand is bright). Varies with sun angle, processing.
- Texture: The frequency of tonal change, creating an impression of smoothness or roughness (e.g., smooth texture of grassland vs. coarse texture of forest canopy).
- Shadow: Provides information about the height and shape of objects, but can also obscure details in the shadow area. Useful for identifying vertical features.
- Site: The location of an object in relation to its surroundings or terrain features (e.g., power plant near water, hilltop settlements).
- Association: The occurrence of certain features in relation to others (e.g., school associated with playgrounds, railway station with tracks).
9. Uses of Aerial Photographs:
- Mapping: Creating topographic maps, thematic maps (land use, soil, vegetation).
- Resource Surveys: Geology (rock types, structures), Forestry (species identification, volume estimation), Agriculture (crop identification, health assessment), Soil surveys.
- Urban Planning: Monitoring urban growth, infrastructure planning, site selection.
- Military Intelligence: Reconnaissance, target identification, terrain analysis.
- Disaster Management: Damage assessment (floods, earthquakes, landslides), planning relief operations.
- Environmental Monitoring: Tracking deforestation, pollution monitoring, coastal erosion studies.
- Archaeology: Identifying ancient sites and features not easily visible on the ground.
- Engineering: Route planning (roads, pipelines), site investigation.
10. Limitations:
- Scale Variation: Especially in oblique photos and vertical photos over variable terrain.
- Distortions: Relief displacement (tall objects lean outwards from the centre), tilt distortion.
- Weather Dependent: Cloud cover, haze, and time of day affect image quality.
- Shadows: Can obscure important details.
- Requires Skill: Interpretation needs training and expertise.
- Cost: Acquisition (flight, camera) can be expensive, though decreasing with drones for smaller areas.
Multiple Choice Questions (MCQs):
-
Which type of aerial photograph is taken with the camera axis intentionally tilted, but the horizon is NOT visible?
a) Vertical Photograph
b) High Oblique Photograph
c) Low Oblique Photograph
d) Orthophotograph -
The geometric centre of a vertical aerial photograph, found by intersecting lines drawn between fiducial marks, is called the:
a) Nadir Point
b) Isocentre
c) Principal Point
d) Exposure Station -
What is the typical percentage of End Lap (Forward Overlap) required for stereoscopic viewing in aerial photography?
a) 10-20%
b) 25-35%
c) 55-65%
d) 80-90% -
If an aerial photograph has a focal length (f) of 150 mm and is taken from a flying height (H) of 3000 m above mean sea level over flat terrain at an elevation (h) of 500 m, what is the scale (S) of the photograph?
a) 1:16,667
b) 1:20,000
c) 1:23,333
d) 1:5,000 -
Which instrument is used to view a pair of overlapping aerial photographs to obtain a 3D perspective of the terrain?
a) Theodolite
b) Planimeter
c) Stereoscope
d) Clinometer -
In photo interpretation, the spatial arrangement of objects, such as a grid of streets or rows in an orchard, refers to which element?
a) Shape
b) Texture
c) Site
d) Pattern -
On a vertical aerial photograph taken over hilly terrain, areas at higher elevations will generally appear at a:
a) Smaller scale than lower areas
b) Larger scale than lower areas
c) Uniform scale across the photo
d) Scale unrelated to elevation -
Which type of aerial photograph covers the largest area but suffers from the most severe scale variations and distortions?
a) Vertical Photograph
b) Low Oblique Photograph
c) High Oblique Photograph
d) Terrestrial Photograph -
The element of photo interpretation that relates to the relative brightness or darkness (or colour) of objects is known as:
a) Texture
b) Shadow
c) Tone/Colour
d) Association -
The main purpose of Side Lap (Lateral Overlap) between adjacent flight lines is to:
a) Allow for 3D viewing
b) Ensure complete ground coverage without gaps between strips
c) Determine the exact flying height
d) Correct for atmospheric distortion
Answer Key for MCQs:
- c) Low Oblique Photograph
- c) Principal Point
- c) 55-65% (Typically around 60%)
- a) 1:16,667 (Calculation: S = f / (H-h) = 0.150 m / (3000 m - 500 m) = 0.150 / 2500 = 1 / 16666.67)
- c) Stereoscope
- d) Pattern
- b) Larger scale than lower areas
- c) High Oblique Photograph
- c) Tone/Colour
- b) Ensure complete ground coverage without gaps between strips
Study these notes thoroughly. Understanding the basics of aerial photographs, their types, geometry, scale, and interpretation is crucial. Let me know if any specific point needs further clarification.