Photogrammetry – Aerial Photography and Procedure in Surveying

What is Aerial Photogrammetry?
Aerial photogrammetry is the branch of surveying that deals with the production of maps such as planimetric or topographic maps by compiling a number of photographs taken in that area. This type of surveying is used to capture images of the ground from an aircraft flying at a certain altitude. Aerial photogrammetry is divided into two main branches: terrestrial photogrammetry and aerial photogrammetry.

Terrestrial Photogrammetry
Terrestrial photogrammetry is the process of taking photographs of the ground from a fixed position on the ground. This type of photogrammetry is typically used to capture images of an area in detail and to map the terrain.

Aerial Photogrammetry
Aerial photogrammetry is the process of taking photographs of the ground from an aircraft flying at a certain altitude. This type of photogrammetry is typically used to capture images of a larger area from a bird’s eye view. Aerial photography is used to create detailed maps of the land and to identify features such as roads, buildings, rivers, and other landmarks.

Terms Used in Aerial Photography

The terms or definitions used in aerial photography are:

Exposure Station

What is an Exposure Station?

An exposure station is a physical location where an aircraft is positioned in space to capture an image through a camera lens. It is typically used in aerial photography and videography to capture images from the sky. It is also used in surveying and mapping to capture images from a variety of angles.

Flying Height

What is Flying Height?

Flying Height is the height of an exposure station above the datum level. It is an important factor that determines the accuracy of a survey. The flying height of an exposure station must be carefully determined in order to ensure accurate survey results.

Altitude

Altitude: What is the Vertical Distance from an Aircraft to the Earth’s Surface?
Altitude is the vertical distance between an aircraft and the earth’s surface or ground. It is a measurement of how high an aircraft is in relation to the ground, measured in feet or metres. Altitude is important for many reasons, including safety, climate, and navigation. It is also used to measure the performance of an aircraft, such as its speed, fuel efficiency, and range. The higher the altitude, the more difficult it is for an aircraft to ascend and descend, since air pressure and temperature decrease with elevation.

Tilt

What is Tilt?

Tilt is a camera angle created by the rotation of an aerial camera around the line of flight (vertical axis). This angle is used to capture images from different perspectives, allowing for a variety of photographs to be taken from the sky.

Swing

What is Swing?

Swing is a type of angle produced when an aerial camera rotates around a horizontal axis that is perpendicular to the direction of flight. It is also known as “tip”.

Principal Point

What is a Principal Plane?
An aerial photograph taken by an aerial camera will have a point of intersection, referred to as a principal plane. This is the point at which the optical axis of the camera intersects with the photograph.

Isocenter

What is an Isocenter?

An isocenter is a point on a photograph at which a bisector of an angle of tilt intersects. This is located at a distance of f tan (t/2) from the angle of tilt.

What is a Bisector of an Angle of Tilt?

A bisector of an angle of tilt is a line that divides an angle of tilt into two equal parts. This line intersects at an isocenter, as described above.

Nadir point

What is a Nadir Point?

A nadir point is a point on an aerial photograph that is pierced by a plumb line when it is dropped from the front nodal point. This point can be used to provide an accurate point of reference on the photograph.

Homologous Points

What are Homologous Points?

Homologous points are a combination of ground points and photo points which demonstrate a connection between an original plane and a projecting plane. This combination is used to accurately map out an area and create an accurate representation of the ground and its features.

Uses of Homologous Points

Homologous points are used in a variety of mapping applications, such as in aerial photography, land surveying, and geographic information systems. By combining ground points and photo points, a more accurate representation of the land can be created and used for a variety of purposes, including land use planning and environmental studies.

Benefits of Homologous Points

The use of homologous points helps to create a more accurate representation of the ground and its features. This is especially useful when mapping large areas or areas with difficult terrain, as it enables better understanding of the land and its features. It also helps to create a more accurate representation of the area for use in land use planning and environmental studies.

Procedure of Aerial Photography

1. Establishing Control Points:
   Creating Control Points for Accurate Mapping
   Control points are essential for creating accurate maps and surveys. They provide reference points to measure the distance and direction between two points. By establishing control points, surveyors and cartographers can measure the elevation, angle, and coordinates of a particular area. They also help in determining the orientation of the map and the accuracy of the map.
2. Flight Planning and Photography:
   Developing Flight Plans for Aerial Photography
   Flight planning is a critical part of aerial photography. This involves the selection of the flight route, altitude, and speed of the aircraft. During the flight, the photographer must be aware of the terrain, weather conditions, and any potential hazards that could affect the quality of the photographs. The photographs are then used to create a map or survey of the area.
3. Photo Interpretation and Stereoscopy:
   Interpreting Aerial Photographs Using Stereoscopy
   Photo interpretation involves the analysis of aerial photographs to extract information about the area being photographed. Stereoscopy is a technique used to create a three-dimensional effect from two photographs of the same area taken from slightly different angles. This helps in extracting further detail from the photographs, such as the shapes and sizes of objects.
4. Parallax and Measurement of Parallax:
   Using Parallax to Measure Distances
   Parallax is the apparent displacement of an object when viewed from two different points. By measuring the parallax between two photographs of the same area, surveyors and cartographers can accurately measure the distance between two points. This is an important technique used in aerial photography and cartography to measure distances and create accurate maps.
5. Construction of Map and Cartography:
   Constructing Maps Using Cartography
   Cartography is the art and science of making maps. It involves the selection of the appropriate scale, projection, and symbols to represent the features of the area being mapped. Cartographers use aerial photographs, ground surveys, and other data sources to create maps. These maps can be used for various purposes, such as navigation, land planning, and resource management.

Establishing Control Points

What are Control Points?
Control points are points on the ground which have known relative positions. They are used to help observe and capture photographs, and are marked as boundaries.

How Many Control Points are Needed?
At least three or four control points are needed in one photograph. The exact number depends on the scale of the map, the flight control, and the cartographical method of mapping being used.

Why do Control Points Need to be Easily Identifiable?
Control points need to be easily identifiable on the photograph in order to ensure that the photograph is accurately captured.

Flight Planning and Photography

Flight Planning: Understanding Height, Coverage, and Intervals
Flight planning is a crucial part of aerial photography. It involves knowing the height that the aircraft should maintain during each shot, the area to be covered in each photograph, the number of photographs and strips to be taken, and the time interval between exposures. This planning depends on several factors, including the area to be surveyed, the focal length of the camera, the overlap, the scale of the photograph, and the ground speed of the aircraft in still air.

Formulae for Altitude and Area
The altitude of the aircraft can be calculated using the formula H = contour interval x C, where C varies from 500-1500. The area covered by one photograph can be computed using the formula (length x scale) x (width x scale).

Calculating Number of Photographs
The number of photographs required to cover a given area can be determined using the formulae N1 x N2, where N1 = L1/((1-Pl)Sl) + 1 and N2 = B1/((1-Pw)Sw) + 1. Here, N1 is the number of photographs in each strip, N2 is the number of strips, L1 is the length of the photograph (in the direction of flight), L2 is the width of the photograph (perpendicular to the direction of flight), Pl is the longitudinal overlap, and Pw is the side overlap. Sl and Sw represent the scales in length and width, respectively.

Photo Interpretation and Stereoscopy

What is Photo Interpretation?
Photo interpretation is the process of analyzing and interpreting aerial photographs of an area in order to create a map. It is done by using an instrument called a stereoscope, which consists of magnifying lenses that allow the user to view a three-dimensional model of the area. In order to achieve accuracy, control stations, elevations, and line lengths must be taken into account while interpreting the photographs.

Types of Stereoscopes Used for Photo Interpretation
There are four types of stereoscopes that are commonly used for photo interpretation: lens stereoscope, mirror stereoscope, scanning mirror stereoscope, and zoom stereoscope. Each type of stereoscope has its own advantages and disadvantages, and can be used for different types of aerial photographs.

Benefits of Photo Interpretation
The main benefit of photo interpretation is that it allows for the identification and interpretation of objects in photographs quickly and accurately. This can be invaluable for mapping, surveying, and other activities that require an understanding of the landscape. Additionally, photo interpretation can be used to identify changes that have taken place in an area over time, such as the construction of new buildings or changes in the terrain.

Shape

Lens and mirror stereoscopes are majorly used for photointerpretation. Shape is one of the key characteristics to be maintained for good photointerpretation. It must be ensured that the shape of the stereoscope is appropriate to capture the desired area.

Size

The size of the lens or mirror stereoscope should be chosen carefully depending upon the area or region to be captured. The size should be large enough to provide a complete view without any distortion.

Pattern

The pattern of the stereoscope should be such that it clearly outlines the objects in the image. This helps to easily identify the features and objects in the image.

Shadow

Shadow is an important characteristic for photointerpretation. The shadow should be properly adjusted to ensure that the objects in the image are clearly visible.

Texture

The texture of the stereoscope should be such that it does not distort the image. It must be ensured that the texture does not interfere with the clarity of the image.

Site

The site where the stereoscope is placed also plays an important role in photointerpretation. The location should be chosen carefully so that the stereoscope can capture the desired area without any obstruction.

Parallax and Measurement of Parallax

Understanding Parallax
Parallax is a displacement of an object in an aerial photograph when the point of observation is shifted to a different angle. In order to accurately measure the third dimension of an object, at least two points of observation are needed. There are two main methods used to measure parallax – floating marks and parallax bars.

Measuring Parallax with Floating Marks
Floating marks are a type of marker used to measure parallax in aerial photographs. The marks can be positioned in a way that allows for the measurement of the third dimension of an object from two different angles. By measuring the displacement of the mark from the two vantage points, the distance of the object from each viewpoint can be calculated.

Measuring Parallax with Parallax Bars
Parallax bars are a type of measuring device used to measure parallax in aerial photographs. The bars are positioned so that they can be aligned with the points of observation. By measuring the displacement of the bar from the two angles, the distance of the object from each viewpoint can be calculated.

Construction of Map and Cartography

Radial Line Method for Plotting Maps
Radial line method is a popular method used for plotting maps. It involves the use of a series of lines radiating out from a central point. Each line is marked with a scale and used as a guideline for plotting points of interest. This method is especially helpful when plotting points that are further away from the central point, as the lines can be used to accurately measure distances.

Slotted Template Method for Plotting Maps
Slotted template method is a technique for plotting maps that uses a template divided into a series of slots. Each slot is labeled with a coordinate, which is used to accurately plot points on the map. This method is particularly useful for plotting points that are close to each other, as the template provides a convenient way to quickly map out points of interest.

Stereoscopic Method for Plotting Maps
Stereoscopic method is a map plotting technique that uses two photographs of a scene taken from slightly different angles. The two photographs are then overlaid, with the differences between them used to create a three-dimensional representation of the scene. This method is useful for accurately plotting contours and elevations, as the three-dimensional representation makes it easier to accurately measure the heights of points of interest.

Radial Line Method

What is Radial Line Method?

Radial Line Method is a graphical technique used to plot maps from vertical photographs. This method incorporates perspective properties, such as fixing a principal point on the photograph and measuring the objects near this point to account for errors in tilt and ground relief. The three rays from known points can be used to accurately position points in two overlapping photographs.

Advantages of Radial Line Method

Radial Line Method is a convenient way to plot maps from vertical photographs and can be used to create planimetric maps. It also eliminates errors of tilt and ground relief by measuring objects near the principal point. Furthermore, the three rays from known points enables the accurate positioning of points in overlapping photographs.

Conclusion

Radial Line Method is an effective and efficient way to plot maps from vertical photographs. It incorporates perspective properties, such as a principal point and three rays from known points, to correct errors in tilt and ground relief and accurately map out points in overlapping photographs.

Slotted Template Method

Slotted Template Method: An Overview
Slotted template method is a mechanical method of plotting maps. This method involves the use of templates which are enlarged images of standard scale photographs. These templates can be made of transparent celluloid sheets, cardboards, etc. This method of plotting maps is known to be more accurate than other methods but is also more expensive.

Steps Involved in Slotted Template Method
The steps involved in the slotted template method are as follows:

1. Preparation of Templates: The first step in this method is to prepare the templates. This requires the use of a standard scale photograph and enlarging it to the required size.
2. Transfer of Principal Points: After preparing the templates, the next step is to transfer the principal points from the photograph onto the template.
3. Selection and Transfer of Minor Control Points: Following the transfer of principal points, the next step is to select and transfer the minor control points from the photograph onto the template.
4. Selection and Transfer of Lateral Control Points: Once the minor control points have been transferred, the lateral control points must also be selected and transferred from the photograph onto the template.
5. Centre Punching: Once the control points have been transferred, the process of centre punching must take place. This involves punching a hole in the exact centre of each control point.
6. Slotting: This involves cutting slots in the template so that the control points can be connected.
7. Assembly of Templates: Once the slots have been cut, the templates must be assembled in order to connect the control points.
8. Completion of Plotting: Finally, the plotting process is completed.

Equipment Used in Slotted Template Method
The equipment used in slotted template method is more expensive than the equipment used in other plotting methods, but it is also more accurate and faster. Some of the equipment used in this method includes transparent celluloid sheets, cardboards, photographs, centre punches, and templates.

Stereoscopic Method

What is Stereoscopic Plotting Method?

Stereoscopic Plotting Method is a technique used for preparing maps with high precision using an instrument called a stereo plotter or multiplex. This method allows the user to view the overlapped area in three dimensions, which helps to construct a spatial model. After the model is created, it is then orthographically projected as a map. This method is commonly used by large mapping organizations due to its accuracy.

Advantages of Stereoscopic Plotting Method

The Stereoscopic Plotting Method offers several advantages, including high precision and accuracy. The instrument used in this method, the stereo plotter or multiplex, helps to view the overlapped area in three dimensions which helps to create a detailed spatial model. Additionally, it can be used by large mapping organizations to ensure accuracy of the maps produced.

Conclusion

The Stereoscopic Plotting Method is a highly accurate and precise way of creating maps. The instrument used in this technique, the stereo plotter or multiplex, helps to view the overlapped area in three dimensions and creates a detailed spatial model. This method is commonly used by large mapping organizations for its accuracy and precision.