Photogrammetric Concept

What is Photogrammetry?

The Photogrammetry is the art, science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images and patterns of Electromagnetic radiant imagery and other phenomena (American Society of Photogrammetry, 1980).

Photogrammetry can be used to measure and interpret information from hardcopy photographs or images. Sometimes the process of measuring information from photography and satellite imagery is considered metric photogrammetry, such as creating DEMs. Interpreting information from photography and imagery is considered interpretative photogrammetry, such
as identifying and discriminating between various tree types as represented on a photograph or image (Wolf, 1983).

Types of Photogrammetry

Photogrammetry can be classified two ways, based on camera location during photography;

• Aerial Photogrammetry
• Terrestrial Photogrammetry,

Aerial Photogrammetry

The Aerial photography, a technique of capture photographing to the Earth’s surface. Aerial photography is the taking of photographs helps to an aircraft, helicopters, balloons, blimps, dirigibles, rockets, parachutes, etc. In recent times use Drone to capture Aerial Photographs.

Terrestrial Photogrammetry

The Terrestrial photography also called Close-range Photogrammetry. Usually this type of photogrammetry is non-topographic, that is the output is not topographic products, like terrain models or topographic maps, but instead drawings, 3D models, measurements, or point clouds.

Image and Data Acquisition

During photographs or image collection, overlapping images are exposed along a direction of flight. Most photogrammetric applications involve the use of overlapping images. In using more than one image, the geometry associated with the camera/sensor, image, and ground can be defined to greater accuracies and precision.

The collection of imagery, each point in the flight path at which the camera exposes the film, or the sensor captures the imagery, is called an exposure station.

The image scale expresses the average ratio between a distance in the image and the same distance on the ground. It is computed as focal length divided by the flying height above the mean ground elevation. For example, with a flying height of 1000 m and a focal length of 20 cm, the image scale would be 1:5000

A strip of photographs consists of images captured along a flight line, normally with an overlap of 60%. All photos in the strip are assumed to be taken at approximately the same flying height and with a constant distance between exposure stations.

A block of photographs consists of a number of parallel strips, normally with a sidelap of 20-30%. Block triangulation techniques are used to transform all of the images in a block and ground points into a homologous coordinate system.

Interior Orientation of Photogrammetry

Interior orientation defines the internal geometry of a camera or sensor as it existed at the time of data capture. It is primarily used to transform the image pixel coordinate system or other image coordinate measurement system to the image space coordinate system.

The internal geometry of a camera is defined by specifying the following variables:

• Principal point
• Focal length
• Fiducial marks
• Lens distortion

Principal point

Principal point is mathematically defined as the intersection of the perpendicular line through the perspective center of the image plane.

Focal length

The length from the principal point to the perspective center is called the focal
length.

Fiducial marks

The image positions of the fiducial marks are measured on the image, and subsequently compared to the calibrated coordinates of each fiducial mark.

Lens distortion

Lens distortion deteriorates the positional accuracy of image points located on the image plane. Two types of radial lens distortion exist: Radial and Tangential lens distortion.

Exterior Orientation of Photogrammetry

Exterior orientation defines the position and angular orientation associated with an image. The variables defining the position and orientation of an image are referred to as the elements of exterior orientation. The elements of exterior orientation define the characteristics associated with an image at the time of exposure or capture.

The positional elements of exterior orientation include:

• Xo
• Yo
• Zo

They define the position of the perspective center (O) with respect to the ground space coordinate system (X, Y, and Z). Zo is commonly referred to as the height of the camera above sea level, which is commonly defined by a datum.

The angular or rotational elements of exterior orientation describe the relationship between the ground space coordinate system (X, Y, and Z) and the image space coordinate system (x, y, and z).

Three rotation angles are commonly used to define angular orientation:

• omega(Ω) – a rotation about the photographic x-axis
• phi(φ) – a rotation about the photographic y-axis
• kappa(κ) – a rotation about the photographic z-axis

Photogrammetry Software’s (Free)