Concepts of Photogrammetry

 

As per the American Society for Photogrammetry and Remote Sensing (1987), photogrammetry is the art, science, and technology of obtaining reliable information about physical objects and the environment by recording, measuring and interpreting photographic images.

 

The word photogrammetry is derived from three Greek words: photos – meaning "light", gramma– meaning "drawing" and metrein meaning "measurement". In a simple way, we can define it as the science of making measurements from photographs.

 

As human eyes are naturally capable of seeing 3D around them in the real world, the stereophotogrammetry technique mimics the same by restoring the 3D positions and orientations of an overlapping image pair. With the filtering of anaglyph red-cyan glasses, 3D glasses or 3D screens, it allows us to see only one corresponding image out of a two-image pair for each eye, which lets our brain generate the 3D from the parallax.

 

 

In 1849, Aimé Laussedat (April 19, 1819 – March 18, 1907) was the first person to use terrestrial photographs for topographic map compilation. He is referred to as the "Father of Photogrammetry".

           Aimé Laussedat

 

In 1480, Leonardo da Vinci wrote " Perspective is nothing else than the seeing of an object behind a sheet of glass, smooth and quite transparent, on the surface of which all the things may be marked that are behind this glass."

 

The principles of perspective and projective geometry form the basis from which photogrammetric theory is developed. In 1858, he experimented with aerial photography supported by a string of kites but abandoned it a couple of years later.

 

In 1862, Laussedat's use of photography for mapping was officially accepted by the Science Academy in Madrid. He also tried balloon photography and is the first person to have captured an image from balloons, but deserted it because of the difficulty of obtaining a sufficient number of photographs to cover all of the areas from one air station. 

 

At the Paris Exposition in 1867, Laussedat exhibited the first known phototheodolite and his plan of Paris derived from his photographic surveys. These maps were comparable to earlier maps compiled from conventional field surveys which showed that this new technology could be used for mapping.

 

In 1855, Nadar (Gaspard Felix Tournachon) used a balloon at 80- meters to obtain the first aerial photograph.

 

The English meteorologist E. D. Archibald was among the first to take successful photographs from kites in 1882.

 

Pigeon photography is an aerial photography technique invented in 1907 by the German apothecary Julius Neubronner.  A homing pigeon was fitted with an aluminum breast harness to which a lightweight time-delayed miniature camera could be attached.

 

The development of the airplane by the Wright brothers in 1903 provided a better camera platform than the terrestrial camera. Wilbur Wright has been identified as the first person to obtain aerial imagery from an airplane.

 

 

 

 

The fundamental principle used by photogrammetry is triangulation. By taking photographs from at least two different locations, so-called “lines of sight” can be developed from each camera to points on the object. These lines of sight are mathematically intersected to produce the 3-dimensional coordinates of the points of interest.

 

 

You might observe that objects seem in different places if you close one of your eyes. One can do this exercise by observing the same object by closing one eye and then the another one. The displacement of an object caused by a change in the point of observation is called Parallax. Stereoscopic parallax is caused by taking photographs of the same object but from the different point of observation. Change in position of an image from one photo to the next is caused by aircraft’s motion.

 

Two important aspects of stereoscopic parallax:

 

The three-dimensional view which results when two overlapping photos (called a stereo pair), are viewed using a stereoscope. Each photograph of the stereo pair provides a slightly different view of the same area, which the brain combines and interprets as a 3-D view.

 

the photographs are usually taken by a series of a parallel passes called flight strip. Photographs are normally exposed in such a way that the area covered by each successive photograph along a flight strip duplicates or overlaps part of the coverage of the previous photograph.

 

This lapping along the flight strip is called end lap and the area of coverage common between two adjacent pairs of photographs called in a flight strip is called stereoscopic overlap (end lap). 

 

 

 

 

The camera is mounted in an aircraft and is usually pointed vertically towards the ground with the camera axis vertical or nearly so. Many photographs are taken with the overlapping concept. Later the processing of these photographs is done using stereo-plotter. These photos are also used in automated processing for Digital Elevation Model (DEM) creation.

 

 

 

Photographs are taken from a fixed, and usually known, position on or near the ground and with the camera axis horizontal or nearly so. The position and orientation of the camera are often measured directly at the time of exposure. The instrument used for exposing such photograph is called photo theodolite.

 

 

In this branch of photogrammetry, the satellites are used to take photographs. With the development of modern-day satellite, a global coverage of satellite imageries is possible in lesser time with a high-resolution data.

 

 

Under this process, the images are studied and identification is done for judging their significance with systematic and careful analysis.

 

 

To know the relative location of a point the precise measurement is done on the photographs. Planimetric mapping and topographical mapping are the best examples of this.

 

 

 

When a vertical aerial photograph is taken, the optical axis of the camera held in a vertical or nearly vertical position.

 

 

Sometime due to unavoidable aircraft tilt, the camera axis is unintentionally tilted from the vertical. Then the resulting photograph is called tilted photograph.  If tilt of the camera axis is less than 3 degree the photograph is called vertical. For tilt more than 3 degree, it is called tilted photograph. These photographs are further classified as below

 

   An oblique photograph in which the apparent horizon appears is termed as high oblique photograph.

 

   In this kind of photograph apparent horizon does not appear.

 

On the basis of the angular field of view of an aerial camera, the photographs can be classified as Normal angle, Wide angle and Super (ultra) wide angle photograph.

 

 

 

Plane table photogrammetry is an extension of the conventional plane table surveying [Konecny, 1985]. Each exposure station was determined by resection and plotted on the plane table. The exposed photos were oriented on the plane table and the directions to the different objects were transferred onto the map sheets.

 

 

Analog instruments are based on the concept of stereo metric vision. 2 photos are relatively oriented to produce a 3D model, where details and contours are drawn. 3D model is realized through projection of the two relatively oriented images. During the early part of the twentieth century, many of the figures in analog stereo plotter manufacturing began to develop their unique brand of instrument.

 

 

Analytical instruments (analytical plotters) are based on the digitization of the homologous coordinates on two photographs identified by stereoscopic vision. Computer software produce three dimensional coordinates of the point which are used for detail plotting and contour drawing in topographic maps.

 

 

In digital photogrammetry the identification of homologous points is eliminated. It is replaced by “correlation” based on computer software and the photogrammetric process is fully automated.

 

 

Aerial Triangulation(AT)  represents the mathematical process of establishing precise and accurate relationships between the individual image coordinate systems and a defined datum and projection (ground).

The main objective of aerial triangulation is to produce from ground control, sufficient points in the photogrammetric models to ensure that each model can be oriented accurately as required for stereo compilation in either orthophoto or line mapping.

There are mainly three stages of aerial triangulation:

 

 

 

 

 

 

One main difference between an aerial photo and an orthophoto map is that an aerial photo shows perspective. The photographer can tilt the camera up and down to give the photo's viewer a sense of scale and height. An orthophoto map corrects for any camera tilt and removes any sense of perspective. For example, in an aerial photo a photographer can make skyscrapers look tall, but in an orthophoto map, they all look the same size — you just see the building's roof from directly above.

 

 

While creating an orthophoto the effect of camera tilt and terrain relief is removed. Both these items distort the map's scale. By removing these effects, a map has a uniform scale.

 

 

Unlike an aerial photo a cartographer can overlay additional information on an orthophoto map. Orthophoto can be used as a background image in GIS and useful in creating base maps

 

 

An orthophoto gets used as either a map or in combination with GIS, an aerial photo gets used more when the photographer wants to show a different perspective of the Earth.