Photogrammetry-Study_Probe

Definitions of Photogrammetry

In very simple words, photogrammetry is a technique to obtain photographs to measure coordinates, or various surface features of the Earth three dimensionally. This technique involves the triangulation or aerial triangulation method as its fundamental principle.

Wolf and Dewitt, in 2000, defined photogrammetry as,
Photogrammetry is the art, science and technology of obtaining reliable information about physical objects and the environment through process of recording, measuring, and interpreting photographic images and patterns of recorder radiant electromagnetic energy and other phenomenon.

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In other words, photogrammetry is all about making accurate measurement of any three dimensional features on the Earth surface from two dimensional imagery. Thus photogrammetry helps to develop digital elevation models, topographic maps, orthophotographs, etc. Not only this, but it also deals with the measurement of height, distance, area, volume etc. of terrain features.


Brief History of Photogrammetry

Photogrammetry is almost contemporary to normal photography, that developed nearly 150 years ago, middle of 19th century. In the very beginning, photography was full based on analogue technique, which is commonly known as film photography, and this traditional optical-mechanical technique was in use for more than a century. Gradually it was developed by the advancement of modern technology. Computer-aided, mathematical algorithm based analytical techniques of photography was constructed. During mid 1970s, the modern digital photography was invented, and accordingly digital imagery and soft copy based photogrammetry appeared.

The idea of creating topographic maps by using photographs was first suggested by an eminent French surveyor Dominique F. Arago in the year 1840.

Origin of the word ‘Photogrammetry’

If the term is broken down into syllables, then it is seen that the term consists of three words– ‘photo’ that means ‘light’, ‘gram’ means ‘drawing’, and ‘metry’ means ‘measurement’.

Prussian architect Albrecht Meydenbaur coined the term ‘photogrammetry’, in his article ‘Die Photometrographie’ in the year 1867.


Types of Photogrammetry

Photogrammetry can be classified into various types based on different aspects. Considering the location of camera, it is basically classified into two broad groupings, viz., Aerial photogrammetry, and Terrestrial photogrammetry.

1. Aerial Photogrammetry

Aerial_Photogrammetry
An illustration of Aerial Photogrammetry

For aerial photogrammetry, a camera onboard an aircraft, points out towards the Earth surface and captures multiple overlapping imagery while passing through the aircraft’s flying path. The captured imagery then processed through traditional stereo-plotter or modern computerized systems. But nowadays, UAV or Unmanned Aerial Vehicle, commonly known as Drone(s), are also being used widely for aerial photogrammetry.

2. Terrestrial Photogrammetry

Terrestrial_Photogrammetry
An illustration of Terrestrial Photogrammetry

Terrestrial photogrammetry is quite opposite to the aerial photogrammetry. In this technique, the camera is placed on the ground, whether by a tripod or hand held, rather than an airborne platform.

But terrestrial photogrammetry technique does not yield a topographic outcome, such as topographic maps, models of terrain features, etc., instead normal measures, drawings or point clouds, etc. This type of photogrammetry is also called ‘Close-range Photogrammetry’ and ‘Image-based Modeling’.


Sometimes in addition to the aforementioned two types, photogrammetry can be further classified as ‘Space Photogrammetry’. In this case, the camera might be located in the ground or any airborne platform (e.g., spacecrafts) but pointed out towards another celestial object, e.g., planets, moon(s), meteorite, etc.


Based on the analysis methods, photogrammetry is also categorised into two types, viz., Interpretative Photogrammetry, and Metric Photogrammetry.


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1. Interpretative Photogrammetry

Interpretative photogrammetry, as the term suggests, involves an interpretation method which helps to delineate an object by analysing captured imagery precisely. The imagery are taken from satellites and help in the process of remote sensing.

2. Metric Photogrammetry

Metric refers to a standard or system of measurement. Thenceforth, metric photogrammetry denotes the technique of measuring quantitative phenomenon on a photograph and to delineate relative location of specific points.

Metric photogrammetry is mainly used in the fields of planimetric mapping and topographic mapping. Planimetric Mapping involves the drawing out the critical features’ data from an aerial photograph. Whereas the Topographic Mapping involves drawing and measurement of distance, area, volume, etc. of topographic features.


Merits of Photogrammetry

  • Aerial photography based photogrammetry covers a large spatial extent of Earth surface.
  • Multiple overlapped imagery with uniform exposure interval are taken in photogrammetry techniques which helps us to create three dimensional views of the captured photographs, and also helps us to study surface features in a better way.
  • Aerial imagery can be preserved as historical evidence for any terrain feature.
  • The sensor involved in photogrammetry techniques can detect and record a broader range of electromagnetic spectrum (from 0.3µm. to 0.9 µm.) which is beyond the human eyes’ sensitivity, that is 0.4 µm. to 0.7 µm. Hence it illustrates a great detail.


Applications of Photogrammetry

Photogrammetry has a wide application in various fields of geography. Some of them are mentioned in the following.
  • As mentioned before, photogrammetry can be applied to map surface topography of Earth, as well as for planimetric mapping.
  • Techniques of photogrammetry are applied to develop a composite picture of Earth surface.
  • Photogrammetry have a special significance in its application to soil classification. Classification of soil thenceforth helps to the advancement of agriculture, delineating areas for forestry, and others.
  • Geological interpretation of an area are facilitated by the involvement of photogrammetry.
  • It is also applicable for the assessment of urban sprawl monitoring, crop damage due to flooding, and many more.



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