Introduction To Photogrammetry

Introduction-to-photogrammetry
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Introduction

Photogrammetry is the science and technology of obtaining spatial measurements and other geometrically reliable derived products from photographs.

 Photogrammetry is an engineering discipline and as such heavily influenced by developments in computer science and electronics. The ever increasing use of computers has had and will continue to have a great impact on photogrammetry. The discipline is as many others, in a constant state of change. This becomes especially evident in the shift from analog to analytical and digital methods.

 Mapping from aerial photographs can take on numerous forms and can employ either hardcopy or softcopy approaches. Traditionally, topographic maps have been produced from hardcopy stereo-pairs in a stereo-plotter device. A stereo-plotter is designed to transfer map information without distortions, from stereo photographs. A similar device can be used to transfer image information, with distortions removed, in the form of an Orthophoto.

 Orthophotos combine the geometric utility of a map with the extra “real-world image” information provided by a photograph. The process of creating an Orthophoto depends on the existence of a reliable DEM for the area being mapped. The DEM is usually prepared photogrammetrically as well. A digital photogrammetric workstation generally provide the integrated functionality for such tasks as generating: DEMs, digital Orthophotos, perspective views, and “fly-throughs” simulations, as well as the extraction of spatially referenced GIS data in two or three dimensions.

Data Acquisition

Data acquisition in photogrammetry is concerned with obtaining reliable information about the properties of surfaces and objects. This is accomplished without physical contact with the objects which is, in essence, the most obvious difference to surveying. The remotely received information can be grouped into four categories 

Geometric information involves the spatial position and the shape of objects. It is the most important information source in photogrammetry.

 Physical information refers to properties of electromagnetic radiation, e.g., radiant energy, wavelength, and polarization. 

Semantic information is related to the meaning of an image. It is usually obtained by interpreting the recorded data. 

Temporal information is related to the change of an object in time, usually obtained by comparing several images which were recorded at different times. 

The remotely sensed objects may range from planets to portions of the earth’s surface, to industrial parts, historical buildings or human bodies. The generic name for data acquisition devices is sensor, consisting of an optical and detector system. The sensor is mounted on a platform. The most typical sensors are cameras where photographic material serves as detectors. They are mounted on airplanes as the most common platforms.

Photogrammetric Products

 The photogrammetric products fall into three categories: photographic products, computational results, and maps.

Photographic Products

Photographic products are derivatives of single photographs or composites of overlapping photographs. During the time of exposure, a latent image is formed which is developed to a negative. At the same time diapositives and paper prints are produced. Enlargements may be quite useful for preliminary design or planning studies. A better approximation to a map is rectifications. A plane rectification involves just tipping and tilting the diapositive so that it will be parallel to the ground. If the ground has a relief, then the rectified photograph still has errors. Only a differentially rectified photograph, better known as orthophoto, is geometrically identical with a map. 

Composites are frequently used as a first base for general planning studies. Photomosaics are best known, but composites with orthophotos, called orthophoto maps are also used, especially now with the possibility to generate them with methods of digital photogrammetry.

Computational Results

Aerial triangulation is a very successful application of photogrammetry. It delivers 3-D positions of points, measured on photographs, in a ground control coordinate system, e.g., state plane coordinate system. Profiles and cross sections are typical products for highway design where earthwork quantities are computed. Inventory calculations of coal piles or mineral deposits are other examples which may require profile and cross section data. The most popular form for representing portions of the earth’s surface is the DEM (Digital Elevation Model). Here, elevations are measured at regularly spaced grid points.

Historical Background

The development of photogrammetry clearly depends on the general development of science and technology. It is interesting to note that the four major phases of photogrammetry are directly related to the technological inventions of photography, airplanes, computers and electronics. 

Photogrammetry had its beginning with the invention of photography by Daguerre and Niepce in 1839. The first generation, from the middle to the end of last century, was very much a pioneering and experimental phase with remarkable achievements in terrestrial and balloon .

The second generation, usually referred to as analog photogrammetry, is characterized by the invention of stereophotogrammetry by Pulfrich (1901). This paved the way for the construction of the first stereoplotter by Orel, in 1908. 

Airplanes and cameras became operational during the first world war. Between the two world wars, the main foundations of aerial survey techniques were built and they stand until today. Analog rectification and stereoplotting instruments, based on mechanical and optical technology, became widely available. Photogrammetry established itself as an efficient surveying and mapping method. 

The basic mathematical theory was known, but the amount of computation was prohibitive for numerical solutions and consequently all the efforts were aimed toward analog methods. Von Gruber is said to have called photogrammetry the art of avoiding computations. With the advent of the computer, the third generation has begun, under the motto of analytical photogrammetry. Schmid was one of the first photogrammetrists who had access to a computer. He developed the basis of analytical photogrammetry in the fifties, using matrix algebra. For the first time a serious attempt was made to employ adjustment theory to photogrammetric measurements. It still took several years before the first operational computer programs became available. Brown developed the first block adjustment program based on bundles in the late sixties, shortly beforeAckermann reported on a program with independent models as the underlying concept. 

As a result, the accuracy performance of aerial triangulation improved by a factor of ten. Apart from aerial triangulation, the analytical plotter is another major invention of the third generation. Again, we observe a time lag between invention and introduction to the photogrammetric practice. Helava invented the analytical plotter in the late fifties. 

However, the first instruments became only available in the seventies on a broad base. The fourth generation, digital photogrammetry, is rapidly emerging as a new discipline in photogrammetry. In contrast to all other phases, digital images are used instead of aerial photographs. With the availability of storage devices which permit rapid access to digital imagery, and special microprocessor chips, digital photogrammetry began in earnest only a few years ago. The field is still in its infancy and has not yet made its way into the photogrammetric practice.

References

[1] Multilingual Dictionary of Remote Sensing and Photogrammetry, ASPRS, 1983, p. 343. 

[2] Manual of Photogrammetry, ASPRS, 4th Ed., 1980, p. 1056. 

[3] Moffit, F.H. and E. Mikhail, 1980. Photogrammetry, 3rd Ed., Harper & Row Publishers, NY. 

[4] Wolf, P., 1980. Elements of Photogrammetry, McGraw Hill Book Co, NY. 

[5] Kraus, K., 1994. Photogrammetry, Verd. Dümmler Verlag, Bonn.