Unit 8 – photogrammetry

1. Photogrammetry is the art and science that uses aerial or terrestrial photographs of the surface of the earth of natural and artificial objects on the earth for such purposes as the making of mosaics, the compilation of various kinds of maps and for other scientific purposes. Photogrammetry is the technology of obtaining reliable information about physical objects and the environment through processes of recording, measuring and interpreting photographic images and patterns of recorded radiant electromagnetic energy and other phenomena.

In a broad sense, photogrammetry involves the following: (1) photographing an object or obtaining the image of the objects, (2) measuring the positions of the object from the imagery and processed photographs, and (3) reducing the imagery to some useful form, such as a digital or topographic map.

Photogrammetry has been the major method of compilation of topographic maps based on measurements and information from aerial and space photographs over several decades. The process of photogrammetry for collecting the digital or analog topographic data consists of the following major steps: (1) planning and acquisition of aerial photography, (2) ground control, (3) aerotriangulation, (4) map compilation, and (5) editing.

Success of the photogrammetric method of data collection depends on the acquisition of proper photographs. Several factors are taken into account when planning for aerial photography. Some of the important factors are:

1. Purpose of photography

2. Scale of photography

3. Overlap between exposures

4. Allowable scale variation

5. Optical and mechanical characteristics of the camera

6. Film base and emulsion type used

7. Flying height used

8. Direction of orientation of topography

9. Relief displacement

There are two kinds of photogrammetry ground or terrestrial photogrammetry that utilizes photographs taken from ground stations with the resulting larger-scaled maps¹; aerial photogrammetry that uses photographs taken from an airplane. Aerial photographs are a unique tool for the analysis of the special characteristics of land. They provide most of the information for evaluating the natural resources, forestry, agriculture, soil, water resources; in engineering for selecting sites for dams, highways, canals, pipelines and airfields; and for urban development; disaster analysis, archeology, meteorology and many others. Commercial survey companies provide air photo acquisition, interpretation and mapping services. The methods of using aerial photographs to make accurate measurements developed into the field of photogrammetry. Photogrammetric methods are now used to produce all topographic and most forestry, geology, land use and soil maps.

2. Photogrammetry is as old as modern photographyand can be dated to the mid-nineteenth century. The first military use of aerial photography was in the American civil war. Photographs were successfully taken using kites and even pigeons as platforms. But the early 1860s photographs had successfully been taken fromcaptive balloons². However for aerial photography to become practical it required a navigable platform³. This platform was supplied by the piloted airplane. The use of aerial photography had a profound effect on military tactics.

3. Photogrammetry is the first remote sensingtechnology ever developed in which geometric properties about objects are determined from photographic images. In the simplest example, the distance between two points that lie on a plane parallel to the photographic image plane can be determined by measuring their distance on the image. A more sophisticated technique, called stereophotogrammetry, makes it possible to estimate the three-dimensional coordinatesof points on an object. These are determined by measurements made in two or more photographic images taken from different positions. Common points are identified on each image. A line of sight (or ray) can be constructed from the camera location to the point on the object. It is the intersection of these rays (triangulation) that determines the three-dimensional location of the point.

4. A recent important application of photogrammetry is called remote sensing⁴. What exactly is remote sensing? Remote sensing is the science of acquiring information about the Earth’s surface without actually being in contact with it. This is done by sensing and recording reflected or emitted energy and processing, analyzing and applying that information. The development of sensing as we know it today began with aerial photography. In much of remote sensing the process involves an interaction between incident radiation⁵ and the targets of interest. Note, however that remote sensing also involves the sensing of emitted energy and the use of non-imaging sensors⁶. Hospitals use imaging technology including CAT scans magnetic resonance imaging (3D imaging of soft tissue) and X-rays for examining our bodies.

Remote sensing process

This is exemplified by the use of imaging systems where the following elements are involved: A) energy source or illumination; B) radiation and the atmosphere; C) interaction with the target; D) recording of energy by the sensor; E) transmission, reception, and processing; F) interpretation and analysis; G) application. These seven elements comprise the remote sensing process from beginning to end.

5. Photogrammetry is used in different fields, such as topographic mapping,architecture,engineering,manufacturing,quality control⁷, policeinvestigation, andgeology, as well as byarchaeologiststo quickly produce plans of large or complex sites and bymeteorologistsas a way to determine the actual wind speed of atornadowhere objective weather data cannot be obtained.

Notes:

1. … larger-scaled maps… – крупно-масштабные карты

2. … captive balloons – аэростат

3. … a navigable platform – навигационная платформа

4. … remote sensing – дистанционное зондирование

5. … incident radiation … – падающая радиация

6. … non-imaging sensors – датчики, не воспроизводящие изображения

7. …quality control… – качество контроля выполняемых работ