Photogrammetry with Application GIS


تحميل عناصر التصوير الجوي وتطبيقاته Photogrammetry with Application GIS

تحميل كتاب عناصر التصوير الجوي وتطبيقاته Photogrammetry with Application GIS ، استكمالا لسلسلة تعلم GIS نقدم لكم في هذه المقالة كتاب عناصر التصوير الجوي وتطبيقات في نظم المعلومات الجغرافية Elements of Photogrammetry with Application in GIS ، من تأليف Paul R. Wolf, Ph.D ، Bon A. Dewitt, Ph.D .

An introduction of contents Photogrammetry with Application GIS

 .Photogrammetry and GIS

Geographic information systems, are widely used and hold a position of prominence in many fields. These computer-based systems enable storing, integrating, manipulating, analyzing, and displaying virtually any type of spatially related information about the environment. They are being used at all levels of government, and by businesses, private industry, and public utilities to assist in planning, design, management, and decision making. Chapter 20 in this book presents examples of applications in geographic information systems. It is important at this juncture, however, to mention the major role that photogrammetry plays in these systems.

An essential element of any GIS is a complex relational database. The information that comprises the database usually includes both natural and cultural features. Specific types of information, objects or layers, within the database may include political boundaries, individual property ownership, transportation networks, utilities, topography, hydrography, soil types, land use, vegetation types, wetlands, etc. To be of use in a GIS, however, all data must be spatially related; i.e., all the data must be in a common geographic frame of reference.

Photogrammetry is ideal for deriving much of this spatial information. As noted in the preceding section, topographic maps, digital elevation models, and digital orthophotos are examples of photogrammetric products which are now commonly employed in developing these spatially related layers of information. By employing photogrammetry the data can be compiled more economically than through ground surveying methods, and this can be achieved with comparable or even greater spatial accuracy. Furthermore, the data are compiled directly in digital format, and thus are compatible for direct entry into GIS databases.

The photogrammetric procedures involved in developing topographic maps, digital elevation models, digital orthophotos, and other products used in GISs are described in later chapters of this text.

Uses of Photogrammetry.

The earliest applications of photogrammetry were in topographic mapping, and today that use is still the most common of photogrammetric activities. At present, the USGS, the federal agency charged with mapping the United States, performs nearly 100 percent of its map compilation photogrammetrically. State departments of transportation also use photogrammetry almost exclusively in preparing their topographic maps. In addition, private engineering and surveying firms prepare many special-purpose topographic maps photogrammetrically. These maps vary in scale from large to small and are used in planning and designing highways, railroads, rapid transit systems, bridges, pipelines, aqueducts, transmission lines, hydroelectric dams, flood control structures, river and harbor improvements, urban renewal projects, etc. A huge quantity of topographic maps are prepared for use in providing spatial data for geographic information systems (see Sec. 1-7). 

Two newer photogrammetric products, orthophotos and digital elevation models (DEMs), are now often used in combination to replace traditional topographic maps. As described in Sec. 13-8, an orthophoto is an aerial photograph that has been modified so that its scale is uniform throughout. Thus orthophotos are equivalent to planimetric maps, but unlike planimetric maps which show features by means of lines and symbols, orthophotos show the actual images of features. For this reason they are more easily interpreted than planimetric maps, and hence are preferred by many users. A DEM, as discussed in Sec. 13-6, consists of an array of points in an area that have had their X, Y, and Z coordinates determined. Thus, they provide a numerical representation of the topography in the area, and contours, cross sections, profiles, etc., can be computed from them. Orthophotos and DEMs are both widely applied in all fields where maps are used, but because they are both in digital form, one of their most common applications, as discussed in Sec. 1-7 and Chap. 20, is their use in connection with GIS.

Photogrammetry has become an exceptionally valuable tool in land surveying. To mention just a few uses in the field, aerial photos can be used as rough base maps for relocating existing property boundaries. If the point of beginning or any corners can be located with respect to ground features that can be identified on the photo, the entire parcel can be plotted on the photo from the property description. All corners can then be located on the photo in relation to identifiable ground features which, when located in the field, greatly assist in finding the actual property corners. Aerial photos can also be used in planning ground surveys. Through stereoscopic viewing, the area can be studied in three dimensions. Access routes to remote areas can be identified, and surveying lines of least resistance through difficult terrain or forests can be found. The photogrammetrist can prepare a map of an area without actually setting foot on the ground—an advantage which circumvents problems of gaining access to private land for ground surveys.