Acquisition of 35-Mm Oblique Photographs for Stereoscopic Analysis and Measurement
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BARRYM. EVANS LARRYMATA Resource Technologies Corporation State College, PA 16801 Acquisition of 35-mm Oblique Photographs for Stereoscopic Analysis and Measurement A Step-by-step approach for obtaining stereoscopic 35-mm oblique aerial photographs and making precise measurements on them is presented. INTRODUCTION 1978; Grumstrump and Meyer, 1977; Hall and Walsh, 1977; Johnson and Mulvaney, 1978; Meyer VER THE LAST 10 to 15 years the use of small- 0 format aerial photographs for a variety of en- and Gerbig, 1974; Milfred and Kiefer, 1976; Miller, vironmental, engineering, and planning applica- 1974; Mintzer and Spragg, 1978; Nash et al., 1977; tions has become quite popular. In particular, hand- heir and Meyer, 1976; trandberg7 held 35-rnm cameras are being used more often by Rather, the emphasis of this Paper is on photo mi'- sion and measurement considerations basic to most scientists, researchers, and technicians who, for rea- sons related to cost, complexity, temporal con- applications of hand-held oblique 35-mm aerial pho- straints, project requirements, and/or personal pref- tographs. erences, have chosen not to contract an aerial survey firm to obtain standard 9- by 9-inch vertical TYPESOF OBLIQUEPHOTOGRAPHS photographs. Aerial photographs can be taken from two basic The intent of this article is to provide a concise perspective or viewing angles: vertical and oblique guide on how to obtain and make precise measure- (see Figure 1). In vertical photography, the camera ABSTRACT:Procedures are provided for obtaining stereoscopic 35-mm aerial pho- tographs with a hand-held camera, and for making precise measurements on them. Emphasis is on photo mission and measurement considerations basic to most ap- plications of oblique 35-mm aerial photography. A variety offlight mission factors including equipment, photo center line and flight line determination, aircraft height, film exposure, and time requirements are discussed. Also presented are procedures on how to construct perspective grids for making photographic mea- surements. ments from "hand-held" 35-mm stereoscopic aerial is pointed straight down toward the center of the photographs. It is directed at potential users who Earth (i.e., the camera's optical axis is held perpen- would like to take their own 35-mm stereoscopic dicular to the ground surface). The angle at which photographs, but are unfamiliar with the proper the optical axis is depressed below the "imaginary procedures for doing so. Specific applications of horizontal" is known as the "depression angle." As 35-mm photographs in such areas as forestry, shown in Figure 1, the camera is held at a depres- agronomy, wildlife management, vegetation anal- sion angle less than 90 degrees for oblique photog- ysis, geology, and environmental monitoring are not raphy. discussed in this paper because these applications Oblique photographs can be categorized as being have been addressed in sufficient detail elsewhere either high obliques or low obliques. Low oblique (e.g., Dalman, 1978; Evans et al., 1983; Fraga, photographs (Figure 2) do not show the horizon, and PHOTOGRAMMETRICENCINEERINC AND REMOTESENSING, 0099-1112/84/5011-1581$02.25/0 Vol. 50, No. 11, November 1984, pp. 1581-1590. O 1984 American Society of Photogra~nmetry VERTICAL LENS OF CAMERA 27 HIGH OBLIQUE AREA COVERED* SHAPE OF GROUND AREA / ! COVERED BY OBLIQUE PHOTOGRAPH FIG.1. Orientation of aerial camera for vertical and FIG.3. High oblique aerial photograph. oblique photography. This table shows the approximate scales at various are generally taken from a depression angle of ap- positions (top 113, center 113, and bottom 113) on a proximately 45 degrees. High oblique photographs, 35-mm photograph taken with a standard 50-mrn which include the horizon, are taken from a depres- lens at various angles from 1000 feet above terrain. sion angle less than 45 degrees (usually around 13 To find the scale when the photograph is taken at degrees). High oblique photographs, such as the other altitudes, multiply the values in the table by one shown in Figure 3, are often called "horizon the appropriate altitude factor (e.g., for an altitude trace" photographs. At a certain depression angle, of 2000 feet, multiply the table value by 2). Proce- the ground horizon will appear to coincide with the dures for determining scale and other photographic upper limit of the camera field of view. Any pho- measurements more accurately are discussed later tograph taken at a depression angle equal to or in the article. greater than this angle will be a low oblique pho- tograph. Conversely, any photograph taken at a depression angle less than this angle will be a high In planning an overflight to acquire 35-mm ste- oblique photograph. The exact depression angle reoscopic coverage with a hand-held camera, con- which separates high obliques from low obliques sideration must be given to a variety of factors in- will vary with the camera field of view, which is a cluding equipment, the length and location of photo function of focal length and film size. center lines and flight lines, aircraft height, film exposure, and mission time requirements. To ac- quire stereoscopic coverage of a given site, it is rec- The scale on an oblique aerial otograph ommended that at least a 60 percent overlap be- tween successive frames along a line of flight be changes from the bottom (foregroundyh to the top (background) of the photograph. Because low obliques are taken at a depression angle closer to vertical photographs than high obliques, low obliques show less scale change than high obliques. Table 1 is a list of scale values for estimating the CAMERA approximate scale of an oblique aerial photograph. DEPRESSION FRAME 35mm 2x2" ANGLE POSITION 50mm lens 80mm lens HIGH Top I"-- INFINITY I"= INFINITY OBLIQUES Center 1" = 2200ft 1" = 10008 Bottom 1"==1100R 1"- 500R LOW 30" Top 1"=1700n 1"=1800R OBLIQUES Center 1"=1000fl I"= 60011 Bonom 1" = 70Mt 1" = 400R 45" TOP 1" = 900~ tw= BOOR Center 1" - 700n I"=5oon Bottom 1" = 600n I"=300R 60° Too 1"- 700R 1" = 500R Center 1" - 600R 1" = 400R Bonom 1"- 500n lV=300n 75" Top 1" = 600R 1" - 400R Center 1" 500R 1" = 300R FIG.2. Low oblique aerial photograph Bonom 1" - 500ft 1"- 300n STEREOSCOPIC ANALYSIS AND MEASUREMENT used. Low obliques, because they are more like ver- the emulsion on films from different batches can tical photographs than high obliques, are recom- react differently to various lighting conditions. mended for stereo pairs. In either case, a forward Some type of filter is generally used when taking overlap greater than 60 percent will improve ste- aerial photographs. Film factors and filmlfilter ad- reoscopic viewing. justments are usually provided by the manufacturer As mentioned, the exact depression angle re- with each roll of film, and filter factors are also quired for high otlique photographs depends upon marked on the filters themselves. Filmlfilter com- the focal length of the camera and the size of the binations commonly used for both small- and large- film. For a given film size, the horizon will appear format aerial photography are listed in Table 2. on the photograph if the angle of depression is less than one-half the lens field of view (see Figure 1). DETERMINING PHOTO CENTER LINE AND FLIGHT A 35-mm camera having a 50-mm lens. for examwle, LINE PARAMETERS has a 27-degree field 07 view. The horizon wili be To ensure proper stereoscopic coverage of a site, at or near the top of a high oblique photograph taken the location of the center of each succeeding pho- with this type of camera system when it is oriented tograph should be planned before taking the pho- at a 13-degree angle below the horizontal plane of tographs. These succeeding locations will form a the aircraft. For a low oblique photograph, the line called the "photo center line," a line along the depression angle would be 14 degrees or greater. ground surface at which the camera will be pointed As mentioned earlier, most low oblique photographs as the aircraft proceeds along a parallel flight line. are taken at angles of approximately 45 degrees. The number of photo center lines required depends With most 35-mm cameras equipped with stan- upon the size of the area. The following five steps, dard 50-mm lenses, it is recommended that photo- along with the values given in Table 3, can be used graphs be taken from an altitude of 2000 feet or less to determine the area encompassed by each pho- to provide for adequate resolution of grobnd fea- tograph, the location of the photo center line, the tures. (In this case, for example, a flying height of number of photographs required for a photo center 2000 feet above terrain will ensure that most objects line, and the location of each photograph. For these greater than one foot in length are readily identi- steps, it is assumed that the camera is held in such fied.) Longer focal length lenses, such as 80- to 200- a way that the orientation of the resulting photo- mm telephoto lenses, will permit photographs to be graphs is similar to that of the photographs shown taken at higher altitudes. However, because they in Figures 2 and 3. are more sensitive to aircraft motion. the use of these lenses at higher altitudes may result in de- Delineate the boundaries of the particular study creased resolution. area on a suitable topographic map (preferably a uscs 1:24,000-scale topographic map). Determine whether the photographs will be high EQUIPMENT: CAMERAS, FILMS, AND FILTERS oblique or low oblique and the appropriate altitude A 35-mm camera equipped with a 50-mm lens is for the aircraft. Column E in Table 3 compares the "usable"area available by perspective and altitude.