PHOTOGRAMMETRY STEREOSCOPY FLIGHT PLANNING PHOTOGRAMMETRIC DEFINITIONS GROUND CONTROL INTRODUCTION

Transcription

1 PHOTOGRAMMETRY STEREOSCOPY FLIGHT PLANNING PHOTOGRAMMETRIC DEFINITIONS GROUND CONTROL INTRODUCTION Before aerial photography and photogrammetry became a reliable mapping tool, planimetric and topographic mapping were primarily the products of the surveyor. Map compilation consisted of control computations and the compilation and assembly of field observations and measurements. Prior to World War II, photogrammetry was recognized mostly as a European science. Other than military and government use, it was not readily practiced in the private or engineering sectors of the United states. Since the domestic introduction of large scale map compilation by photogrammetric methods, the surveyor's role has continually changed in parallel with the rapid improvement of photogrammetric instrumentation and techniques. Today, a project comprising as few as five to six acres becomes an economic consideration with photogrammetric methods. It is now generally accepted that photogrammetric mapping from aerial photographs is the best mapping procedure yet developed for both large and small scale projects. It is faster and less expensive than any other method and provides more complete and accurate detail, supported by evidence that is historically retained in the wealth of detail of the aerial photograph. As versatile and diversified as photogrammetry has become so has the surveyor's role in supplying the quality foundation on which good mapping is based. It is impossible to make a map from aerial photographs without the underlying data provided by field surveys. FIELD SURVEYS Field surveys are required to: 1. Provide the basic horizontal and vertical control needed to determine the scale, azimuths, and basis of data for the photogrammetric process, 2. Provide mapping of certain desert or plain areas, sandy beaches, or snow where photographs do not show the ground surface well.

2 3. Provide mapping of deep canyons or high obstructions that conceal the ground surface in the photographs. 4. Provide mapping of areas covered with dense conifer or tropical rain forests. 5. Provide as-built and sub-surface structure detail. 6. Provide mapping of boundary and land net features. 7. Secure information which cannot be obtained solely from the observation of aerial photographs, and. 8. Obtain precise supplemental map data such as cross-sections, field edit and map accuracy verification. SURVEYING ISSUES In the interest of the complimentary roles between the surveyor and photogrammetrist, this lesson is intended to address the most often asked questions about photogrammetric surveys and to provide assistance in effecting the most applicable map product in an economic and timely manner. There are many unique and diverse applications in the field of photogrammetry. Each project must be tailored to the specifics of the user; from small "ad-hoc" projects to comprehensive mapping programs of large regional areas. Although it is not the intent to expand on the theories of photogrammetry, some basic fundamentals must be considered if the surveyor and photogrammetrist are to formulate their most effective plan of operation. Questions the surveyor is most often confronted with in a photogrammetric project are: How to design the most expedient mapping plan for a given project? How many photographs (stereo models) will be required to cover a given project area? How many horizontal and vertical control points will be required? Will the use and accuracy of aerotriangulation satisfy the extension of photo control? What must the distribution of control points be? What size targets will be required? What map accuracy can be expected?

3 DEFINITION Photogrammetry is defined as the art, science, and technology of obtaining reliable information and measurements form aerial photography. SCALE By design, the successful execution of any photogrammetric project depends upon good quality photography. Aerial photographs taken with a frame camera are commonly classified as either vertical or oblique. Vertical photographs having a 9-inch by 9-inch format are the most common type taken for photogrammetric work. Basically, there are two types of aerial cameras presently used for photogrammetric mapping. They are: 1. The 6" focal length, wide angle lens, designed for universal mapping. 2. The 31/2" focal length, super-wide angle lens, designed to accomplish high altitude, reconnaissance-type mapping using a single engine aircraft. Of these available camera types, the 6-inch focal length lens provides the best compromise between stereo-photo geometrical strength, scale and ground coverage. This is the focal length that is in most common use throughout the world in photogrammetry today. The comparative measure of stereoscopic geometrical strength in stereo photography is generally expressed in terms of base to height ratio (B/H). Geometrical strength increases with increased B/H ratios; for the larger these ratios become, the greater the angles of intersection of corresponding light rays; thus increasing the stereoplotter pointing accuracy. For this reason, the 31/2-inch focal length lens provides better photography from a geometrical strength point of view, and in certain instances is used to increase the vertical accuracy in large scale mapping. In order for the photography to satisfactorily serve its intended purposes, the photographic flight mission must be carefully planned and faithfully executed according to the flight plan. A flight plan generally consists of two items: 1. A flight map which shows where the photos are to be taken. 2. Specifications which give the details on how to take the photos including requirements such as camera and film requirements, scale, flying height and a flight schedule which will coincide with completion of control pre-marking.

4 3. Figure 1 How an Aerial Photograph is taken The scale of vertical photograph is the ratio of photographic distance to the distance it represents on the ground. In as much as a single frame perspective photograph represents a plane system, it follows that the scale of a vertical photograph will change throughout the photograph with variations in the elevation of the ground. As flying height above the terrain increases, scale decreases; as ground elevation increases, scale increases. These are important points to remember when considering photographic scale. In photogrammetric work it is convenient to use an average photo scale which applies to the average terrain elevation in the project area. The required flying height above average terrain (AMT) and above mean sea level (ASL) can be readily calculated once the required photo scale has been selected and the camera focal length is known. Height above mean terrain (in feet) = Camera focal length in inches x scale of Photography ft/in = feet AMT Height above sea level (in feet) = Camera focal length in inches x scale of Photography ft/in + avg. terrain = feet ASL. Vertical aerial photographic coverage of an area is normally taken as a series of overlapping flight strips. To assure stereoscopic coverage of an area, a forward overlap of 55 to 60 percent, and a side lap of 30 percent is required.

5 Figure2 shows how stereoscopic overlaps are obtained Figure 3 Overlap Area and Stereo Model The neat-model for any overlapping stereo pair is the area between adjacent principal points and extending in the y direction to the middle of the side lap area. For 60 percent forward overlap and 30 percent side lap, the photographic area of the neat-model is 3.6 inches by 6.3 inches. In photogrammetric mapping, the actual area that is compiled, per stereo-pair, is generally limited to the neat-model area. Therefore, to roughly estimate the

6 number of stereo models that must be compiled in order to map a given area, the area to be covered must be divided by the area of each neat-model. Once the photo scale is selected, the area covered by a 9-inch square single vertical aerial photo may be readily calculated. PLOTTING INSTRUMENTS There are many types of photogrammetric plotting instruments in use today, each make and model having its own special features, advantages, disadvantages, and certain inherent precision. The three most commonly recognized designs are: 1. The double projection stereo-plotters. 2. The optical-train universal stereo-plotters. 3. The analytical stereo-plotters. 4. Softcopy stereo-plotters Although the design characteristics of the various stereo-plotting instruments may vary significantly, they all operate on the principal of stereovision. Stereovision occurs when two photographs are made of the same object from different positions in space, and then instrumented so that he right-hand photograph is seen by the right eye, and the left-hand photograph is seen by the left eye. The perspective intersection of light rays of corresponding images in the photographic pair is expressed as a three dimensional model and allows the observer the ability to view depth perception. The stereo-plotting instrument in turn, makes it possible for an observer to place the photographs in their proper geometric relationship with respect to their true position on the ground and to locate and plot planimetric features and contour lines by viewing the photographs in three dimension. More technically defined, model orientation of two stereo photographs in the plotting instrument is the re-construction of intersecting light rays of identical images from two separate photo stations in space. The intersection of light rays from corresponding image points is sometimes referred to as the picture plane of the stereo-model. For stereoscopic measurement of corresponding images in the stereo-plotter, the intersection of light rays in the instrument's optical-train is referenced by a black or white dot known as the floating mark. This floating mark is also seen stereoscopically in respect to the re-construction of image rays, and is viewed in direct relationship to the photographic image scale. The size of the floating mark will vary from.025 mm to over.080 mm, depending on the type and accuracy of the respective stereo-plotting instrument. The nomenclature of the stereoplotter and floating mark accuracy is important to the surveyor, since the size of aerial targets or photo identities must correspond closely in image size. In other words, the floating mark can "swim" within a target too large, or totally obliterate a target too small.

7 It is generally found that the greater the precision (C-Factor) in the plotting instrument, the higher the allowable flying height. This relationship is of great value due to the fact that increasing the flying height in turn increases the ground coverage per photograph, and therefore reduces the necessary ground control. Due to the fact that vertical accuracy is usually the limiting factor in the photogrammetric process, the flying height is often derived by the contour interval of the finished map. This relationship is expressed as a precision factor, referred to as the "C" factor of the photogrammetric equipment, and applies to 6-inch focal length aerial photography only. "C factor" = Flying Height above Mean Terrain /Contour Interval OR C = H/CI C-factors as given by the instrument manufacturers should be considered as applicable to the calibration specifications of specific equipment, and not to the inherent errors of the photogrammetric process. Contour plotting accuracy of course, depends not only on the stereo-plotting instrument, but also upon the characteristics of the aerial camera, the quality of the photography, method of photo control, operator pointing ability, and many other errors inherent to the photogrammetric process. All of these factors combined give what may be referred to as an "effective C-Factor." Accordingly, the instrument rated C-Factor should be reduced approximately 20 to 25 percent for large scale precision plotting. For example, an instrument with a C-Factor of 1500 should probably be considered to have an effective C- Factor more in the range of about 1200 for plotting large scale design mapping where grading plans or earth quantities are a consideration. Applying this equation, an instrument having a C-Factor of 1200, can theoretically plot a 1-foot contour interval from photography taken at 1200 feet above terrain. SOURCES OF AERIAL PHOTOGRAPHS Aerial photographs that are used for topographic mapping purposes are invariably flown specifically to satisfy the requirements of each individual project. Aerial photographs two or more years old usually have little value if the latest topographic and cultural details are to be accurately mapped. However, there are many engineering and scientific studies that actually require the use of dated (or historical) aerial photographs. The National Cartographic Information Center (NCIC) of the U.S. Geological Survey maintains a summary record of existing, in-progress, and planned aerial photography in the United States. Included in the record are aerial photographs acquired or to be acquired by agencies of the federal and state governments as well as some commercial firms. The record provides information on the locations of the photographic coverage, scales, types of camera, dates of photography, and addresses for making purchase inquiry. The summary record is continuously updated by NCIC and is an excellent source for information concerning the availability of existing aerial photography for a given location.

8 CHECKING CONTOURS FROM AERIAL PHOTOGRAPHY The standard method of checking contours produced by photogrammetric methods is to run check cross sections across the contour map and compare the check section with the profile from the contour map.