Close-Range Photogrammetry James R. Williamson 123 Photogrammetry Pearland, Texas, USA With the advent of commercial satellite imagery of our planet, our moon and the planets in our solar system have been photographed in detail. There are some who have forgotten that image collections made with hand-held cameras have a more immediate down to earth use and the images are not as expensive in collecting. Small format analytical and graphical photogrammetry has been in use for well over 75 years. In general, it is not unusual for the common person to think one of the first uses for small format imagery is that of photographs for intelligence purposes - military or civilian use, makes no difference. As exciting as that may be, one of the more dramatic uses of small format collections is in litigation, i.e., private property damage and vehicle personal injury accidents. From the time of the first photographic map it has been determined that photographic images of man-made objects are really two dimensional drawings of three dimensional objects. By applying the mathematics of projections, and perspective, these two dimensional drawings can be used to recreate the original three dimensional scene and thus the site of a vehicular accident can be reconstructed graphically to be used in the reconstruction of the accident events. Many years ago the standard commercial method of collecting the photography was with large format cameras of one kind/type/brand or another, and the purpose was that of cartography (map making). The view was somewhat obvious - straight down from an aircraft (platform) flying in regular patterns/flight-lines and at a constant altitude. Over the years not much has changed in photogrammetry, except the camera, the camera platform, the method of collection, processing, the analysis procedures and the time from start to finish. As the photogrammetry industry grew there were some exceptions and the majority of those were on platforms that could be considered off-world platforms within our solar system. Most of the major commercial use of photogrammetry was in the field of cartography. Believe it or not photogrammetry has always been a science to be reckoned with in our World’s scientific endeavors. There were photogrammetry projects that involved very sophisticated cameras, collections, delivery, and procedures to get the final product - even with handheld cameras (close-range photogrammetry) which is the arena of my life. A little over 40 years ago, when I started working with close-range photogrammetry, the time from collection to finished project could very well be a matter of weeks or months. After the great advancements in computers and the ingenious digital camera the start to finish time of various photogrammetry projects became shorter. In the advancement of close-range photogrammetry, computer programs, and digital cameras, there are now cameras and computer programs that allow the user to provide complete detailed dimensional information, from collection to finished project, in a matter of hours. It is the use of digital imagery in modern times, with an assist from having good usable geometry in the photography, that makes close-range photogrammetry a very useful tool in the analysis of accident photographs. Although the more adventurous photogrammetry projects seem to be those connected with off-world collections, which actually started from collections made from balloons. It is the high profile (pardon the pun) photogrammetry that has sparked the art and science of modern day computerized photogrammetry, however one should never forget the fundamentals that began with the analysis of one photograph at a time. There is a wealth of information in accident photography and to obtain that information requires knowledge and experience in the fundamentals of single-photo perspective. The fundamentals are the things that never seem to change and more so with close-range photogrammetry. By definition, close-range photogrammetry is meant to be when the distance (range) from the camera to the object of interest can be from several feet to about 1,000 feet (about 1 to 300 meters). In close-range graphical photogrammetry there are basically four types of perspective imagery: true or direct scale, one-point perspective, two-point perspective and three-point perspective. Actually, these procedures are well established and very useful in the analysis vehicle accident photographs. The first type, direct scale, is the perspective imagery captured with the camera image plane being parallel to a common plane of the object of interest such that the lines in the common plane are of a “true” scale. The direct scale image is usually of a constant plane, such as a wall, the side of a vehicle, or an object with no vanishing lines. The second type of perspective imagery is also where the image plane is parallel to an infinite number of planes leading up to and even beyond the object of interest - this is one-point perspective where all lines perpendicular to the image plane converge to a common point, usually the principal point on the principal ray of the camera. A quick visual reference for one-point perspective is the image of a hall where the image plane is parallel to the end wall of the hall. This same geometry can be obtained when collecting an image of a garage or a basketball court. The simple criteria for this perspective image is that the camera is held such that the principal ray of the lens is parallel to the horizontal reference plane (ground/floor) and the principal ray is perpendicular to the plane parallel to the image plane. It is possible with one-point perspective in a photograph/image to develop a scale model of horizontal and vertical planes shown in the imagery. The remaining two types of perspective imagery deal with the two and three dimensions of object space. Thus, starting again, with one-point perspective which is fairly easy for everyone to visualize. You (the camera) are standing at a camera position in the middle of an endless plane. Resting on this plane, in front of us (the camera), is a very large and tall rectangular building. This large building is twice as long as it is wide, with doors and windows along each floor. The door frames are of the same size and as are all of the window openings. What we see is what the camera “sees,” one side of the building. We collect a one-point perspective image and the camera (our right eye) is oriented such that the image plane is parallel to the longest side of the rectangular building. All vertical lines are perpendicular to all of the horizontal lines, thus all vertical lines are parallel and all horizontal lines are parallel. By knowing the width of any window opening, or the width of a door frame, or the distance between any two windows, and so on, we can determine the dimensions of all items in this image of the building. In effect we have a very detailed orthographic elevation view of the side of the building. Moving our camera position such that instead of looking directly into the long side of the building we are looking directly at the closest corner of the building with the short side receding to our right and the long side receding to our left. All of the vertical lines are parallel to each other and to the image plane. The horizontal lines of the short side of the building (to the right) will appear to converge to a common point (a vanishing point right) somewhere to our camera station right. That is, all of the horizontal lines on the short side of the building will converge to this vanishing point right. The same perspective geometry applies to the long side whose horizontal lines will converge left to a common point (a vanishing point left). Since we have not rotated our camera about the principal line of the camera lens (principal ray) and have kept the bottom image frame edge parallel to our flat plane, our two vanishing points will rest on a horizontal line through the middle of our image frame. This is a simple description of two-point perspective. To achieve the next perspective type we use the same camera location of the two-point camera location and simply rotate our camera up at an angle greater than six degrees. Now, none of the vertical and horizontal lines of our large building are parallel to the image plane. We still have the horizontal lines converging to the vanishing point right and vanishing point left, as before, plus we now have vertical lines from both sides of the building converging to a common point (vanishing point up). This is a simple description of three-point perspective. An example of the use of close-range photogrammetry is that of a tractor-trailer (18 wheeler) and two four wheeled vehicles. To describe the many details of this graphical analysis is beyond the scope of this article, however there were many single-photo perspective analyses completed to provide the data necessary to produce the two dimensional plot of the accident site for the use in the accident reconstruction. One of the photographs that were scanned and used in the analysis, along with a plot developed from a number of photographs are shown. This is a typical simple image to plot analysis, whereas there are many such cases involving more than 20 vehicles where the accident scene covers as much as a half a mile (more than 800 meters) in length. In the situation of the accident imagery shown it is the roadway lane lines and the overpass (bridge) that provide the lines defining this image as two-point perspective. With this information the stripes dividing the lanes were used to define rectangles of known dimensions. With this information the plot plan view was developed. In addition to this information the investigating police officers provided dimensional information about lane widths, stripe lengths, bridge column diameters and separation between columns, along with several other dimensions of the accident scene. This field data (dimensions) and the photographs taken by the police were required to generate this basic plot as shown. Those who have used close-range photogrammetry procedures such as the one shown in this article know full well that only the tip of the work effort has been shown/discussed. In the close-range photogrammetry analysis of accident photography it is the geometry contained in the photograph that needs to be identified to enable the photogrammetrist to select the correct method/procedure. It is quite possible for the photograph to contain perspective geometry for more than one perspective analysis, or multiple applications of the same type of perspective geometry. In this short description of the four types of perspective geometry, the purpose was to provide information to help identify what could be available. In future articles the application of the methods will be presented with graphic examples. Photographs/digital-images provide a wealth of knowledge concerning the man-made objects shown. They provide the best data base available and the resulting analysis, graphical or analytical, provide the proof. | ||
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