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.
