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PhotogrammetryThe principles of photogrametry can be traced back to the works of Leonardo da Vinci in the late fifteenth century. Through his work in the fields of geometry, optics, mechanics, and geophysiscs, da Vinci graphically illustrated the concepts of aerodynamics and optical projection. It is not until the mid 1800s that the field of photogrammetry came into its own, particularly within the European community. The term photogrammetry was first coined in 1855 by the European geographer Kersten, and was in common use throughout Europe by 1900.The term, however, did not win wide acceptance in the United States until 1934.In that year, the American Society of Photogrammetry (ASP) was founded. Types of Photogrammetry The ASP currently consider photogrammetry to be the art, science, and technology of surveying and measuring by photographic and other energy-emitting processes. The procedure is extensively used in topographic mapping, and can be applied from the ground as well as in the air. Aerial photogrammetry, though, is the procedure most commonly used in mapping. It has all but eliminated the need for extensive field surveys. Ground, or terrestrial, photogrammetry is used only as a supplement and complement to the aerial process, or in areas with unusual physical characteristics. Like other professional disciplines, photogrammetry consists of a variety of specialty areas. For example, there are the two broad areas if terrestrial (ground) and aerial (air) photogrammetry. Other specialty areas of photogrammetry are defined as to the technology or energyemitting process used. Some of the types of photogrammetry used in the mapping and topographic field are discussed here.Radargrammetry is the use of radar as a measuring device to describe the physical characteristic of an area of the earths surface. X-ray photogrammetry employ X-ray to collect surveying data, and the use of motion pictures for surveying is called cinephotogrammetry.Hologrammetry is the use of holographs images projected by the use of coherent light systems such as lasers to measure surface characteristics. In monoscopic photogrammetry, single images or photographs are use or surveying purposes. Finally there is space or satellite photogrammetry, in which spacecraft or satellites are utilized as utilized as a platform for taking surface measurements. This type is also referred to as extraterrestrial photogrammetry.Aerial Photography: The aerial photograph is perhaps the most familiar product of the photogrammetry field. It application in the mapping and topographic dada collection process combines the use of scientific and artistic procedures and techniques. Aerial photogrammetry, then, is the use of photographic images taken from an airborne base for surveying purpose.The success and accuracy of an aerial photography mission is dependent upon a number of important factors , including the following:1. The use of correct photographic equipment, lenses, and supplies;2. Appropriate correct photographic materials, such as film, print and duplication materials, reflection print materials, and plates; 3. The photographic team (pilot, photographer, and film processor ) ;4. Weather conditions;5. The position of the sun when the photographs are taken.Classifications of Aerial Photography Aerial photography is a complex and dynamic profession, as is the photogram metric field in general. Aerial photography requires a vast array of equipment, technologies, and procedures. Therefore, to classify the various types of aerial photographs used in the mapping and topographic field, The ASP has identified four criteria: the orientation of the camera axis, the tens systems, the spectral range, and the mode of scanning these criteria are discussed in this section. Orientation of Camera Axis This refers to the angle at which the camera is in tentionally positioned at the time of the photograph. Ad shown in fig.23-1, these are two orientations of axis, used in aerial photography. These are the vertical orientation axis and the oblique orientation axis.With a vertical orientation axis, the photograph is taken when the camera is intentionally positioned as nearly vertical-or as close to 90 to the earths surface-as possible. Because of the earths curvature and aeronautical factors, it is impossible to obtain a true theoretical vertical picture (hence the qualifier “as nearly vertical as possible”). For practical mapping purposes, however, the “near vertical” is more than sufficient.The vertical orientation axis is the most common type of aerial photograph used in mapping. This type of photograph cam be easily converted into map drawings: it fives a sense of an ex fisting map or chart. The three major advantages of using the vertical orientation axis are that measurements can be easily taken off the photograph and transferred, surface objects and landmarks can be easily identifies, and the amount of hidden ground (areas not observable in the photograph) will be minimal.Oblique orientation axis photographs are images intentionally registered off vertical, and in the oblique, technically, this means that the camera lens angle is aimed between true horizontal and vertical. These photographs can be further subclassified into high oblique and low oblique. high oblique photographs pertain to the optical axis of the camera being at a high angle to the vertical, while low-oblique photographs have the optical axis of the camera at a low angle to the vertical.Oblique photographs have a limited application for mapping. They are only of practical use in situations requiring small-scale drawings. The two advantages of the oblique photographs are that they provide a stereoscopic or three-dimensional perception of the photographed area, and they can provide a larger area coverage than is provided by vertical photographs .there are, however, three significant disadvantages to using these photographs for map and topographic drawings. Fist, there is a loss of imagery and resolution. Second, the scale constantly reduces as one observes images a way from the camera. Third disadvantage is that highly skilled personnel and specialized equipment are needed to generate an accurate and useful product, resulting in higher cost.Lens Systems These are classified by the number and configuration of the camera lenses used for aerial photographs. Within this framework, all lens systems, and the multiple lens systems.The single-lens system is the most popular and frequently used system in aerial photography. It is applicable for either vertical or oblique photographs. The single-lens system usually employs a 153 millimeter (mm) focal length with a 228228 mm format , while using only one lens per shot. By comparison, super wide-angle lens cameras are seldom used. If used, however, it is for photographic missions over low-relief terrain.Multiple-lens systems consist of two or more lenses. These systems can be sub classified by the type of mounting used. The fist mounting is where the lenses are separately mounting in different cameras and the shutters are synchronized for simultaneous exposures. In each case, the lenses are fixed at different optical axes so that the photographs can be used on a stereoscope. The accuracy, and usefulness of photographs taken with a multiple-lens system are dependent upon the care and accuracy, of the calibration and retention of the axis angles.Another term used for the multiple-lens system is multispectral systems consist of two or more cameras, simultaneous exposures of an area of land, and the choice of different films and/ or filter combinations.Spectral Range This criterion refers to the entire range of light on the spectrum. The vast majority, of aerial photograph is limited to the range of the spectrum that can be observed by the human eye. This is called the optical range .in some situations, aerial photographs are used to record images within the infrared range.Spectral is measured in terms of micrometers (um). As shown in Fig.23-2, the optical range is from 0.4 um to 0.8um um, while the infrared range starts at 0.8 um. With the use of photographic film capable of recording optical range image, it is also possible to record some infrared images at the lower end of the scale (i.e., 0.8 um and 0.9 um). Photography further into the infrared range requires the use of special film and procedures. Because infrared is used to photograph temperature difference, it is also referred to as thermal photography. Mode of Scanning This is the last criterion used in classifying aerial photographs. There are three scanning modes used in aerial photography: the single frame, panoramic and continuous strip. In this sense, scanning refers to how the camera lens functions during the photographic process.Of the three scanning modes, the single-frame camera, the entire frame (format) is exposed through a lens that is fixed in relation to the focal plane. These cameras are the easiest to use and the least expensive. They also provide the most accurate data to transfer onto maps and topographic drawings. There are, however, two disadvantages to single frame photographs. First, these is a resolution as one moves away from the center of the photography.The problems associated with single frame photographs can be solved with the use of panoramic or continuous-strip cameras. Panoramic photography combines a high resolution at the center of the picture over the total angle scanned, which in some cases can be from horizon to horizon. Panoramic photography is accomplished by using narrow angles ,fast lens systems, by scanning the lens system though large angles across the flight path, and by using normal-width film and advancing it parallel to the scanning direction at ground speed.Continuous-strip photography eliminates the need for a conventional shutter system. The images are exposed on the film as the film passes continuously over a narrow slit in the focal plane of the lens at ground speed. Ideal for low-altitude reconnaissance photography, continuous-strip photographs provide a sharp picture scanned in one long strip.Ground control points The application of ground control points is a system used to ensure the accuracy of aerial photograph interpretation ground control points represent a series of fixed references which establish positions and elevations, ground control points are also used for correlation various-map features. Control is classified into four orders which represent the degree of accuracy and precision. The first order signifies the highest degree of precision or quality.The precision of the control system is based upon the use of the following seven types of controls.1. Basic control is determined in the field and is permanently marked, or monumental. It is based upon horizontal and vertical control of the third order or higher.2. Horizontal control is control is control relative to positions referenced to geographic parallels and meridians; that is, references with horizontal positions only (e.g. latitude, longitude, or plane coordinate axis). 3. Vertical control is usually made in reference to sea level and ground elevation.4. Astronomical control is control established by astronomical observation.5. Geodetic control are those control that account for curvature of the earths surface.6. Ground control is frequently associated with basic control and geodetic control. these are references that have been established by ground or field surveys.7. Supplemental control is made when additional, or subordinate, surveys are conducted to correlate the aerial photograph with geodetic control, thus ensuring positive identification of ground features and monuments.Applications of Aerial Photography To the casual observer, it may appear that aerial photography is only good for providing picture of the earths surface from an overhead perspective. Nothing could be further from the truth. There are a number of applications for aerial photography in the mapping and topographic field. These applications are useful to people in various professions, including engineers, geologist, geographers, planners, city administrators, agriculture specialists, lawyers, and economists. All applications, however, fall into four major categories. These are photo interpretation, stereo compilation, orthophotography, and analytical aero triangulation.Photointerpretation is a process used to analyze aerial photographs for the identification and measurement of surface objects and features. When used in an engineering study, this process usually emphasizes the relationships between surface objects and features and the project itself. Any person who is trained in photo interpretation can obtain a substantial amount of information. when applied to specific professions, this information can mean the difference between financial success and failure.Stereo compilation is the process of extracting information from a stereo model. A stereo model is a three-dimensional image or model that is formed when the projecting rays of an overlapping pair of photographs intersect. The three-dimensional model, then, is based upon the use of aerial photographs, taken at slightly different angles, of the same area of land.The purpose of a srereocompilation is to extract precisely located feature information from aerial photography. The stereo model created is characterized by vertical “stretching” or exaggeration that emphasizes the difference of contour features, and makes if easier to produce accurate map features. Hence, stereo compilation makes it possible to provide great accuracy of geodetic control data.Orthophotography is the use of an aerial photograph as the final map product, It represents an immensely useful substitute for small-scale maps. This process is widely used by geographers and by agencies such as the United States Geological Survey.The process of orthophotography eliminates the need for transferring all measurements and information from the aerial photograph to a drawing. The map produced by orthophotography is called a photomap. The process begins with a perspective photograph. The displacements of all images due to tilt and relief are removed; this is known as differential rectification. Ail information not shown on the photomap, such as scale, names, and elevations, are either drawn, scribed, or overprinted during reproduction.Analytical aero triangulation is a procedure required to supplement ground control points which are too far apart for photogram metric compilation needs. Analytical aero triangulation means that the coordinates of photo control points are produced by mathematical procedures rather than by analogue methods. This is a highly technical and analytic procedure used to establish precise photogram metric data. 摄影测量学最早提出摄影测量学概念可以追溯到15世纪后期Lenarda da vinci的著作中，但是他研究的领域主要集中在几何学、光学、物理学及地球物理学，因此他主要从空间动力学和光学投影方面阐述摄影测量的概念。直到19世纪摄影测量学才真正形成，尤其在欧洲学术界的重视下，摄影测量学逐渐兴起。“摄影测量学”这一术语在1855年被欧洲地理学家Kersten首次提出，在1900年被正式普遍的运用。尽管如此，直到1934年这个“外来词”被美国人广泛接受。并在那一年，美国摄影测量协会（ASP）正式成立。ASP通常认为摄影测量是集艺术、自然科学、测量技术，航空技术和其他一些相关技术的优化综合。该技术被广泛运用于测绘地形图上，而且无论是在地上还是在空中能够进行，航空测量是绘制地图的最常用的方法，它不需要大量的野外测量。摄影测量可以为地面测量过程提供补充和完善，特别是一些特殊地形地区。像其他的专业学科一样，摄影测量学包括许多的特殊区域，例如它包括陆地摄影测量和航空摄影测量两部分。摄影测量对一些特殊区域是通过技术和技术优化的过程来划分的。我们这里讨论的是一些绘图和地形方面常用的摄影测量类型。雷达图像测量学即利用雷达作为测量设备来描述地表某一区域的物理特征的科学,X射线摄影测量是利用X射线来收集测量数据，将动态图像用于测量的方法称为电信摄影测量。全息摄影测量师利用全息相片（利用诸如激光等相干的光学系统投影的影像来获取测量所需要的地表特征信息，在空间摄影测量里，单独的影像或相片被用于测量，最后，剩下的就是太空或者说是卫星摄影测量利用宇宙飞船或卫星做平台来进行地表测量的测量方法。这种类型也被称为地球外摄影测量。航空摄影测量：航摄相片可能是摄影测量领域中最熟悉的产品。通过科学的和艺术的程序和技术结合后，它被应用在绘图和地形图数据的采集过程。航空摄影测量即是利用获得的图像来达到测量的目的。航空摄影测量任务和精度的成功完成依赖于许多重要的方面来决定的，它包括如下方面：1. 正确的摄影仪器设备，镜头和补偿的使用；2. 恰当的选择和利用摄影材料，例如胶卷、洗印和复制材料、影像洗印材料以及感光板；3. 摄影测量队的组成（飞行员、摄影师、和胶卷处理员）；4. 天气环境情况；5. 当拍摄时太阳所处的位置状况；航空摄影测量的分类：航空摄影一般来说是在摄影领域是一项复杂而且动态的行业。航空摄影测量需要大量的仪器、技术和过程。因此，ASP任明了四个标准：视准轴的定向、透镜系统、光谱限度及扫描方式，用来分类各种各样的航摄相片面性以便用于绘图和地形领域。我们在这章里讨论这些标准。视准轴的定向 这就涉及到摄像机摄像时刻放置的角度，在航空摄影测量中实际上我们应用到了两个方面的轴线，这就是垂直方向和倾斜方向轴线。轴的方向垂直于地面时，摄像拍照时的光轴方向与地面方向尽可能处置或者是与地面成90，由于地球曲率和航空的因素，我们不可能得到真正的理论上的垂直相片（因此限定为“尽可能垂直”）尽管如此，对于一般的测绘工作，“尽可能垂直”已经足够了。在绘图工作中，垂直投影相片是航空摄像中最为常用的一种类型，这种类型的相片可以被轻易的转变到地图绘图中。它给我们提供了地图的基本轮廓。应用垂直投影相片的三个主要优点就是可以使测量工作免除了相片之间的转换和合补充，表面物质和地面目标可以轻易的识别出来，隐藏地貌（相片上不能观察到的区域）最少。倾斜相片是有意地将相片倾斜而不是将它水平定位，这意味着摄影机的棱镜角度处于水平和垂直之间，这些相片按倾斜程度进一步的可分为高倾斜和低倾斜。高倾斜照片属于摄影机的光学轴与垂直面成一个较大的角度，而低倾斜度照片即摄影机的光轴与垂直面成一个角度。倾斜投影相片对于地图投影具有一定的局限性，实际上它们仅仅用在小比例尺测图中。倾斜相片有两个优点：一是可以提供一个立体观测或三维立体感的区域。二是倾斜的摄影覆盖的面积比垂直摄影所提供的面积大，然而，用这些相片进行地形图的绘制有三个缺点，需要高技能的工作人员和专业的设备，只有那样才能操作出精确而有用的产品，同时也需要很高的费用。棱镜系统 它是按照运用于航空摄影测量棱镜的数量和结构分类的，全部棱镜系统可以分为两大类：单棱镜系统和多棱镜系统。当棱镜系统是在航空摄影测量中运用最多最频繁的一种系统。它适用于垂直摄影或倾斜摄影。单棱镜系统由一个焦距为153mm型号为228228的棱镜构成，然而一节只用一个棱镜，相比之下宽度很大的棱镜摄像机很少用到。如果用到的时候，也是被用到难以处理的地形的摄影。多棱镜系统包括两个和两个以上的多棱镜，这些系统要以按照它应用的配置类型分类，第一类配置就是棱镜安置在不同的摄像机中，但是快门控制着它同时曝光。第二类配置棱镜安装在同一摄像机体内，而且用同样的快门同步曝光。在每种情况下透镜被安装在不同的光学组合中，以便能够用立体视像来观察相片，用放大透镜系统得到的照片的精度可用程