GIS Development Staff
An aerial photograph presents a “bird’s-eye view” of the features on the ground as seen from the air. They are different from ordinary photographs as they present an aerial view of the objects i.e. objects are portrayed from an overhead position. These photographs are also recorded in infra-red wavelengths of light. Although both maps and aerial photos present a “bird’s-eye” view of the earth, aerial photographs are NOT maps. Maps are orthogonal representations of the earth’s surface, meaning that they are directionally and geometrically accurate (at least within the limitations imposed by projecting a 3-dimensional object onto 2 dimensions). Aerial photos, on the other hand, display a high degree of radial distortion. That is, the topography is distorted, and until corrections are made for the distortion, measurements made from a photograph are not accurate. Nevertheless, aerial photography is a powerful tool for studying the earth’s environment.
The history of aerial photography dates back to 1858, when Nadar took photographs of the ground from a rising balloon. During World War I, aerial photography became recognised as an operational military reconnaissance tool and airplanes photographed enemy territories and movements. After the war, the potential use of this technology for civilian purposes was acknowledged and it began to be used for making maps, retrieving information regarding the topography, landform, vegetation and cultural features present on the surface of the earth. Since then, countries have been photographing their areas on a regular basis, both for military and civilian purposes. With the advance in technology, better instruments began to be used for obtaining good quality photographic data. Soon remote sensing satellites came into picture, which could also provide good quality data in a repetitive cycle and the data was available at much cheaper price than their counterparts. Because of this technology, the importance of aerial photography began to be questioned. The scenario is fast changing now. As most of the world has been mapped at scales of 1:50,000 and some even less, the demand for higher resolution photographs is increasing day by day for mapping areas at much smaller scales. This increase corresponds to the rapid increase in industrial and urban development. In this respect aerial photography is proving to be a boon as it can provide us with very high-resolution photographs with ease. Some aspects of aerial photography are covered below.
Catching a view
In aerial photography the photographs are taken in “runs” in the direction of flight in such a way that there is approximately 60% overlap between adjacent photographs. The plane is flown in such a way that there is 30% sidelap between adjacent runs. Aerial photographs are classified on the basis of orientation of the camera axis: Vertical Aerial Photography and Oblique Aerial Photography; on the basis of the lens system: Single lens photography, Trimetrogon (Three lens photography), Continuous strip photography and on the basis of films, filters and equipment: Black and white (panchromatic) photography, colour photography, colour infra-red photography etc. Vertical Aerial Photographs are taken with the camera axis pointing vertically down or nearly so at the time of exposure, whereas Oblique Aerial Photographs are taken with the camera axis inclined with respect to the vertical. In panchromatic photography, the film is exposed through minus blue or yellow colour permitting the recording of wavelengths from only parts of the visible range of the spectrum. This type of photography is most suited to general photo interpretation. Infra-red photography records wavelengths of only red and infra-red part of the visible spectrum and is best suited for forest studies and for water vegetation discrimination. Similarly, colour infra-red photography records spectral colours and infra-red in combination resulting in false colours. This type of photography is helpful in vegetation studies related with discrimination of plants and crops etc.
Geometry of Photography
Photogrammetry is a science in which the aerial photographs serve the basis for the making reliable measurements for preparation of maps, digital elevation models etc. A few features of photogrammetric operations are listed below:
- Helps in determination of horizontal ground distances and angles made from measurements on a photograph.
- Relief displacements and measurement of image parallax help in determination of object heights.
- Helps in preparation of maps in stereoplotters.
- Based on the above, preparation of digital elevation models and orthophotographs.
Mapping the photograph
Photogrammetric procedures are utilised for the preparation of maps. In the aerial photographs certain ground control points are identified. The precise geographic location of such points is then found out. In ideal case there should be at least three such marked points on ground in each aerial photograph. With the help of such three points, the locations of all the other points are determined. The points identified on the photograph are transferred to a base map of suitable scale and projection. The plotted points on the photograph are pin pricked and positioned in a plotting machine and adjusted for scale, tilt and other distortions until the points on the photograph and the base map coincide. Thus, an accurate model of the terrain is recreated. This whole procedure is carried out on a stereoplotting machine. The created model can be viewed stereoscopically and used to create a planimetric map having no relief distortions. Topographic contours can also be plotted on the map as the height of the features on the model can be easily determined and transferred onto the map.
Digital Methods in Aerial Photography
Although the photographs provide an excellent resolution, in today’s digital world, the analogue data is of little use until it is processed and converted into digital form. Digital data has many advantages as the data can be processed with ease, enhanced, analysed and presented in an easily understandable form through the application of computers. In photogrammetry, aerial photographs are utilised to form a three-dimensional model which when further processed can yield mapping data. This process involves the usage of computers and thus it is beneficial to store the raw data in the a digital format so that each image point (pixel) can be distinguished in terms of its position and brightness level. The most important advantage of data in a digital format is that it can be directly utilised for analysis in a GIS environment. There are two different approaches to obtain aerial photographs in digital format (a) scanning of existing aerial photograph and (b) direct digital acquisition.
Existing aerial photographs can be scanned using scanner. In this process the aerial photograph is scanned by either moving the charge coupled device (CCD) sensor head over the photo or moving the photo carriage over a static sensor. Once the image is scanned, the digital form of the image can be easily imported to a software where the data can be processed for GIS or remote sensing application. The recent trend in digital aerial photography is the use of digital cameras. Although still in its early stages, this technology will prove to be a major breakthrough in the field of aerial photography. Digital cameras use the CCD technology in which the patterns of blacks, greys and whites are electronically recorded as digital values, each representing the brightness of a specific point within the image. The main advantage of this technology includes its size, weight, dynamic range, optical sensitivity, stability, linearity and durability. Currently, the major disadvantages of this technology are its exorbitant price and the limited size of the CCD sensor array which can be used only for small format applications.
Using GPS to enhance Aerial Photography
Knowledge of exact locations of several features on the surface of the earth helps in registration of the photograph. Registration of a photograph means that a coordinate system can be overlaid on the photograph such that location of each feature on the photograph can be represented by a set of co-ordinate points. Prior to registration, the exact coordinates of the features that are seen on a photograph are determined by surveying techniques. Enough targets must be placed to ensure that a sufficient number of them (usually three or more) appear in each photograph. The role of GPS comes in the exact location of features on the ground. Recent advances in handheld Global Positioning System (GPS) receivers have made the task of precisely locating features fast, easy, and inexpensive.
Earlier the use of GPS receivers for registration of features on photograph was not advised as it would provide inaccurate results, correct to only 100 meters, which could be corrected upto 10 meters using correction algorithms. However, recently GPS receivers have been prepared which are accurate within a few centimetres. The data provided by the GPS requires post-processing on a computer before it can be used, but this once-difficult procedure has become much simpler now. So to obtain adequate ground control, we only have to select a number of naturally occurring features on the ground and acquire GPS positions for them. Once the photographs are registered then they “fit” within some standard coordinate system such as latitude/longitude or Universal Transverse Mercator (UTM).
The above method works very well when the photographed area is easily accessible. However, large areas like dense forests, steep hills etc. are not accessible. If these areas have to be covered by an aerial photo then the process of photo registration is not very accurate. It is only possible to approximately register such a photograph in the absence of traditional ground control. This is possible if the camera’s position and orientation (latitude, longitude, and direction of travel) are known at the time the photograph is taken. Modern GPS receivers can help in recording such information so that data having accuracy within 100 meters is saved and is transferred to a computer for correction such that the corrected data accurate to within 2 to 5 meters is obtained.
In order to register such photographs, the flight path along with the exact time the photograph is taken is recorded. The record of the exact time each photo is taken helps to find the corresponding inflight position. A system is developed such that each time a photo is taken, a GPS records position and time data.
After the flight, the record of the complete flight path along with the times each photo was taken is exported to a computer and subsequent processing and differential correction of the flight path data gives out data having an accuracy of 2-5 meters. Then the two datasets are merged to provide the position data for each photograph.
GIS Database with Aerial Photographs
Each point in a digital aerial photograph represents a geographic location with a characteristic value associated with it. This information forms the basis of the geographic data related with the point. A collection of such data for all the associated points in the aerial photograph results in a database that can be used for analysis of the photograph in a GIS environment. Registered photographs can be incorporated into a geographic information system (GIS) and used to create or update maps. Aerial Photographs which are not digital, require manual scanning of the photograph. Once the photograph is scanned, it results in a digital image. But this image does not represent any data as it is just a collection of pixels of varying values. To define the data present in the digital image, either photogrammetric operations are carried out or the photograph undergoes digitisation process. Both these operations provide the image a well-defined data structure wherein each point provides attribute as well as spatial data associated with the point. Once such a database forms, GIS operations can be undertaken for analysis work related to the area represented by the image.
Recent Trends in Aerial Photography
- Small Format Aerial Photography Except a very few areas, aerial photography of most of the parts of the earth have been carried. With the advent of satellite remote sensing, even these areas have been covered. With rapid urbanisation and industrialisation the need of the hour is very high resolution maps i.e. large scale maps. In this respect small format aerial photography is playing a key role. In this type of photography a small format (35mm) automatic camera is used. This type of camera allows for hands-free operation and is light enough to be lifted by a large kite. Critical camera settings like focus, shutter speed etc. are controlled by a microchip and light meter within the camera body. Simple acquisition of airphotos can be attained by enabling the camera’s timer function to take photograph after a few seconds.
- Low Altitude Remotely Piloted Vehicle (RPV) Aerial Photography The latest improvement in aerial photography is the utilisation of a remotely piloted vehicle (RPV) for aerial photography. Utilizing an unmanned, remotely piloted vehicle, offers cost effective alternatives to traditional aerial photography. Since RPV’s are not limited by the same restrictions as full-size aircraft, they can be operated at altitudes of less than 500 feet, even in residential areas where full-size aircraft are restricted to a minimum altitude of 1000 feet. As a result of the freedom associated with this method of aerial photography unique, low cost aerial photographs from previously impossible perspectives for a multitude of applications can be obtained.