Environmental remote sensing devices can be mounted on a variety of platforms. Hand-held cameras can be used to acquire (usually) oblique photo images and hand-held radiometers can be used to measure the reflectance characteristics of a surface. If a wider field of view is required, the camera, radiometer or scanner can be mounted on a tower or cherry picker (trucks used by hydro department to repair electrical lines or replace burnt out street lamps). These platforms are commonly used to collect radiometer data representing the reflectance of different surfaces or land cover types. For mapping and analysis of spatil patterns, however, we usually rely on remote sensing devices mounted on low or high altitude aircraft or on satellites. In general, the higher the altitude of the platform, the smaller the scale of the resulting remote sensing imagery, although scale is also dependent on the configuration of the remote sensing device.
Photographic Camera/Film Systems
The term "photographic" refers to systems that use films coated with photo-sensitive silver halide emulsions to record an image. Silver halide crystals are structurally changed when exposed to light, producing a latent image on the film. When the exposed film is placed in a developer solution, the silver halide crystals turn to black metalic silver. The speed of this transformation depends upon the intensity of light striking the silver halide crystals at the instant of exposure: crystals exposed to intense light turn black very quickly while crystals exposed to less intense light turn black more slowly. The development process can be arrested by rinsing the film to remove all traces of the developer solution. The result is a continuous tone negative (bright spots appear black and vice versa) image..
Photographic camera/film systems have been used since the first decades of the 20th century to collect spatial data. Aerial photographs are the primary data input for production of topographic maps, although various types of "ground truth" information are needed to verify interpretation of the airphoto imagery and to ensure accurate transformation of the image data into map or GIS database format. Photographic films are sensitive to reflected EMR in wavelengths ranging from the mid-ultraviolet to the near-IR. The camera's entire field of view is recorded instaneously. The film detects and records the EMR reflected from surfaces within the field of view as a continuous tone image.
Digital cameras are a recent development. Like the traditional photgraphic camera, they use a lens to focus reflected EMR but use an array of EMR sensors rather than photographic film to record the image. Sensor arrays can have different spatial resolutions, ranging from 512 by 512 for a 35 mm equivalent image up to 2048 by 2048 for applications requiring finer spatial detail. The sensors detect reflected EMR for each wavelength for which they are calibrated. The resulting image is comprised of picture elements or pixels, each of which records a brightness value for the spatial field of view it detects. A 2048 by 2048 pixel image contains 4.2 million pixels!
A trimetregon camera is acutally an array of three cameras that take simultaneous overlapping images of the terrain. This type of camera is used to take airphotos in areas of mountainous terrain. The central camera in the array takes a vertical airphoto while the left and right cameras record oblique images of adjacent terrain. This type of camera is used to obtain images of steep valleys. By flying along the valleys and collecting overlapping images of the floor and sides of the valleys, trimetregon cameras can overcome the problems that are associated with normal parallel traverse airphoto coverage in areas with high local relief.
Electro-optical scanners used in both airborne and satellite remote sensing are somewhat similar to digital cameras in that they use an array of electronic sensors, in combination with mirror/lens optical devices to scan a scene and record an image. Each sensor in the array produces an electrical signal for each wavelength detected. The electrical signals can be recorded on magnetic tape. In the case of satellite sensors, the continuous electrical signals are usually converted into digital numbers representing up to 256 gray levels before being transmitted to Earth-based receiving stations. Optical-electrical scanners offer the potential of real time data acquistion since there is no delay while film is being developed and prints produced for distribution.
Active vs Passive Remote Sensors
Remote sensing devices can be classified according to whether they are active or passive devices. Passive remote sensing devices detect reflected EMR while active remove sensing devices emit a signal and detect the intensity of the signal reflected back off an object. A photographic camera used with available light and Landsat MSS, Landsat Thematic Mapper, or SPOT satellite imagary are examples of passive remote sensing systems. A photographic camera used with a flash attachment, radar and sonar are examples of active remote sensing systems.