Remote Sensing - Imaging Systems

Aerial Photography

Image 2 : SeaWiFS Imagery of Saharan Dust and the Canary Islands (SeaWiFS)

Aerial photography involves the use of film or digital cameras onboard airplanes or other aerial platforms. Typically this type of photography uses black-and-white (panchromatic), normal color, or infrared films. Just like any ordinary camera, the aerial camera records exposures or frames of photography one at a time [See Image 2].

Aerial photographs are either vertical or oblique. Vertical photos are taken of the area directly below the aircraft. Oblique photos are taken at an angle other than 90 to the ground and may or may not include the horizon.

Stereo pairs of photographs allow for 3-dimensional analysis of an area. These photos typically have 60% overlap which, when viewed through a stereoscope gives the appearance of three-dimensions. This is of great benefit to the photo interpreter.

An Orthophoto is an airphoto product in which the displacements caused by tilt of the camera and terrain relief have been removed. This creates the effect of the camera being directly over every point in the image simultaneously.

Image 3 : Infra-Red Line Scanner Image

Aerial imaging systems can be categorized in various ways. Each of these is not simply another method of getting an "aerial photograph". Rather, it is a way of obtaining various, differing kinds of information about the ground. For example, an Infra-Red Line Scanner (IRLS) can show, from the heat signature on the ground, where vehicles or aircraft were recently parked, but are now gone [See Image 3]. That is, this remote sensing device is showing us evidence of past events, which could not be photographed. The most common active aerial imaging system in use today is Radar; this stands for Radio Detection and Ranging [See Image 4]. The moving target indicator (MTI) display of a side-looking airborne radar (SLAR or SAR), will clearly highlight vehicles or other equipment which are moving any faster than 5 kilometers per hour even at night. Clearly, these are unique kinds of information, which could not be simply "photographed". The use of Infa-Red Line Scanner and Radar allows humanitarian demining managers evaluate their information needs about the terrain and how best to obtain these data layers.

Image 4: Radar Image of the Hudson Strait (RADARSAT)

Active versus Passive Systems

One of the most common ways of categorizing aerial imaging systems is to consider whether they are "active" or "passive". The standard aerial camera, called a "frame camera", usually operates as a passive system. This means that it does not send out any energy, but only receives energy. This energy, which is the reflected sunlight from the ground, is used to expose the film at the back of the camera. On the other hand, if a strobe system is mounted under the belly of the aircraft, the aerial camera becomes an active remote sensing system. It could acquire aerial imagery day or night. The majority of the aerial imaging systems in use today are passive. Exceptions to this include: all radar devices, and some of the newest Lidar sensors; "Lidar" stands for Light Detection and Ranging which involves transmitting laser light to the ground and recording the reflections. It can be used to create new and very detailed digital elevation data sets for the ground.






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