Remote Sensing - Imaging Systems
2 : SeaWiFS Imagery of Saharan Dust and the Canary Islands
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.
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
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.