Remote Sensing - A Primer of GIS: Fundamental Geographic and Cartographic Concepts - page 164

lution inhibits the ability to detect certain wavelengths and to distinguish
features. A finer spectral resolution sensor can be used to represent different
features based on the distinct wavelength patterns detected by the sensor.

Classification
Considering these three types of resolutions and other sensor and environ-
mental characteristics, an operator can make a choice about how to classify
the pixels from a scene using either supervised or unsupervised classifica-
tion. Supervised classification means that the operator participates in an inter-
active process that assigns pixels to categories. Unsupervised classification
occurs automatically without instructions from the operator.

Types of Sensors

This discussion of principles focuses on satellite-based remote sensing tech-
nology. This is only part of the available remote sensing technologies. The
same technologies used for satellites, or adaptations thereof, are often used
for remote sensing technologies used by airplanes, helicopters, and in some
case hand-held formats.

Photography
Photography is the most common remote sensing technology. In fact, some
of the first military remote sensing satellites used cameras with film in the
1960s. The film was dropped out of the satellite in a special heat-resistance
reentry container with a parachute and picked up in the air by an airplane.
Satellites still use cameras, but most of the images are now captured and
stored digitally. Satellite sensor technologies using photography are “pan-
chromatic” or sensitive to the full visible spectrum. The potential resolutions
of photographic images are very high, but may be limited by data acquisition
costs. Many governments and companies use aerial photography as a means
of data collection. Using ground reference points and calculations to remove
subtle changes in the airplane's movements, two aerial photographs taken
simultaneously can be used to make a stereoscopic image. They are a very
useful type of remote sensing because when viewed with some additional
equipment like a stereoscope, it is possible for most people to distinguish
heights and elevation changes. A single photographic image that also has the
effects

of elevation change removed (called planimetric) is called an

“orthophoto” and is georeferenced to a coordinate system.

Infrared
Usually when we refer to photographic remote sensing we mean recording
EMR in the visible wavelength spectrum, but this can be broadened to
include infrared. This can be done with the chemical applied to photo-