Remote sensing is the collection of data about an object from a distance. Humans and many other types of animals accomplish this task with the aid of their eyes, smell, or hearing. Geographers use remote sensing to monitor and measure phenomena in the Earth's lithosphere, biosphere, hydrosphere, and atmosphere. Remote sensing of the environment by geographers is usually done with the help of mechanical devices known as remote sensors. These gadgets have a greatly improved ability to receive and record information about an object without physical contact. These sensors are often positioned away from the object of interest using helicopters, planes, and satellites. Most sensing devices record information about an object by measuring the electromagnetic radiation it reflects and emits. 

            Remote sensing imagery has many applications in mapping land use and cover, agriculture, soil mapping, forestry, city planning, archaeological investigations, military observation, and geomorphologic surveying, among others. For example, foresters use aerial photographs to prepare forest cover maps, locate potential access roads, and estimate the volume of harvested trees. Specialized photography using color infrared film has also been used to detect disease and insect damage in forest trees. 

            The simplest form of remote sensing uses a photographic camera for imaging. This camera contains film that records information from visible or near-infrared wavelengths (Table 2.1). In the late 1800s, cameras were mounted in balloons or kites to take oblique aerial landscape photographs. During World War I, aerial photography played an essential role in gathering information on enemy troop positions and movements. These photographs were often taken from airplanes. After the war, civilian use of aerial photography from planes began with the systematic vertical imaging of large areas of Canada, the United States, and Europe. Many of these images were used to construct topographic and other reference maps of the natural and human-made features on Earth's surface. 

    

            The development of color photography following World War II gave a more natural depiction of surface objects. Color aerial photography also significantly increased the amount of information gathered about an object. The human eye can differentiate many more shades of color than tones of gray (Figures 2.38 and 2.39). In 1942, Kodak developed color infrared film, which records wavelengths in the near-infrared part of the electromagnetic spectrum. This film type showed good haze penetration and the ability to determine vegetation type and health.

Satellite Remote Sensing 

            In the 1960s, a revolution in remote sensing technology began with the deployment of space satellites. From their high vantage point, satellites have a significantly extended view of the Earth's surface. The first meteorological satellite, TIROS-1 (Figure 2.40), was launched by the United States using an Atlas rocket on April 1, 1960. This early weather satellite used vidicon cameras to scan broad areas of the Earth's surface. Satellite remote sensors generally do not use conventional film to produce their images. Instead, these sensors digitally capture the images using a device similar to a television camera. Once captured, this data is transmitted electronically to receiving stations on Earth's surface. The image in Figure 2.41 is from TIROS-7 of a mid-latitude cyclone off the coast of New Zealand.

            Today, the GOES (Geostationary Operational Environmental Satellite) system of satellites provides most of the remotely sensed weather information for North America. Two satellites are employed in a geostationary orbit to cover the entire continent and adjacent oceans. The western half of North America and the eastern Pacific Ocean are monitored by GOES-10, which is directly above the equator and 135° West longitude. The eastern half of North America and the western Atlantic are covered by GOES-8. The GOES-8 satellite is located over the equator and at 75° West longitude. Advanced sensors aboard the GOES satellite produce a continuous data stream, allowing images to be viewed at any time. The imaging sensor produces visible and infrared images of the Earth's terrestrial surface and oceans (Figure 2.42). Infrared images can depict weather even during the night. Another sensor aboard the satellite can determine vertical temperature profiles, vertical moisture profiles, total precipitable water, and atmospheric stability.

            In the 1970s, the second revolution in remote sensing technology began with the USA's deployment of the Landsat satellites. Since 1972, several generations of Landsat satellites with their image sensors have provided continuous coverage of the Earth for about 50 years. Currently, Landsat satellites orbit Earth at an altitude of approximately 700 kilometers. The spatial resolution of objects on the ground surface is 79 x 56 meters. Complete coverage of the globe requires 233 orbits and occurs every 16 days. Landsat's primary imaging sensor, called a multispectral scanner, records a zone of the Earth's surface that is 185 km wide in four wavelength bands: band 4 at 0.5 to 0.6 µm (micrometers), band 5 at 0.6 to 0.7 µm, band 6 at 0.7 to 0.8 µm, and band 7 at 0.8 to 1.1 µm. Bands 4 and 5 receive the green and red wavelengths in the visible light range of the electromagnetic spectrum. The last two bands on the scanner image are in the near-infrared. A second sensing system was added to Landsat satellites launched after 1982. This imaging system, the Thematic Mapper, records seven wavelength bands from the visible to the far-infrared portions of the electromagnetic spectrum (Figure 2.43). In addition, the ground resolution of this sensor was enhanced to 30 x 20 m (98 x 66 ft). This modification provides greatly improved clarity for imaged objects.

            The usefulness of satellites for remote sensing has led several other organizations to launch their own devices. In France, the SPOT (Systeme de l'Observation de la Terre) satellite program has launched five satellites since 1986. Since 1986, SPOT satellites have produced more than 10 million images of our planet. The first four SPOT satellites used two different sensing systems to capture images of Earth's surface. One sensing system produces black-and-white panchromatic images from the visible band (0.51 to 0.73 µm) with a ground resolution of 10 x 10 m (33 x 33 ft). The other sensing system is a multispectral imaging device that captures green, red, and reflected infrared bands at 20 x 20 m (66 x 66 ft) (Figure 2.44). SPOT-5, launched on May 4, 2002, was much improved compared to its siblings. This satellite has a ground resolution as fine as 2.5 x 2.5 m (8.2 x 8.2 ft) in both panchromatic mode and multispectral operation.    

            The Canadian Space Agency launched Radarsat-1 in November 1995. As a remote sensing device, Radarsat is quite different from the Landsat and SPOT satellites. Radarsat is anactive remote sensingsystem that transmits and receives microwave emissions. In contrast, Landsat and SPOT sensors measure reflected radiation at wavelengths roughly equivalent to those our eyes detect (passive remote sensing). Radarsat's microwave energy penetrates clouds, rain, dust, or haze and can produce images in total darkness. Radarsat images have resolutions ranging from 8 to 100 m. This sensor has found important applications in crop monitoring, defense surveillance, disaster monitoring, geologic resource mapping, sea-ice mapping, and monitoring, oil slick detection, and digital elevation modeling (Figure 2.45). An improved Radarsat-2 was launched in the spring of 2007.

FIGURE 2.38  Differentiating shades in color and tones of gray. The rows of color tiles are replicated on the right as complementary gray tones. On the left, we can make out 18 to 20 different shades of color. On the right, only 7 shades of gray can be distinguished.  Image Copyright: Michael Pidwirny.

FIGURE 2.43  The Landsat 7 Enhanced Thematic Mapper instrument. Image Source: NASA.

FIGURE 2.39  Comparison of black and white and color images of the same scene. Note how the increased number of tones found on the color image makes the scene much easier to interpret.  Modified Image Source: University of California at Berkley - Earth Sciences and Map Library.

FIGURE 2.40  TIROS-1 meteorological satellite was launched from Cape Canaveral, Florida, on April 1, 1960. Image Source: NASA.

FIGURE 2.41  TIROS-7 image of a mid-latitude cyclone off the coast of New Zealand, August 24, 1964.  Image Source: NASA - Looking at Earth From Space.

FIGURE 2.44  SPOT false-color image of central Pennsylvania. Image Source: Wikimedia Commons.