Concept | Technology | Research Team

The era of civilian remote sensing from satellites began with the launch of LANDSAT I (then called ERTS-1 for Earth Resources Technology Satellite) in July, 1972, which bore the MSS (MultiSpectral Scanner) sensor into orbit with 4 spectral bands (half in the visible and half in the reflective infrared wavelength region) and a spatial resolution of 80-meters. For the first time from space, geologists were able to map a limited number of compositional differences, such as ferric oxides as a class of minerals, in surface soils and rocks. Ten years later, in 1982, the LANDSAT TM (Thematic Mapper) sensor was orbited with 7 spectral bands (6 with 30-meter resolution in the visible/reflective infrared wavelength regions and 1 with 120-meter resolution in the thermal infrared wavelength region), producing significantly better capabilities in compositional mapping. In the mid-80's, the French SPOT satellite was launched with 4 spectral bands and the first digital stereo capability (imaging the same area on the ground from two different observation angles), at spatial resolutions of 10-meters and 20-meters. The significance of stereo data is that height or elevation information can be extracted from digitized stereo images. In the 1990's, additional satellites-some with special resolutions ranging from 3 meters to 5 meter-have been launched by Japan, Europe, Russia, and India.

Soon (i.e., 1999-2001), two to four advanced technology civilian satellites, which will significantly broaden the number of applications for satellite remote sensing will be orbited. Two of them (one from Space Technology Development Corp. with 20-meter spatial resolution and the other from Orbital Sciences with 8-meter resolution) will be hyperspectral sensors with an excess of 200 spectral bands each. These sensors will greatly aid any problem, such as geological mapping, that depends on unique chemical compositional differences among mappable units. Although multispectral information is useful for discrimination of buildings from trees or other chlorophyll-rich objects, two spectral bands alone (red and reflective infrared) are sufficient for that task. Though helpful, hyperspectral sensors do not appear to be as important as other types of sensors for purposes of remote estimates.

The other two satellites will offer stereo viewing with 1-meter spatial resolution, but with modest numbers of spectral bands (5 or less). These sensors, one called IKONOS by Space Imaging Inc. and the other called OrbView 3 by Orbital Sciences, Inc., will greatly help determine height at high spatial resolutions.

The remote sensing task of greatest importance to the estimation of population density is the mapping of the three-dimensional boundaries of buildings. Volume, which can be obtained by multiplying a building's height times its area coverage, is one of the most important characterizations of a building. A parking lot, for example, may have the same asphalt covering and the same area as a building, but height easily discriminates between building and parking lot. The greatest difference between a warehouse and an office building is usually its height-to-area ratio. The square footage of living space in a family home is proportional to its volume (height times area covered). Some have argued that a spatial resolution of 0.3-0.5 meters is best for the acquisition of stereoscopic image data from which building perimeter, area, volume, and height can be extracted. On the one hand, this argument is correct if either visualization of the buildings or survey-type positional accuracy is required. On the other hand, it may not be necessary since a 3 to 5 band multispectral sensor with 1-meter resolution is likely sufficient to discriminate buildings from everything else and to estimate building volume well enough to classify buildings as homes, warehouses, office buildings, malls, etc. In short, 1-meter stereo images from a satellite sensor with 3 to 5 spectral bands may provide more than adequate information from which to estimate population.