Remotely sensing image data are diverse, but most have some common characteristics. One is that the data represent measurements of electromagnetic energy (sonar is an exception to this rule). Another is that the data can be compared in terms of spatial, radiometric, spectral, and temporal resolution. We stressed that a key advantage of multispectral remote sensing is that objects that "look" the same in the one band of the electromagnetic spectrum may be easier to tell apart when viewed in multiple bands.
This chapter identified a couple of key trends in the remote sensing field. One is the miniaturization and, some would say, democratization of aerial and space-based platforms (UAVs and small satellites). Another is the emergence of object-based analysis of high resolution multispectral imagery, and the corresponding decline of pixel-based classification techniques.
Throughout the chapter, we suggested that earth imaging is analogous to desktop document scanning, only a lot more complicated. Earth's shape, rotation, and semi-transparent atmosphere, along with aircraft flightpaths and satellite orbits, necessitate geometric and radiometric corrections, as well as image enhancements. Finally, we pointed out that the desktop scanner analogy is more fitting for active remote sensing like radar and lidar than it is for passive sensors that measure solar radiation emitted by the Sun and reflected or re-emitted by Earth.
Analysts in many fields have adopted land remote sensing data for a wide array applications like land use and land cover mapping, geological resource exploration, precision farming, archeological investigations, and even validating the computational models used to predict global environmental change. Once the exclusive domain of government agencies, an industry survey suggests that the gross revenue earned by private land remote sensing firms exceeded $7 billion (U.S.) in 2010 (ASPRS, 2011).
The fact that remote sensing is first and foremost a surveillance technology cannot be overlooked. State-of-the-art spy satellites operated by government agencies, high resolution commercial sensors, and now cameras mounted on UAVs are challenging traditional conceptions of privacy. In a historical precedent, remotely sensed data were pivotal in the case of an Arizona farmer who was fined for growing cotton illegally (Kerber, 1998). Was the farmer right to claim that remote sensing constituted unreasonable search? More serious, perhaps, is the potential impact of the remote sensing industry on defense policy of the United States and other countries. Some analysts foresee that "the military will be called upon to defend American interests in space much as navies were formed to protect sea commerce in the 1700s" (Newman, 1999).
Geospatial professionals should be mindful and conscientious about the ethical implications of remote sensing technologies. However, the potential of these technologies and methods to help us to become more knowledgeable, and thus more effective stewards of our home planet, is compelling. Several challenges must be addressed before remote sensing can fulfill this potential. One is the need to produce affordable, high-resolution data suitable for local scale mapping—the scale at which most land use decisions are made. UAV-based aerial imaging seems to have great potential in this context. Another is the need to further develop object-based image analysis techniques that will improve the accuracy and cost-effectiveness of information derived from remotely sensed imagery.
Of course this brief overview cannot adequately convey the depth and dynamism of the remote sensing field. For those interested in learning more, we suggested specialized Penn State courses in remote sensing, image analysis, lidar, and even unmanned aerial systems (UAS). Meanwhile, if you really want to geek out, check out the Earth Observation Portal, which provides a searchable database of over 600 in-depth articles of satellite missions from 1959 to 2020, as well as a complementary database of airborne sensors containing detailed information of almost 40 flight campaigns from the last 20 years.