James S. Aber
Remote Sensing Basics
Remote sensing is the observation and measurement of objects from a distance, i.e. instruments or recorders are not in direct contact with objects under investigation. Remote sensing depends upon measuring some kind of energy that is emitted, transmitted, or reflected from an object in order to determine certain physical properties of the object. Electromagnetic, acoustic, and seismic energy may be utilized for different kinds of remote sensing.
Taken from USGS EROS Center.
We are concerned primarily with electromagnetic energy emitted or reflected from the Earth's surface and detected at some altitude above the ground. The electromagnetic spectrum is, thus, the starting point for understanding remote sensing. Passive remote sensing is based on detecting available (background) electromagnetic energy from natural sources. Most methods, such as aerial photography and multispectral scanning, are passive in nature. Active remote sensing, in contrast, depends on an artificial "light" source, such as radar (microwaves) or lidar (laser), to illuminate the scene.
Sunlight is the main source of energy at the Earth's surface with most energy in the ultraviolet, visible, and short infrared portions of the spectrum. The Earth is a much weaker source of energy at longer wavelengths of thermal infrared and microwaves. All passive remote sensing is based on these two energy sources.
- Ultraviolet, visible, and short infrared (<3 µm) are mainly reflected solar energy.
- Mid-IR, thermal-IR, and microwave radiations (>3 µm) are mostly emitted from the Earth's surface.
Atmospheric Effects The atmosphere is not equally transparent to all wavelengths of electromagnetic energy. Thus, atmospheric effects are an important part of remote sensing. The atmosphere affects transmission of electromagnetic energy in two important ways.
Major atmospheric windows include wavelength ranges: 0.3 to 1.1 µm, 3 to 5 µm, 8 to 14 µm, and >0.8 cm. Various photographic and electronic detectors cover these windows.
- Scattering -- in clear air, scattering is wavelength dependent with shorter wavelengths scattered more strongly than longer. This is why the sky is blue. Larger particles (dust, water droplets) may partly or completely block reflected solar energy.
- Absorption -- certain gas molecules (CO2, H2O, O2, O3) selectively absorb particular wavelengths of electromagnetic energy. This results in atmospheric windows, which are wavelength intervals that have relatively little absorption.
Spectral Information Traditional interpretation of remote-sensing images is done by visual examination, on the basis of such attributes as: size, shape, pattern, color, texture, shadow, context, etc. This approach yields a great deal of information about objects at the Earth's surface. Each object has a unique combination of reflected, emitted, and transmitted energy--its so-called spectral signature. This includes both visible light as well as longer and shorter wavelengths of electromagnetic energy, which can be analyzed statistically to identify specific materials.
For more information, see the USGS Spectroscopy Lab.
Spectral signatures are how we recognize the natural or false color of objects. This is a powerful method for classifying and interpreting remote sensing data, but classification techniques must be used with some caution. The spectral signatures of objects are not constant; signatures vary with time of year, amount of solar illumination, geographic locations, and many other factors. Thus, a classification technique that works well in one situation may not be appropriate in other circumstances.
The multi-concept is an important working method in remote sensing, as applied to data acquisition and analysis--multilevel (height), multivantage (viewpoint), multispectral (bands), multidetector (sensor), and multitemporal (date). Typically this means various types of aerial photography as well as different kinds of space-based imagery.
Manned platforms range from sailplanes to space flights. Unmanned platforms include balloons, drones, rockets, and satellites. At ESU, we have developed a specialty for unmanned kite and blimp aerial photography. The various types of remote-sensing data may be combined with other kinds of ground-based information, so-called ground truth, to achieve an integrated, multi-source portrayal of surficial natural and cultural land cover and use.
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EB/ES/GE 351 © J.S. Aber (2016).