CHAPTER 6 SECTIONS > Scientific Theory | Spatial | Temporal | Radiometric | The Landsat Niche
Scientific Theory of Measurements
When solar energy strikes an object, five types of interaction are possible.
The energy is:
- Transmitted - The energy passes through with a change in velocity
as determined by the index of refraction for the two media in question.
- Absorbed - The energy is given up to the object through electron
or molecular reactions.
- Reflected - The energy is returned unchanged with the angle
of incidence equal to the angle of reflection. Reflectance is the ratio
of reflected energy to that incident on a body. The wavelength reflected
(not absorbed) determines the color of an object.
- Scattered - The direction of energy propagation is randomly changed.
Rayleigh and Mie scatter are the two most important types of scatter
in the atmosphere.
- Emitted - Actually, the energy is first absorbed, then re-emitted,
usually at longer wavelengths. The object heats up.
The Landsat-7 system is designed to collect 7 bands or channels of reflected
energy and one channel of emitted energy. A well calibrated ETM+ enables
one to convert the raw solar energy collected by the sensor to absolute
units of radiance. Radiance refers to the flux of energy (primarily irradiant
or incident energy) per solid angle leaving a unit surface area in a given
direction. Radiance corresponds to brightness in a given direction toward
the sensor, and is often confused with reflectance, which is the ratio
of reflected versus total power energy. Radiance is what is measured at
the sensor and is somewhat dependent on reflectance.
The eight bands of ETM+ data are used to discriminate between Earth
surface materials through the development of spectral signatures. For
any given material, the amount of emitted and reflected radiation varies
by wavelength. These variations are used to establish the signature reflectance
fingerprint for that material. The basic premise of using spectral signatures
is that similar objects or classes of objects will have similar interactive
properties with electrmagnetic radiation at any given wavelength. Conversely,
different objects will have different interactive properties. A plot of
the collective interactive mechanisms (scattering, emittance, reflectance,
and absorption) at wavelengths on the electromagnetic spectrum should,
according to the basic premise, result in a unique curve, or spectral
signature, that is diagnostic of the object or class of objects. A signature
on such a graph can be defined as reflectance as a function of wavelength.
Four such signatures are illustrated in Figure 6.1.
Figure 6.1 - Spectral Reflectance Curves of Four Different Targets
ETM+ data can be used to plot spectral signatures although the data
are limited to eight data points within the spectral range of .45 to 12.5
µm. More useful is plotting the ETM+ spectral signatures in multi-dimensional
feature space. The four surface materials shown in Figure 6.1 are plotted
in Figure 6.2 using just two of the ETM+ spectral bands. (GL representing
grasslands, PW representing pinewoods, RS representing red sand, and SW
representing silty water) may be characterized as distinct.
Figure 6.2 - Spectral Separability Using Just Two Bands
Each of the materials has been plotted according to its percent reflectance
for two of the wavelengths or spectral bands. When more than two wavelengths
are involved, the plots in multi-dimensional space tend to increase the
separability among different materials. This spectral separation forms
the basis for multispectral analysis where the goal is to define the bounds
of accurately identified data point clusters.