sensing to the study of vegetation phenology is presented. After establishing a
theoretical foundation for LSP, methods of deriving and validating LSP metrics
are discussed. This chapter concludes with a discussion of major research findings
and current and future research directions.
4.2 Theoretical Basis
The emission of electromagnetic radiation from the Sun and its reflection from the
Earth’s surface provide a basis for satellite remote sensing and, consequently, for
LSP. Therefore, our discussion of the theoretical basis for LSP begins by
describing briefly the nature of electromagnetic radiation. We then present the
reflectance characteristics of foliage, which are subsequently used to describe how
the timing of recurring changes in vegetation can be studied using satellite remote
sensing.
Electromagnetic radiation (hereafter referred to as ‘‘radiation’’) is energy
emitted in the form of waves by objects possessing heat (Anderson and Strahler
2008). The amount and wavelengths of radiation are of particular interest in the
field of remote sensing, which focuses on monitoring and analyzing characteristics
of the Earth from afar by measuring its emitted or reflected radiation. The amount
of radiation emitted by an object is proportional to its temperature, with warmer
temperatures leading to larger emissions of radiation and cooler temperatures
resulting in lower emissions of radiation. The peak wavelength of emitted radiation is inversely proportional to the temperature of the emitting object. Because
different objects have different temperatures, radiation can be emitted at a variety
of wavelengths. The various wavelengths of radiation are classified commonly into
different categories, or types, of radiation and represented as a continuous spectrum beginning with those that are shortest and ending with those that are longest.
Progressing from shorter to longer wavelengths, the general types of radiation are
gamma-rays, x-rays, ultraviolet radiation, visible light, near-infrared radiation,
middle infrared radiation, thermal infrared radiation, microwaves, and radio waves
(Shellito 2012).
Measurements of emitted and reflected radiation have facilitated the development of numerous methods to monitor the Earth from afar. In the context of land
surface phenology, methods of documenting and analyzing the timing of seasonal
changes in vegetation have relied primarily on visible and near-infrared radiation
(NIR) emitted by the Sun. Variable amounts of visible light and NIR emitted by
the Sun are transmitted through the atmosphere and received by the Earth’s surface. When the Earth receives this radiation from the Sun, it is absorbed, transmitted, or reflected. Sensors (often called radiometers or spectroradiometers)
mounted on satellites orbiting the Earth effectively measure the amount of visible
light and NIR reflected from the land surface. The measured reflectances of visible
light and NIR from a specified areal unit on the Earth’s surface are assigned digital
4 Land Surface Phenology
101
theoretical foundation for LSP, methods of deriving and validating LSP metrics
are discussed. This chapter concludes with a discussion of major research findings
and current and future research directions.
4.2 Theoretical Basis
The emission of electromagnetic radiation from the Sun and its reflection from the
Earth’s surface provide a basis for satellite remote sensing and, consequently, for
LSP. Therefore, our discussion of the theoretical basis for LSP begins by
describing briefly the nature of electromagnetic radiation. We then present the
reflectance characteristics of foliage, which are subsequently used to describe how
the timing of recurring changes in vegetation can be studied using satellite remote
sensing.
Electromagnetic radiation (hereafter referred to as ‘‘radiation’’) is energy
emitted in the form of waves by objects possessing heat (Anderson and Strahler
2008). The amount and wavelengths of radiation are of particular interest in the
field of remote sensing, which focuses on monitoring and analyzing characteristics
of the Earth from afar by measuring its emitted or reflected radiation. The amount
of radiation emitted by an object is proportional to its temperature, with warmer
temperatures leading to larger emissions of radiation and cooler temperatures
resulting in lower emissions of radiation. The peak wavelength of emitted radiation is inversely proportional to the temperature of the emitting object. Because
different objects have different temperatures, radiation can be emitted at a variety
of wavelengths. The various wavelengths of radiation are classified commonly into
different categories, or types, of radiation and represented as a continuous spectrum beginning with those that are shortest and ending with those that are longest.
Progressing from shorter to longer wavelengths, the general types of radiation are
gamma-rays, x-rays, ultraviolet radiation, visible light, near-infrared radiation,
middle infrared radiation, thermal infrared radiation, microwaves, and radio waves
(Shellito 2012).
Measurements of emitted and reflected radiation have facilitated the development of numerous methods to monitor the Earth from afar. In the context of land
surface phenology, methods of documenting and analyzing the timing of seasonal
changes in vegetation have relied primarily on visible and near-infrared radiation
(NIR) emitted by the Sun. Variable amounts of visible light and NIR emitted by
the Sun are transmitted through the atmosphere and received by the Earth’s surface. When the Earth receives this radiation from the Sun, it is absorbed, transmitted, or reflected. Sensors (often called radiometers or spectroradiometers)
mounted on satellites orbiting the Earth effectively measure the amount of visible
light and NIR reflected from the land surface. The measured reflectances of visible
light and NIR from a specified areal unit on the Earth’s surface are assigned digital
4 Land Surface Phenology
101