Interaction of EMR with Earth Surface Features and Spectral Signatures

Interaction with the Target (C)

Introduction: Spectral Signature

• The reflectance behavior of an object over various wavelengths of the EMS (Electromagnetic Spectrum) is called its spectral reflectance signature or spectral signature.
• Detecting spectral signatures allows us to separate features and understand the size and shape of objects.
• Spectral signatures change over time and space.
• The relationship between incident solar energy and the spectral composition of reflected energy provides information about:
  • Biogeochemical nature of the surface (leaf chemistry, soil mineralogy, water content).
  • Physical and structural characteristics (canopy height, leaf area, soil roughness).
• Remote sensing measurements often consist of mixed signals due to the variety of surface materials (soil, vegetation, water, litter) within the sensor's field of view (Chuvieco & Huete, 2009).
• Spectral signatures can be represented graphically:
  • X-axis: Reflected EMR's frequencies (or wavelength).
  • Y-axis: Intensities.
  • Different objects absorb different parts of solar radiation, resulting in different reflected rays.

Spectral Reflectance (Vegetation, Soil, and Water)

• Different materials exhibit different spectral reflectance curves:
  • Water: higher reflectance in the visible spectrum and low reflectance in the infrared spectrum.
  • Vegetation: Low reflectance in the blue and red regions (due to chlorophyll absorption), high reflectance in the green and near-infrared (NIR) regions.
  • Soil: Generally increasing reflectance with increasing wavelength.
• Examples of spectral reflectance signatures for Earth surface materials (snow, sand, vegetation, cement, water) are shown on a graph with % reflectance on the Y axis and wavelength (µm) along the X axis with the visible (VIS), near infrared (NIR) and shortwave infrared (SWIR) regions labeled.

Factors Affecting Spectral Reflectance

• Factors influencing an object's spectral reflectance pattern:
  1. Surface Roughness / texture of the features
  2. Organic or Inorganic content in features
  3. Moisture content in the features

Surface Roughness / Texture

• The reflectance pattern is influenced by surface roughness (rough/smooth).
• Specular & Diffuse Reflections
• Diffuse reflection dominates when
  • the wavelength is much smaller than the surface variations, OR
  • the particles size that make up the surface.

Interaction EMR with Vegetation

• Plant chlorophyll absorbs energy in the blue ($0.45µm$) and red ($0.65µm$) wavelengths, reflecting green energy.
  • Healthy vegetation appears green.
• Stressed plants absorb less energy in the blue and red bands.
  • Increased red reflectance combined with green energy makes the plant appear yellow.
• Reflectance in healthy vegetation increases in the $0.7 - 1.3µm$ range (IR region).
• Absorption is minimal, and reflectance is from the internal structure of plant leaves.
  • This structure varies between plant species, allowing species discrimination in the IR region.
  • Vegetation stress alters reflectance in the IR region, enabling its detection.
• Water in the leaf absorbs energy at $1.4µm$, $1.9µm$ and $2.7µm$, causing dips in reflectance.
• Reflectance characteristics depend on leaf properties (chlorophyll, orientation, structure of leaf canopy).
• The proportion of radiation reflected depends on leaf pigmentation, leaf thickness and composition, and water content.

Vegetation Spectral Signature (Example)

• The curve is low in the blue and red bands with a peak in the green band.
  • Chlorophyll absorption causes this pattern.
  • Absorption of blue and red provides energy for photosynthesis.
• The curve rises sharply between the red and near-IR (NIR) bands due to interactions between internal leaf structure and EMR.
  • This curve represents active vegetation.
• The steep rise in reflectance occurs at around $0.7µm$, called the RED EDGE point.
  • Its property is important in the study of the photosynthetic state of vegetation.
• The shape of the spectral reflectance curve is useful for:
  • Distinguishing vegetated and non-vegetated areas.
  • Differentiating between species.
  • Estimating physical properties (Leaf Area Index-LAI, Biomass, crop yield).
• Differences between vegetation types and their growth rates can be assessed using multi-temporal imagery.

Interaction of EMR with Soil

• Factors affecting soil reflectance:
  • Moisture content (most important).
  • Soil texture (proportion of sand, silt, and clay).
  • Surface roughness/structure.
  • Presence of iron oxide.
  • Organic matter content.
  • Soil mineralogy.
• Moisture in soil decreases reflectance.
  • This effect is greatest in the water absorption bands at about $1.4µm$, $1.9µm$, $2.2µm$ and $2.7 µm$.
• Soil texture is related to moisture content.
  • Sandy soil (low moisture) has high reflectance, while poorly drained soils have lower reflectance.
• Soil roughness, organic matter, and iron oxide reduce reflectance.
• The curve rises in reflectance with increasing wavelength.
• Bare soil reflectance depends on color, moisture content, and carbonate and iron oxide content.

Interaction of EMR with Water Bodies

• Water absorbs energy in the infrared, resulting in zero reflectance in the IR band.
  • Useful for locating and mapping water bodies.
• Various conditions of water bodies can be distinguished in visible bands.
• Reflectance properties of water are a function of:
  • Interaction with the water's surface (specular reflection).
  • Suspended material in water.
  • Bottom of the water body.
• Water has lower reflectance compared to vegetation and soil.
  • Vegetation may reflect up to $50$, soils up to $30-40$, while water reflects at most $10$ of the incoming radiation.
  • Beyond $1.2µm$, all energy is absorbed.
• Most of the radiant flux is either absorbed or transmitted, not reflected.
• At visible wavelengths, little energy is absorbed, a small amount (under $5$) is reflected, and most is transmitted.
• Water absorbs strongly at near-infrared wavelengths.
• Clear water absorbs little energy in the less than $0.6µm$ band.
  • High transmittance in the blue-green band.
• Changes in turbidity affect transmittance and reflectance.
  • Turbidity is due to organic and inorganic material presence.
• Water containing suspended sediments from soil erosion has higher visible reflectance than clear water.
• Reflectance changes with chlorophyll concentration.
• Increased chlorophyll decreases reflectance in the blue band and increases it in the green band.
  • Useful in algae detection via remote sensing.
• Pollutants like oil and industrial wastes can also be detected.
• The spectral curve shows a reduction in reflectance as wavelength increases and is zero in the NIR band.

Ground Truth

• In Situ Data Collection
  • Spectroradiometer measurement.
  • Global Positioning System (GPS) measurement.
• Ground sensors: Field Spectrometers Applications
  • Hand-held spectrometers for use in laboratories or the field
  • Data recorded for single sample unit
  • Able to control acquisition parameters
  • Coordinate with ground observations
• Invasive weeds mapping in tropical forests

Conclusions

• Object properties in remote sensing can be determined by analyzing their spectral signatures.
• Spectral signatures are the pattern of the Electromagnetic spectrum reflected by an object.
  • Represented graphically with reflected EMR's frequency (x-axis) and intensities (y-axis).
• Reflectance in healthy vegetation increases in the $0.7 - 1.3µm$ (IR region).
• Soil reflectance depends on six main factors, with moisture content being the most important.
  • Reflectance decreases due to water absorption bands at about $1.4µm$, $1.9µm$, $2.2µm$ and $2.7 µm$.
• Clear water absorbs little energy in less than $0.6µm$ band.
  • High transmittance in the blue-green band.