MEV 024: Unit 07 – Introduction to remote sensing

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 UNIT 7: INTRODUCTION TO REMOTE SENSING

7.1 Introduction

Remote sensing is the science of obtaining information about objects or areas from a distance, typically using satellite or airborne sensors. It plays a crucial role in environmental monitoring, agriculture, forestry, disaster management, urban planning, and climate studies. Remote sensing technologies allow for the collection of data over large areas, across different timescales, and under varying environmental conditions without physical contact.

As an indispensable tool in modern geospatial analysis, remote sensing helps in capturing patterns, detecting changes, and deriving valuable insights about Earth’s surface and atmosphere. This unit introduces the fundamental concepts of remote sensing, including electromagnetic radiation (EMR), sensor types, and image interpretation.

7.2 Objectives

After studying this unit, you will be able to:

  • Understand the definition and process of remote sensing.
  • Explain the basic principles of electromagnetic radiation and its interaction with the Earth's atmosphere and surface.
  • Identify the spectral signatures of different Earth surface features.
  • Differentiate between various types of remote sensing and sensors.
  • Recognize the advantages and limitations of remote sensing technology.

7.3 Remote Sensing

7.3.1 Definition

Remote sensing refers to the technique of acquiring information about an object, area, or phenomenon without making physical contact, typically through sensors mounted on satellites or aircraft. It involves the detection and measurement of radiation reflected or emitted from Earth's surface.

7.3.2 Remote Sensing Process

The remote sensing process involves a series of steps:

  1. Energy Source: Provides electromagnetic energy to the target (usually the Sun).
  2. Radiation and Atmosphere: EMR travels through the atmosphere and may be scattered or absorbed.
  3. Interaction with the Target: EMR interacts with Earth features (e.g., vegetation, soil).
  4. Recording by Sensor: Sensors detect the reflected/emitted radiation.
  5. Transmission and Processing: Data is transmitted to receiving stations and processed.
  6. Interpretation and Analysis: Data is analyzed using software to extract meaningful information.

7.3.3 Advantages and Limitations

Advantages:

  • Covers large and inaccessible areas.
  • Enables frequent data acquisition.
  • Useful for long-term environmental monitoring.
  • Multispectral capabilities offer detailed feature analysis.

Limitations:

  • Affected by atmospheric conditions (especially optical sensors).
  • Requires expertise in data processing and interpretation.
  • High-resolution data may be expensive or restricted.

7.4 Electromagnetic Radiation (EMR)

7.4.1 Models of Electromagnetic Radiation

Electromagnetic radiation exhibits both wave-like and particle-like properties, modeled by:

  • Wave Model: EMR travels as sinusoidal waves characterized by wavelength and frequency.
  • Particle Model: EMR consists of discrete energy packets called photons.

7.4.2 Electromagnetic Spectrum

The electromagnetic spectrum (EMS) includes various types of radiation based on wavelength:

Region

Wavelength Range

Ultraviolet (UV)

0.01 – 0.4 µm

Visible

0.4 – 0.7 µm

Near Infrared (NIR)

0.7 – 1.3 µm

Shortwave IR (SWIR)

1.3 – 3 µm

Thermal IR

3 – 14 µm

Microwave

> 1 mm

Remote sensing mainly uses visible, infrared, and microwave regions.

7.4.3 Radiation Laws

Key laws governing EMR:

  • Planck’s Law: Radiation emitted by a blackbody is a function of wavelength and temperature.
  • Wien’s Displacement Law: Peak emission wavelength shifts with temperature.
  • Stefan–Boltzmann Law: Total emitted energy increases with the fourth power of temperature.

7.5 EMR Interactions with Atmosphere and the Earth Surface

7.5.1 EMR–Atmosphere Interaction

As EMR travels through the atmosphere, it undergoes:

  • Scattering: Deflection by air molecules (Rayleigh, Mie, and non-selective).
  • Absorption: Certain gases (e.g., ozone, CO₂, water vapor) absorb EMR.
  • Transmission: Remaining energy reaches the Earth's surface.

Atmospheric windows refer to wavelength ranges where EMR passes with minimal absorption.

7.5.2 Interaction of EMR with Earth Surface

When EMR strikes the surface, three phenomena occur:

  • Reflection: EMR bounces back (measured by sensors).
  • Absorption: Energy is absorbed and converted to heat.
  • Transmission: Energy passes through (mainly in water bodies).

The proportion of these interactions depends on the material and wavelength.

7.6 Spectral Signatures of Earth Surface Features

Spectral signatures refer to the specific reflectance characteristics of materials across wavelengths.

7.6.1 Vegetation

  • High reflectance in NIR region due to internal leaf structure.
  • Absorption in red and blue due to chlorophyll.
  • Healthy vegetation shows a distinctive "red edge" between red and NIR bands.

7.6.2 Soil

  • Reflectance varies with moisture, organic matter, and texture.
  • Wet soils are darker (lower reflectance).
  • Dry, sandy soils have higher reflectance in visible and NIR.

7.6.3 Water

  • Strong absorption in infrared and microwave regions.
  • Clear water reflects little energy; turbid or algae-rich water may reflect more in visible bands.

7.7 Types of Remote Sensing

Remote sensing can be classified based on the source and platform:

  • Ground-based: Close-range observations (e.g., field spectrometers).
  • Aerial: Aircraft-mounted sensors (e.g., drones, planes).
  • Satellite-based: Wide-area monitoring from space.

7.8 Types of Remote Sensors

7.8.1 Passive and Active Remote Sensing

  • Passive sensors detect natural radiation (e.g., optical, thermal).
    • Example: Landsat, MODIS.
  • Active sensors emit their own energy and detect its reflection.
    • Example: RADAR, LiDAR.

7.8.2 Imaging and Non-imaging Systems

  • Imaging sensors produce two-dimensional images (e.g., cameras, scanners).
  • Non-imaging sensors collect point data or profiles (e.g., spectroradiometers).

7.8.3 Multispectral, Thermal, and Microwave Imaging

  • Multispectral sensors capture data in discrete spectral bands (e.g., Sentinel-2).
  • Thermal sensors detect emitted radiation to monitor temperature (e.g., ASTER).
  • Microwave sensors operate under all weather, day/night conditions (e.g., SAR).

7.9 Let Us Sum Up

  • Remote sensing involves acquiring information about Earth's surface using sensors without direct contact.
  • The process relies on electromagnetic radiation and its interaction with the atmosphere and surface.
  • Different materials have unique spectral signatures that aid in their identification.
  • Sensors vary by platform, functionality (passive/active), and spectral resolution.
  • Remote sensing is a foundational technology for environmental monitoring, agriculture, urban planning, and disaster assessment.

7.10 Key Words

  • Remote Sensing: Technique to collect information without direct contact.
  • Electromagnetic Radiation (EMR): Energy that travels as waves or photons.
  • Spectral Signature: Reflectance pattern across wavelengths unique to a surface feature.
  • Passive Sensor: Detects natural radiation.
  • Active Sensor: Emits its own signal and measures its return.
  • Multispectral: Captures multiple spectral bands.
  • Atmospheric Window: EMR wavelengths minimally absorbed by the atmosphere.

 

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