Interpreting Aerial Photographs to Identify Natural Hazards

Charles E. Glass

• Publisher: Elsevier
• Year: 2013
• Print ISBN:
  9780124200180
• eBook ISBN:
  9780124200289
• Topic: Physical Sciences
• Subtopic: Digital Media

Authored by a world-renowned aerial photography and remote sensing expert, Geographic Aerial Photography: Identifying Earth-Surface Hazards Through Image Interpretation is the most practical and authoritative reference available for any professional or student looking for a reference on how to recognize, analyze, interpret and avoid – or successfully plan for – dangerous contingencies. Whether they are related to natural terrain, geology, vegetation, hydrology or land use patterns – it's critical for you to be able to recognize dangerous conditions when and where they exist. Failure to adequately recognize and characterize geomorphic, geologic, and hydrologic dangers on the ground using aerial photography is one of the major factors contributing to due to natural hazards and disasters, damage to architectural structures, and often the subsequent loss of human life as a result. Aerial photographs provide one of the most prevalent, inexpensive and under-utilized tools to those with the knowledge and expertise to interpret them. - Authored by one of the world's experts in aerial photography and remote sensing, with more than 35 years of experience in research and instruction - Features more than 100 color photographs to vividly explore the fundamental principles of aerial photography - Chapter tables underscore key concepts including channel size and shape characteristics, image scales, reverse fault values, and strike-slip fault systems

Part I

Establishing Baselines: Look for the Butterfly

Part I Establishing Baselines: Look for the Butterfly

Chapter 1 Getting Started

Chapter 2 More About Photographs Than You Ever Cared to Know

Chapter 3 Textural Information

Chapter 4 Color Information

Part I

Establishing Baselines: Look for the Butterfly

I once read a novel, the name of which I can no longer remember, in which a computer specialist was looking out the window on the top floor of a high-rise hotel. He noticed a butterfly fly past the window. What an anomaly! What was a butterfly doing up there? There were no flowers for it—no mates either. This impressed the computer specialist—and obviously me as well—for when pouring over reams of data from then on, the specialist always looked for the butterfly—that small, yet significant, anomaly that just didn’t belong. This will be your job when looking for hazardous ground conditions using aerial photographs. Always look for the butterfly.

We will see that dangers are revealed on aerial photographs primarily because they depart subtly from the norm; they are anomalous features that depart from what we expect to see in natural terrain. So searching for dangerous ground conditions on aerial photographs is similar to a forensic specialist searching for clues to a crime.

Look for a moment at the aerial photograph shown in Figure I.1. This is our subject. This is what we will spend the rest of our time studying. What you see in Figure I.1—or more precisely what you think you see—depends a lot on how much time you’ve already spent looking at aerial photographs (your experience). Everyone will agree that aerial photographs frequently contain complex patterns—patterns caused by reflection of sunlight from landscape features. We will use these patterns to glean from aerial photographs information about the terrain, geology, vegetation, and land use within the area covered by the photograph. This information will comprise the norm, or the background, from which we will search for butterflies, anomalous patterns that constitute natural hazards. Perhaps you have already seen some natural hazards on Figure I.1. If you haven’t, don’t be concerned, you soon will.

Figure I.1 The color vertical aerial photograph is of Owens Valley, California. North is toward the top. Place a bookmark on this photograph, or print it, as we will be referring to it later.

Chapter 1

Getting Started

1.1 Interpretation Begins with the Sun

To begin, let us consider the sunlight that we use to illuminate our subject. This will help us understand aerial photographs vis a vis other forms of remotely sensed images.

Sunlight is simply electromagnetic radiation. Electromagnetic radiation comprises a continuous spectrum of energy extending from long-wavelength (low-frequency) radio waves to short-wavelength (high-frequency) gamma and cosmic waves. A cartoon depicting a portion of the electromagnetic spectrum is presented in Figure 1.1. The spectrum is arbitrarily divided into the following regions: x-ray, ultraviolet, visible, infrared, microwave (radar), and radio. Boundaries between these regions are rather arbitrary, and some regions (ultraviolet, visible, and infrared are examples) are further divided into subregions, so you will find a fair amount of disagreement about region boundaries within the literature.

Figure 1.1 A portion of the electromagnetic spectrum.

One region of the electromagnetic spectrum that is tightly defined, though, is the visible region, because it is defined by the response of the human eye. Our eyes are sensitive to electromagnetic radiation having wavelengths ranging from approximately 0.4 μm (violet) to approximately 0.7 μm (red).

Through some stroke of luck, evolution, or genius this range of electromagnetic energy also corresponds to the location of the Sun’s peak energy output. How fortunate for us; it enables us to see our world. This e-book will deal nearly exclusively with the visible spectrum, even though sensors have been developed that are capable of imaging in other, more exotic, regions. Why are we to be discriminated against and forced to remain in the “low-tech” visible region? There are several good reasons to do so.

1.2 Film

The main advantage of focusing on the visible portion of the spectrum is that film emulsions, color and black-and-white, are available there. Why is this important?

1. There are many commercial aerial photography firms that use film.

2. Time-lapse aerial photo coverage is available for many areas (dating back to the 1930s in some areas).

3. Films are inexpensive ($15–20 per exposure depending on number of exposures).

4. Films have high geometric fidelity (few distortions).

5. Colors and gray tones are familiar to us. After all, we have used the visible portion of the electromagnetic spectrum since birth.

6. Films have high spatial resolution.

Although aerial photography firms throughout the world still use film and camera systems, many modern firms also offer high-resolution digital aerial imaging. Although the digital acquisition systems sometimes use scanners, the geometric fidelity is excellent on modern systems. Digital products provided for interpretation, however, most often involve scanning the digital data onto film. Hence the advantages of digital imaging in the visible portion of the spectrum are essentially the same as those mentioned above.

1.3 Target Interactions

So what happens when sunlight strikes a target? Three things happen as follows:

1. Sunlight is reflected back from the target surface. Once reflected, the energy can be sensed by a camera, your eye, or a scanner of some kind. The amount of energy reflected from a target is determined by the reflectance of the target. Some colors may reflect more energy than others. If most of the reflected energy is in the red portion of the spectrum, for example, the object will have a reddish hue. The color of a target depends on its spectral reflectance (reflectance in different portions of the spectrum).

2. Sunlight is absorbed by the target. Absorption occurs when a photon (packet) of energy excites atoms at the surface of the target forcing electrons to jump away from the nucleus into more distant orbits, thus absorbing the energy of the photon. This new atomic arrangement keeps the energy within the surface atoms, and prevents reflection. It also causes the atoms to vibrate more rapidly, increasing the thermometric temperature of the surface. Usually the new electron configuration is relatively unstable, and the electron will eventually drop back into its original orbit. When this happens, the energy originally absorbed is emitted by the surface atoms at longer wavelengths, frequently in the thermal infrared region. The amount of sunlight absorbed is determined by the target’s absorptance and the amount of energy absorbed by different colors its spectral absorptance. Iron minerals, for example, absorb strongly in the blue and green portion of the visible spectrum, thus little blue and green energy is available for reflection. This results in the reddish appearance of iron minerals such as iron oxides.

3. Sunlight is transmitted by the target. Transmitted energy is propagated through the target where it may interact with another object below. The energy transmitted is determined by a target’s spectral transmittance.

There may be some confusion about the terms reflectance, absorptance, and transmittance used above relative to the terms reflection, reflectivity and similar terms for absorption, absorptivity and transmission and transmissivity. I’ll try to clear this up using the reflection process and expanding to the others intuitively.

Reflection is a process in which energy is reflected from a surface, the amount of energy reflected depends on the wavelength of energy, the roughness of the surface, the angle of incidence of the energy at the surface, and the electrical properties of the surface (reflectivity), among others.

Reflectivity is an intrinsic property of a material. It depends on the electrical properties of the material. It is a property that one can find in the Handbook of Physics and Chemistry. Gold, for example, has a constant reflectivity regardless of the character of its surface.

Reflectance is an intrinsic property of a surface. It depends on the reflectivity of the material composing the surface, and the other surface properties mentioned above. The reflectance of a smooth, pure gold surface, for example, may vary from the reflectance of a corroded or abraded gold surface.

Look down at the earth for a moment. Can you see into it? I can’t. So I think we can safely say that for most earth materials we deal with at aerial photography scales the transmittance is zero. Then

In this equation, ρ(λ) is the spectral reflectance and α(λ) is the spectral absorptance, at wavelength λ (color). This is a trivial equation that says that if the electromagnetic energy is not reflected at a surface, it is abs...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Figures
• List of Tables
• Part I: Establishing Baselines: Look for the Butterfly
• Part II: Recognizing Hazards on the Ground
• Part III: Spectral Ranges Beyond Visible
• Appendix. A Selection of Aerial Photographs for You to Interpret