Basics of Remote Sensing

.title[
# Basics of Remote Sensing
]
.subtitle[
## 🗺
]
.author[
### <strong>Dr. Ankit Deshmukh</strong>
]
.institute[
### Pandit Deendayal Energy University, Gandhinagar
]
.date[
### 25 August 2022
]

---

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# .white[“The most precious resource we all have is time.”] <br /> .f3.gold[~ Steve Jobs ~]

---
# Remote Sensing 101

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---
# Remote Sensing 101
.pull-left[
<img src="images/Picture1.png" width="744" style="display: block; margin: auto;" />
]

- **Cartography:** an attempted to automate the manually dependent map-making process by substituting the drawing work by vector digitization.

- **Computer graphics** An applications of digital vector data apart from cartography, particularly in the design of buildings, machines, and facilities.

- **Databases** A general mathematical structure according to which the problems of computer graphics and computer cartography could be handled.

- .light-red[**Remote sensing** It creates immense amounts of digital image data in need of geocoded rectification and analysis.]
]

---
# History

]

- Remote sensing is identification of objects by indirect means using naturally existing/ artificially created force field.

- In 1972, Landsat 1 (USA), became the first remote sensing satellite with a world coverage at 80 m pixels in four spectral visible and near-infrared channels.

- Higher spatial resolution was achieved by the French Spot satellites launched since 1986 with pixel sizes of 10 m.

- The Indian satellites IRS 1C and 1D reached panchromatic pixel sizes of 6 m in 1996. 
]

---

# Principles of remote sensing

- The radiation propagates through a vacuum with the speed of light.

- It reaches an object, where it interacts with the matter of this object.

- Part of the energy is reflected toward the sensor.

- At the sensor, the intensity of the incoming radiation is quantized and stored.

- The stored energy values are transformed into images.

- Image processing techniques applied to analyze and extract object information.

]

---
# Law of Electromagnetic Radiation (EMR)

- Electromagnetic energy is radiated by any body having a temperature higher than –273°C (or 0 K), the absolute zero temperature.

- A body radiates energy in all frequencies. The relation between frequency, ν, and wavelength, λ, is expressible as:

.red.f2[$$ \lambda = \frac{c}{\nu}$$]

> with `$\lambda$` expressed in meters and frequency in cycles per seconds (i.e., Hertz).

---

# Electromagnetic spectrum

---
# Energy interactions in the atmosphere

1. Composition of the atmosphere

2. Interactions of EMR with the atmospheric particles

## 1. Composition of the atmosphere 
<div class="figure" style="text-align: center">
<img src="images/Compo.png" alt="Table: Composition of the Earth’s atmosphere (from Gibbson, 2000)" width="50%" />
<p class="caption">Table: Composition of the Earth’s atmosphere (from Gibbson, 2000)</p>
</div>
---
.pull-left[
## 2. Energy Interactions

The intensity and the spectral composition of the incident radiation are altered by the atmospheric effects.

### Atmospheric interaction depends on the:
- Properties of the radiation such as magnitude and wavelength
- Atmospheric conditions
- Path length

### Interaction with the atmospheric particles
- Scattering
- Absorption
]

<img src="images/EMR-int.png" width="787" style="display: block; margin: auto;" />
]

---

# Energy Interactions: .red[Scattering]

1. Scattering -Rayleigh Scattering
2. Mie Scattering

<img src="images/Sunset.png" width="1492" style="display: block; margin: auto;" />
--
<br /> 
.red[Scattering causes yellow-orange sky].blue[, blue sky, and White clouds.]

---

# Rayleigh Scattering
.pull-left[
- Scattering caused by the atmospheric molecules and other tiny particles.

- Also known as selective scattering or molecular scattering it dependents on the wavelength.

- Occurs when particles are much smaller than the wavelengths of the radiation.
> Particle size less than (1/10)th of the wavelength.

- Intensity of the scattered light is inversely proportional to the fourth power of wavelength. 
> Shorter wavelengths are scattered more than longer wavelengths.

- Scattering of the visible bands is caused mainly by the molecules of Oxygen and Nitrogen
]

.pull-right[
<img src="images/R-scatter.jpg" width="120%" style="display: block; margin: auto;" />
.footnote[source: http://cleanenergywiki.org]
]

---

# .f1[Rayleigh Scattering of the Visible Part of the EM spectrum.]
Scattering of the visible bands is caused mainly by the molecules of Oxygen and Nitrogen.
--
.co-note[
- Blue (shorter wavelength) is scattered more
- Blue light is scattered around four times the red light
- UV light is scattered about 16 times the red light
- A "blue" sky is a manifestation of Rayleigh scatter
]
--
<br /> <br /> 
.co-imp[
- Orange or red color during sunrise and sunset
- Sun rays have to travel a longer path
- Complete scattering (and absorption) of shorter wavelength radiation
- Only the longer wavelength (orange and red) are visible
]

---
# Mie Scattering
.co-tip[The grey/white color of the clouds is caused by Mie scattering by water droplets, which are of a comparable size to the wavelengths of visible light.]

<img src="images/mie.png" width="803" style="display: block; margin: auto;" />
--

- Occurs when the wavelengths of the energy is almost equal to the diameter of the atmospheric particles.

--
- Usually caused by the aerosol particles such as dust, smoke and pollens.

--
- Gas molecules are too small to cause `Mie scattering` of the radiation commonly used for remote sensing.

--
- Longer wavelengths also get scattered compared to Rayleigh scatter.

---
# Absorption
.pull-left[
<img src="images/Road.jpg" width="80%" style="display: block; margin: auto;" />
]
.pull-right[
<img src="images/loss.jpg" width="120%" style="display: block; margin: auto;" />
]

---

# Absorption
- Absorption is a process in which the incident energy is retained by particles in the atmosphere. 
- Unlike scattering, atmospheric absorption causes an effective loss of energy

### Absorption depends on

- Wavelength of the energy
- Atmospheric composition
- Arrangement of the gaseous molecules and their energy level

### The most efficient absorber of solar radiation are:
- water vapor, 
- carbon dioxide, and 
- ozone

### Gaseous components are selective absorber of EMR:
- Absorb EMR in specific wavelength bands.
- Depends on the arrangement of the gaseous molecules and energy levels.

---
# Energy Interaction at the Object
Interaction of incident electromagnetic energy with matter depends on the molecular and atomic structure of the object.

Energy can directionally reflected, scattered, transmitted, or absorbed. 
The ratio between the reflected (in all directions), transmitted, and absorbed fluxes radiation and the incoming radiation is described as:

1. Reflection coefficient, `$\rho_\lambda = \frac{\phi_{\lambda \ reflected}}{\phi_{\lambda \ incoming}}$`

1. Transmission coefficient, `$\tau_\lambda = \frac{\phi_{\lambda \ transmitted}}{\phi_{\lambda \ incoming}}$`

1. Absorption coefficient, `$\alpha_\lambda = \frac{\phi_{\lambda \ absorbed}}{\phi_{\lambda \ incoming}}$`

.i.dark-green[
The normal reflection coefficients vary for different object types, for example, for green light:
- Coniferous forest, 1% 
- Water, 3% 
- Road, 8%
- Sand, 25%
- New snow, 78% 
]
---

# A short introduction of .purple[Photogrammetry]
Photogrammetry is defined as  the science, art, and technology of  obtaining reliable information from photographs.

<img src="images/Photogrammetry.webp" width="50%" style="display: block; margin: auto;" />
.footnote[https://newmexicouas.com/photogrammetry]
---
# Uses of Photogrammetry
.b.purple[Photogrammetry is now the principal method employed in topographic mapping and compiling other forms of spatial data.]

- Advantages of Photogrammetry are: 
  - speed of collecting spatial data in an area, 
  - relatively low cost, 
  - ease of obtaining topographic details, 
  - especially in inaccessible areas,
  - reduced likelihood of omitting details in spatial data collection.

- Land surveying to compute coordinates of section corners, boundary corners, or points of evidence that help locate these corners.

- Photogrammetry is used to map shorelines in hydrographic surveying.

- Data for modern land and geographic information systems.

---
# Photogrammetry classification

### 1. Metrical Photogrammetry
- To determining spatial information including distances, elevations, areas, volumes, and data for compiling topographic maps from measurements made on photographs.

.co-warn[Aerial photographs (exposed from aircraft) are normally used, although in certain special applications, terrestrial photos (taken from earth-based cameras) are employed.]

### 2. Interpretative Photogrammetry
- Involves recognizing objects from their photographic images and judging their significance.

- Critical factors considered in identifying objects are the shapes, sizes, patterns, shadows, tones, and textures of their images.

---
# Metrical Photogrammetry → Aerial Photogrammetry
<img src="images/Field.png" width="68%" style="display: block; margin: auto;" />
---
# Types of Aerial Photographs

1. **Vertical Photograph:** Taken with the camera axis aimed vertically downward or as nearly vertical as possible.

2. **Oblique Photograph:** Made with the camera axis intentionally inclined at an angle between the horizontal and vertical.

.pull-left[
<div class="figure" style="text-align: center">
<img src="images/Vert-img.png" alt="Vertical photographs are the principal mode of obtaining imagery for photogrammetric work." width="70%" />
<p class="caption">Vertical photographs are the principal mode of obtaining imagery for photogrammetric work.</p>
</div>
]

.pull-right[
<div class="figure" style="text-align: center">
<img src="images/Obl-img.png" alt="Oblique aerial photograph showing Madison, Wisconsin; Seldom used for metrical applications." width="70%" />
<p class="caption">Oblique aerial photograph showing Madison, Wisconsin; Seldom used for metrical applications.</p>
</div>
]

---
# Photomosaic

It is common to combine several vertical photographs in the block of photography to create a photomosaic.

<div class="figure" style="text-align: center">
<img src="images/Photomosaic.png" alt="Block of Aerial Photography Compiled into an Uncontrolled Photomosaic" width="2159" />
<p class="caption">Block of Aerial Photography Compiled into an Uncontrolled Photomosaic</p>
</div>
---
# Getting started with .dark-green[QGIS]

- For windows: https://qgis.org/downloads/QGIS-OSGeo4W-3.22.9-2.msi
- For Mac: https://qgis.org/downloads/macos/qgis-macos-ltr.dmg
- For Linux: https://qgis.org/en/site/forusers/alldownloads.html#linux

---

.footnote[
.light-red[
Slides created via the R package [Xaringan](https://github.com/yihui/xaringan). The chakra comes from [remark.js](https://remarkjs.com/), [R Markdown](https://rmarkdown.rstudio.com/)
]]