introduction to gis: lecture #9 (remote sensing) introduction to remote sensing history emr ems...

Introduction to GIS: Lecture #9 (Remote Sensing)

Introduction to Remote Sensing

History

Radiation Characteristics

Spectral Signatures

LANDSAT Imagery

Remote Sensing

A technique of obtaining information about objects through the analysis of data collected by special instruments that are not in physical contact with the objects of investigation.

Reconnaissance from a distance.

History

1839 - first photograph

1858 - first photo from a balloon

1903 - first plane1909 first photo from a plane

1903-4 -B/W infrared film

WW I and WW II

1960 - space

Electromagnetic Radiation (EMR)

wavelength

frequency

Electromagnetic Radiation

Wavelength

Frequency (how many times peak passes per second)

Light - can be thought of as a wave in the 'electromagnetic field' of the universe

A wave can be characterized by its wavelength or its frequency

Remote sensing is concerned with the measurement of EMR returned by the earth’s natural and cultural features that first receive energy from the sun or an artificial source such as a radar transmitter.

Because different objects return different types and amounts of EMR, the objective in remote sensing is to detect these differences with the appropriate instruments.

This, in turn, makes it possible for us to identify and assess a broad range of surficial features and their conditions.

Electromagnetic Spectrum

Ranges From:Gamma rays (short wavelength, high frequency and high energy content)

To:Passive radio waves (long wavelength, low frequencies, and low energy content).

A spectral band is composed of some defined group of continuous spectral lines, where a line represents a single wavelength or frequency. The boundaries between most of the bands are arbitrarily defined because each portion overlaps adjacent portions.

centimeter = .01 meters

millimeter = .001 meters

micrometer = .000,000,1 meters

nanometer = .000,000,000,1 meters

angstrom = .000,000,000,01 meters

The EM Spectrum

Blue green yellow red

1020 Hz 1018 Hz 1016 Hz 1014 Hz 1012 Hz 1010 Hz

1 pm 10pm 10 nm 1 micron 100 microns 1 mm 100 mm

vi-si-ble

Gamma Rays

X-Rays UV N. IR

Microwaves

TV FMRadiowaves

0.4 m 0.5 m 0.6 m 0.7 m

Far IR

Different wavelengths of light can be grouped together into different types

Visible light contains light from 0.4 to 0.7 micrometers Infrared light from 0.1 micrometers to 1 millimeter

Radiation

R/S Spectral Regions

Ultraviolet (UV)

Visible

Infrared (IR)

Microwave

Traditionally, the most common used region of the EMS in remote sensing has been the visible band. Its wavelength span is from 0.4 to 0.7 micrometers, limits established by the sensitivity of the human eye.

Visible Light

Composed of colors (different wavelengths)

These familiar colors range from violet (shortest wavelength) through indigo, blue, green, yellow, orange and red (ROYGBIV).

The visible spectrum is also viewed as being composed of three equal-wavelength segments that represent the additive primary colors;

Blue (0.4 to 0.5 micrometers)

Green (0.5 to 0.6 micrometers)

Red ( 0.6 to 0.7 micrometers)

Primary Colors

A primary color is one that cannot be made from any other color. All colors perceived by the human optical system can be produced by combining the proper proportions of light representing the three primaries. This principle forms the basis for the operation of the color TV.

The chlorophyll of healthy grass selectively absorbs more of the blue and red wavelengths of white light and reflects relatively more of the green wavelengths to our eyes.

Infrared (IR) Band

The infrared (IR) band has wavelengths between red visible light (0.7 micrometers) and microwaves at 1,000 micrometers. Infrared means “below the red.”

In remote sensing the IR band is usually divided into two components that are based on basic property differences;

Reflected IR band

Emitted/Thermal IR band

Reflected IR

The reflected IR band represents reflected solar radiation which behaves like visible light. Its wavelength span is from 0.7 to about 3 micrometers.

Thermal IR (Heat)

The dominant type of energy in the thermal IR band is heat energy, which is continuously emitted by the atmosphere and all objects on the earth’s surface. Its wavelength span is from about 3 micrometers to 1,000 micrometers or 0.1 centimeters.

Microwave Band

The microwave band falls between the IR and radio bands and has a wavelength range extending from approximately 0.1 centimeters to 1 meter.

Microwave Band

At the proper wavelengths microwave radiation can pass through;

- clouds

- precipitation

- tree canopies

- dry surficial deposits such as;

- sand and

- fine-grained alluvium

Microwave Sensors

Passive Microwave - detect natural microwave radiation that is emitted from the earth’s surface.

RADAR - propagates artificial microwave radiation to the surface and detects the reflected component.

Solar and Terrestrial Radiation

Most remote sensing systems are designed to detect;

solar radiation which passes through the atmosphere and is reflected in varying degrees by the earth’s surface features.

terrestrial radiation which is continuously emitted by these same features.

99% of the sun’s radiation falls between wavelengths of 0.2 and 5.6 micrometers.

80% is contained in wavelengths between 0.4 and 1.5 micrometers (visible and reflected IR), to which the atmosphere is quite transparent.

Maximum radiation occurs at a wavelength of 0.48 micrometers in the visible band.

About half the solar radiation passes through the earth’s atmosphere and is absorbed in varying degrees by surface features of the earth.

Most of this absorbed radiation is transformed into low-temperature heat (warming the surface), which is continuously emitted back into the atmosphere at longer thermal IR wavelengths.

The earth’s land and water surface has an ambient

temperature of about 300oK (80oF)

Because the wavelengths covering most of the earth’s energy output are several times longer than those covering most of the solar output, terrestrial radiation is frequently called longwave radiation and solar radiation is termed shortwave radiation.

Longwave radiation is also emitted by;

- the atmosphere’s gasses and clouds and

- from artificially heated objects on the earth’s surface such as

- from buildings

- steam lines

- certain industrial effluents.

EMR manifests itself only through its interactions with matter which can be in the form of;

a liquid

a solid

Radiation-Matter Interactions

When EMR strikes matter, EMR may be;transmitted

reflected

scattered

absorbed

The amount on interaction depends upon;the composition and physical properties of the medium.

the wavelength or frequency of the incident radiation.

the angle at which the incident radiation strikes a surface.

Transmission

Transmission is the process by which incident radiation passes through matter without measurable attenuation. The substance is thus transparent to the radiation.

Transmission

Transmission through material media of different densities (such as air to water) causes the radiation to be refracted or deflected from a straight-line path with an accompanying change in its velocity and wavelength; frequency always remains constant.

Reflection

Reflection (also called specular reflection) is the process where incident radiation “bounces off” the surface of the substance in a single, predictable direction.

Reflection

The angle of reflection is always equal and opposite to the angle of incidence.

Reflection is caused by surfaces that are smooth relative to the wavelength of the incident radiation. These smooth mirror-like surfaces are called specular reflectors.

Specular reflection causes no change to either EMR velocity or wavelength.

Scattering

Scattering (also called diffuse reflection) occurs when incident radiation is dispersed or spread out unpredictable in many different directions, including the direction from which it originated.

Scattering

In the real world, scattering is much more common than reflection.

The scattering process occurs with surfaces that are rough relative to the wavelengths of incident radiation.

Such surfaces are called diffuse reflectors. EMR velocity and wavelength are not affected by the scattering process.

Absorption

Absorption is the process by which incident radiation is taken in by the medium. For this to occur, the substance must be opaque to the incident radiation.

EMR - Atmosphere Interactions

Areas of the spectrum where specific wavelengths can pass relatively unimpeded through the atmosphere are called transmission bands or atmospheric windows.

Absorption bands define those areas where specific wavelengths are totally or partially blocked.

To observe the earth’s surface different remote sensing instruments have been designed to operate within the windows where the atmosphere will transmit sufficient radiation for detection.

EMR interacts with the atmosphere in the following ways;

it may be absorbed and re-radiated at longer wavelengths, which causes the air temperature to rise.

it may be reflected and scattered without change to either its velocity or wavelength.

it may be transmitted in a straight-line path directly through the atmosphere.

Atmospheric Absorption and Transmission

Significant absorbers of EMR in the atmosphere;

oxygen

nitrogen

carbon dioxide

water vapor

Atmospheric Absorption and Transmission

Atmospheric Scattering

EMR within certain sections of the UV, visible and reflected IR bands is scattered by the atmosphere.

Important scattering agents include;

gas molecules

suspended particulates

clouds

In addition, clouds absorb most of the longwave radiation emitted by the earth’s surface, essentially closing the thermal IR windows.

This is why cloudy nights tend to be warmer than clear nights. Only microwave radiation with wavelengths longer than about 0.9 cm is capable of penetrating clouds.

Important scattering agents include;gas molecules

suspended particulates

clouds

There are three types of atmospheric scattering important to remote sensing;

Rayleigh or molecular scattering

Mie or non-molecular scattering

Non-selective scattering

Rayleigh or Molecular Scattering

primarily caused by oxygen and nitrogen molecules whose diameters are, at least, 0.1 times smaller than the affected wavelengths.

Rayleigh scattering is highly selective being inversely proportional to the fourth power of the wavelength.

occurs when there are sufficient particles in the atmosphere that have diameters from about 0.1 to about 10 times larger than the wavelengths under consideration.

Important Mie scattering agents include;water vapor

volcanic materials

salt from evaporated sea spray

is found in the lower atmosphere when there are sufficient numbers of suspended aerosols having diameters at least 10 times larger than the wavelengths under consideration.

Important nonscattering agents include;

larger Mie particles

water droplets

ice crystals

depends upon wavelength. Within the visible band, colorless water droplets and ice crystals scatter all wavelengths equally well, causing, for example, the sunlit surfaces of clouds to appear brilliant white.

Skylight and Haze

The clear sky is a source of illumination because its gasses preferentially scatter the shorter wavelengths of sunlight.

This diffuse radiation is called sunlight or sky radiation.

Skylight and Haze

To our eyes sky radiation is manifested as haze which causes a reduction in visibility and also causes distant landscapes to take on a soft, blue-gray appearance.

Atmospheric haze has important ramifications in remote sensing.

Skylight and Haze

In the short wavelength region, radiation reaching an airborne or spaceborne sensor consists of two components;

radiation that is scattered by the earth’s surface and then reaches the sensor without being affected by the intervening atmosphere.radiation that is scattered by the atmosphere, either before or after it reaches the earth’s surface.

Skylight and Haze

The radiation scattered by the atmosphere contains no information about the earth’s surface, and it acts as a masking agent when a remote sensing system records these wavelengths.

Skylight and Haze

The net effect of this extra illumination, or non-image forming “haze light”, is a loss of detail and a reduction in scene contrast.

Haze is visualized as a fog-like veil in black and white photos and as an overall blueish tint in a color photo.

EMR - Surface Interactions

The natural and cultural features of the earth’s surface interact differently with solar radiation.

Albedo or Spectral Reflectance is the percentage radiation reflected by an object.

EMR - Surface Interactions

Spectral Signatures

Every natural and synthetic object reflects and emits EMR over a range of wavelengths in its own characteristic manner according , in large measure, to its chemical composition and physical state.

Spectral Signatures

Spectral signatures are the distinctive reflectance and emittance properties of objects.

Within some limited spectral region, a particular object will usually exhibit a unique spectral response pattern that differs from that of other objects.

Spectral Signatures

Remote sensing depends upon operation in wavelength regions of the spectrum where these detectable differences in reflected and emitted radiation occur.

Spectral Signatures

The diagnostic response patterns of that make it possible to discriminate objects (spectral signatures) often lie beyond the narrow confines of the visible spectrum where no detectable differences occur.

Spectral Signatures

Detectors translate the sensed radiation into electrical energy which is used to drive invisible-to-visible translation devices.

Spectral Signatures

Radiometer measurements are used to prepare spectral signature curves which are line plots showing the radiation intensity for various features as a function of wavelength.

Here are typical spectral signature curves for three common materials; vegetation, soil and water.

Spectral Signatures

LANDSAT Imagery

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