GIS Final Remote Sensing

Remote sensing

• Measuring properties of an object without making physical contact with the object

• Collecting information related to the reflected/emitted electromagnetic energy from a target by a device a considerable distance away from that target from an aircraft/spacecraft

Passive systems

Record signals and energy already in the environment

Active systems

Generate and record their own pulses

Examples of passive systems

• Aerial photography

• Satellite images

• IR images

Examples of active systems

• RADAR (radio detection and ranging)

• LiDAR (light detection and ranging)

How do active systems work?

By creating their own signal, they can time the signals return, and generate range (distance information)

Why are different wavelengths important?

• Objects appear whatever color they emit, or the inverse of the color they absorb (plants use red light and appear green)

• Some wavelengths aren’t visible to humans but are still important

Different wavelengths of electromagnetic radiation

• Ionizing radiation

• Ultraviolet radiation

• Visible light

• Infrared radiation

• Microwaves and radio waves

Ionizing radiation

• X-rays, gamma rays, etc…

• Ability to change biological tissue

• Mostly absorbed by ozone

• Not usually useful for remote sensing

Ultraviolet radiation

• 3nm-400nm

• Just smaller than visible wavelengths

• Usually not useful for remote sensing du to atmospheric absorption

Visible light

• Red, orange, yellow, green, blue, violet (400nm-700nm)

• Shorter wavelengths (blue/violet) are more strongly attenuated (reflected/scattered) by the atmosphere, making them less useful in remote sensing

• Panchromatic: Measures all wavelengths at once (image based on the value of all emitted radiation)

Infrared radiation

• Broad spectral range (700nm-1,000,000nm)

• Not visible, but can be photographed

• Extremely useful to study land cover/vegetation

Microwaves and radio waves

• 1nm-1m (usually described by frequency instead of wavelength)

All electromagnetic radiation passes through the atmosphere and is either…

• Scattered prior to reaching the target

• Transmitted through the target

• Absorbed by the target

• Reflected by the target

Relationship between path length and atmospheric interference

The longer the path from target to sensor, the greater potential atmospheric affects (only an issue for satellites)

Spectral signature

A collection of wavelengths (band) that indicates a certain kind of object/substance being represented by a pixel

Spectral reflectance

• A measure of brightness stored in pixels represented through brightness value (BV) or digital number (DN)

• Changes in this value between bands gives clues to what kind substances pixels represent

Band

A collection of wavelengths

Multispectral

Covering multiple bands

Multispectral images

• Multiple bands stacked together used in tandem with red/green/blue color guns (RGB) to assign each color to an image

• The colors we see depend on how we assign colors in a software to create color composites

CIR (color infrared) renderings

• Specific type of way to view images

• Infrared on red, red on green, green on blue, and the blue band on nothing

• Provides maximum contrast and high detail, particularly for vegetation (healthy vs dead)

Aerial imagery

• Common

• Relatively inexpensive

• Easily interpreted

• Covers smaller area

• Larger scale

• Subject to terrain/tilt error (scales often inaccurate as a result)

Satellite imagery

• Large area coverage

• Covers greater spectral range

• Repeated coverage to detect changes

• Easier to georeference (less terrain/tilt error)

• Resolution is coarser than aerial

Measuring electromagnetic radiation

• Gases in Earth’s atmosphere absorb some solar radiation

• Bands of energy that pass through the atmosphere rather than absorbed are referred to as “atmospheric windows”

What wavelengths of electromagnetic radiation do urban areas emit?

Thermal infrared

What wavelengths of electromagnetic radiation do vegetation emit?

Near infrared