Remote Sensing Unit 1

active remote sensing

an electromagnetic ray is emitted, then reflected back to a sensor and recorded

LiDAR

a form of active remote sensing; laser emitter and sensor units are attached to aircraft, which aims lasers at the ground and records the time it takes for the laser to bounce back to convert to point clouds

point cloud

a discrete set of data points in space which may represent a 3D shape or object

ground penetrating radar (GPR)

a form of active sensing; consists of a transmitter, a receiver, and antennae that are used to see below ground without excavating pits

passive remote sensing

sensing that uses the sun’s energy reflecting off the earth to a sensor

satellite digital imagery

a form of passive sensing; landsat and other satellites take images of the Earth’s surface using measurements of actual reflectance values, including both visible and non-visible bands of the EM spectrum

pixel

the smallest area that makes up a component of a digital image; larger pixels = less refined detail = larger area

resolution

the amount of information available in a satellite imagery; higher resolution = smaller pixel = finer detail

multispectral remote sensing

sensing based on the principal that energy from the sun is broken up into multiple components (ie colors/light spectrum); collect data in multiple spectral bands (4-30ish)

wavelength

the distance between successive crests of the wave, typ. in m, cm, mm, etc

frequency

the number oc oscillations completed per second measured in hertz (Hz)

gamma rays

>ten-trillionths of a meter wavelength w/ extreme penetrating capabilities generated by radioactive atoms

x-ray

10-billionths of a meter to 10-trillionths of a meter wavelength w/ great penetrating power and extensive medical applications

ultraviolet rays

wvl of 400-billionth to 10-billions of a meter that can burn skin and are largely blocked by the ozone layer

visible rays

400-700 nm wavelength, best for identifying objects by visible colors

infrared rays

wavelengths extend from visible range to 1mm, including thermal radiation, with applications in locating thermal pollution and crop health

microwave radiation

range from 1mm to 30cm used for heating food and radar weather applications

radio waves

range from >1 cm to tens or hundreds of meters, used to transmit radio signals and create images through darkness or clouds

spectral irradiance

the radiant flux received by a surface per unit area per unit of frequency or wavelength

spectral irradiance

come back

transmission

the movement of light through a material; length dependent

reflection

the process of radiation bouncing off an object; angle at which radiation hits and obect = radiation is reflected at the same angle

absorption

EM energy is absorbed and converted int another form of energy; tends to occur in absorption bands

scattering

the unpredictable reflection of radiation by atmospheric particles; largely occurs high in the atmosphere

refraction

the bending of light when it passes from one medium to another; dependent on angle with vertical, distance through mediums, and air density

diffuse reflection

combined diffuse and partial specular possible depending on the surface

apparent optical property (AOPs)

a property that can be explained as a function of two inherent optical property (e.g. reflectance (R = scattering/(absorption+scattering)))

inherent optical properties (IOPs)

cannot be explained as a function of other properties (e.g. absorption and scattering)

ground-based platform

hand-held of mounted on tripods, towers, etc

airborne platforms

most airplanes with camera or digital sensors attached

RGB

3-band systems

RGBN

traditional Landsat bands

superspectral

covers the entire spectral range

hyperspectral

covers many bands (typ. over 100)

stereogram

two of more overlapping sets of air photos properly positioned for viewing with a stereoscope

aerial photo striping

occurs from sensor malfunction, calibration errors , or instrument design

aerial photo saturation

too much light reaches the detector as a result of a long integration time or shutter aperture that is too wide

scan line correctors (SLC’s)

a mechanism placed behind the primary optics designed to compensate for the forward motion that creates a zigzag pattern while sideways scanning, stitching together a rectilinear image

spatial resolution

distance between independent measurements, or the physical dimension that represents a pixel of the image

temporal resolution

the amount of time it takes a satellite to revisit a particular location on the Earth’s surface; related to orbit pattern, swath width, speed of satellite, and orbital overlap; higher = imaged more frequently

Instantaneous Field-Of-View (IFOV)

the area on the Earth’s surface recorded by a single sensor element at any moment in time; size sets the pixel width

Ground Sample Distance (GSD)

distance on ground determining one dimension of a pixel, which is affected by flight speed; shoudl be set to match the IFOV

low-Earth orbit satellites

orbit roughly pole to pole while the Earth spins underneath, continually scanning a thin ribbon of data

geostationary satellites

in sync with the orbit of the Earth and can look at the same spot so long as they remain in orbit

metadata

information about the data itself; Landsat metadata typ includes date taken, GPS coordinates, path/row, name of sensor, cloudiness level, and other facts about the digital image

objective image interpretation

a type of interpretation that usually involves numbers

subjective image interpretation

interpretation involving elements of recognition, including knowledge of subject matter

site

the location of an object in relation to its environment

color stimulus calculation

light source x Object Diffuse Spectral Reflectance = color stimulus (aka spectral power distribution)

CIE 1979 Color Gamut

provides a definition of human color vision based on the tristimulus response function of the human eye

bit depth

how many levels a pixel can hold

histogram equalization

spread the range of brightness values but preserves the peaks and valleys in the histogram, allowing for higher contrast

primary color theory

remote sensing is energy we cannot see being reflected and recorded; to represent that energy, we must use colors we can see

additive primary theory

based on the wavelengths of light and on the assumption that we always start with black then add the primary colors of RBG to make all other colors; to create white, all light is reflected back to the eye

subtractive color theory (aka CYMK model)

based on the light absorbing quality of ink printed onto paper; white light strikes the inks, is partially absorbed and partially reflected; a perfect mix of cyan, magenta, and yellow absorbs all light resulting in black