Low Impact Development Center

Anacostia Remote Sensing Background Information

The completion of this project will succeed in providing a pathway for the exposure to and use of high technology solutions by Bay citizens' groups, governments, and stakeholders. As part of this effort, a brief remote sensing primer is included. After some basic definitions, a description of the Landsat satellite system is provided.

Canada Centre for Remote Sensing (CCRS) Graphic

More detailed information can be found on the websites listed below. These were the primary sources for the information and graphics used here.

1) Canada Centre for Remote Sensing (CCRS) on-line tutorials
http://ccrs.nrcan.gc.ca/resource/index_e.php

2) NASA's on-line educational resources
http://gcmd.nasa.gov/records/remote_sensing_tutorial-00.html

Earth Remote Sensing

The technology of acquiring data and information about the Earth and its environment from a distance without physical contact. This is done by sensing and recording reflected or emitted energy and then processing, analyzing and applying the information.

Satellite

A free-flying object that orbits the Earth, another planet, or the sun. The moon is a natural satellite, whereas man-made satellites include those platforms launched for remote sensing, communication, and telemetry (location and navigation) purposes.

Near-polar Orbit

Near-polar Orbit Graphic

source: CCRS

A remote sensing platform designed to follow a north-south orbit which, in conjunction with the Earth's west-east rotation, allows coverage of most of the Earth's surface over a certain period of time.

Swath

source: CCRS

The area observed by a satellite as it orbits the earth. Swaths for spaceborne sensors are generally between tens and hundreds of kilometers wide. As the satellite orbits the Earth from pole to pole, its east-west position doesn't change. However, as seen from the Earth, the satellite appears to be shifting westward because the Earth is rotating beneath it. This apparent movement allows the satellite swath to cover a new area with each consecutive pass.

Passive Sensors

Remote sensing systems that measure naturally available energy. In contrast, radar, which is an active sensor, transmits its own energy to the object being detected and waits for the return signal.

Electromagnetic Spectrum

Electromagnetic Spectrum Graphic

source: NASA

Light is a form of electromagnetic radiation. Other forms include radio waves, microwaves, infrared radiation, ultraviolet rays, X-rays, and gamma rays. These differ by their wavelength, which is directly related to the amount of energy being carried. The shorter the wavelength of the radiation, the higher the energy. All of these wavelengths are known collectively as the electromagnetic spectrum. Visible light represents only a very small portion of this spectrum. There is a lot of radiation around us which is invisible to our eyes but can be detected by other remote sensing instruments and used to our advantage.

Infrared (IR) Radiation

Can be divided into the reflected IR and the emitted or thermal IR. Radiation in the reflected IR region is very similar to visible energy. Vegetation cover and condition can be inferred from remotely sensed visible and near-infrared signals. In the longer wavelength far-infrared region of the spectrum, emissions from the Earth's atmosphere and surface offer information about atmospheric and surface temperatures.

Atmospheric Window

Atmospheric gases, such as water vapor and carbon dioxide, absorb electromagnetic energy in specific regions of the spectrum, influencing at what wavelengths a satellite is able to detect energy from the Earth's surface. The atmospheric windows are those areas of the spectrum that are least influenced by atmospheric absorption and thus most useful to earth remote sensors.

Band

A relatively narrow region of the electromagnetic spectrum in which a remote sensor is designed to detect energy.

Spectral Resolution

The ability of a sensor to define fine wavelength intervals. The finer the spectral resolution, the narrower the wavelength range for a particular band. For example, black and white film records wavelengths extending over much, or all, of the visible portion of the electromagnetic spectrum. Its spectral resolution is fairly coarse, as the various wavelengths of the visible spectrum are not individually distinguished.

Multi-spectral Sensors

A remote sensing system that records energy over several separate wavelength intervals at various spectral resolutions. Landsat is a multi-spectral sensor; its bands are described below.

Spectral Response

Determined by measuring the energy that is reflected or emitted by targets on the Earth's surface over a variety of different wavelengths. Response patterns of different features can be compared in order to distinguish between and classify objects.

Pixel

source: CCRS

"Picture element" - a matrix of these composes most remote sensing images. Pixels are normally square and represent a certain area of the earth's surface in digital format. Each pixel actually contains each band's "brightness value" for the radiation detected from the particular area.

Spatial Resolution

The area represented by each pixel in an image. The smaller the area represented, the more detailed the image. Images where you can only discern large features have coarse or low resolution. In fine or high resolution images, small objects can be detected. Military sensors for example, are designed to view as much detail as possible, and therefore have very fine resolution. Commercial satellites provide imagery with resolutions varying from a few meters to several kilometers.

http://landsat.usgs.gov/

Landsat TM and ETM will serve as the primary satellite sensors used in this study. The Thematic Mapper (TM) sensor was first launched on Landsat 4 in 1982. The most recent Landsat - Landsat 7, launched in 1999 - carries the Enhanced Thematic Mapper (ETM) sensor as a joint venture between USGS and NASA. This 8-band multi-spectral sensor improves upon the TM by providing a panchromatic band with 15-meter spatial resolution (essentially a black and white aerial photograph) and increased spatial resolution for the thermal infrared channel. The pixel sizes are 49 feet (15 meters) in the panchromatic band; 98 feet (30 meters) in the 6 visible, near and short-wave infrared bands; and 197 feet (60 meters) in the thermal infrared band. The LANDSAT World-Wide-Reference system catalogues the world's landmass into 57,784 scenes, each 115 miles (183 kilometers) wide by 106 miles (170 kilometers) long. This instrument provides powerful capability for monitoring and studying water bodies, vegetation, soils, urban heat islands, riparian buffers, urban development, etc.

Band Number	Spectral Range (microns)	Ground Resolution (m)	Application
1	.45 to .52 (blue)	30	soil/vegetation discrimination; bathymetry/coastal mapping; cultural/urban feature identification
2	.52 to .60 (green)	30	green vegetation mapping (measures reflectance peak); cultural/urban feature identification
3	.63 to .69 (red)	30	vegetated vs. non-vegetated and plant species discrimination (plant chlorophyll absorption); cultural/urban feature identification
4	.75 to .90 (near-IR)	30	identification of plant/vegetation types, health, and biomass content; water body delineation; soil moisture
5	1.55 to 1.75 (shortwave IR)	30	sensitive to moisture in soil and vegetation; discriminating snow and cloud-covered areas
6	10.40 to 12.5 (thermal IR)	60	vegetation stress and soil moisture discrimination related to thermal radiation; thermal mapping (urban, water)
7	2.09 to 2.35 (shortwave IR)	30	discrimination of mineral and rock types; sensitive to vegetation moisture content
Pan	.52 to .90 (visible)	15	basic mapping

adapted from CCRS