The surface of the Earth is always changing. Some changes like earthquakes, volcanoes, floods, and landslides happen quickly and other changes, such as most erosional processes, happen slowly over time. It’s often hard to see these changes from ground level. A much broader view is needed, and multiple views that provide a record of change over time are especially helpful. Earthshots shows you how satellite data are used to track these changes.
The Landsat series of Earth-observing satellites has acquired data for monitoring the planet’s landmasses since 1972. The vast archive containing millions of Landsat scenes is managed at the U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center in Sioux Falls, SD. The images displayed in Earthshots are examples of Landsat data that help scientists worldwide understand more about how both people and nature are changing the landscape.
Each Earthshots page features a different location from around the world and explains the changes that the satellite images reveal. For example, the Mount St. Helens page shows what the mountain looked like before and after the 1980 eruption. Furthermore, it shows recent images that demonstrate how the forest is recovering. The images at the right are three of the Landsat images from that page.
Remote sensing means observing something from a distance. Satellites observe the Earth from space and help scientists study large tracts of land and how that land changes over time.
The sensors onboard the Landsat satellites use reflected light to detect electromagnetic energy on the Earth’s surface. The level of energy is represented by the electromagnetic spectrum, which is the range of energy that comes from the Sun. The light from the Sun that we can see is only a small part of the electromagnetic spectrum and includes the colors of the rainbow. Satellite sensors record this information in different portions of the electromagnetic spectrum, which is measured in wavelengths. Landsat satellite sensors detect both visible and infrared light.
When satellite images are made, these “invisible” types of light are assigned visible colors to represent them so that our eyes can see the data.
Since 1972, the Landsat satellites have been imaging Earth’s land areas. Landsat represents the world’s longest continuously acquired collection of space-based moderate resolution land remote sensing data. Landsat imagery provides a unique resource for those who work in agriculture, geology, forestry, regional planning, education, mapping, and global change research. Landsat images are also invaluable for emergency response and disaster relief.
Landsat satellites image the Earth’s surface along the satellite’s ground track in a 185-kilometer wide (115-mile wide) swath as the satellite moves in a descending orbit (moving from north to south) over the sunlit side of the Earth.
The two satellites still in operation, Landsats 7 and 8, are in a polar orbit 705 kilometers (438 miles) above the Earth’s surface. They complete one orbit every 99 minutes, for 14 ½ orbits per day. This means it takes 16 days for each satellite to obtain imagery over the entire globe.
The images you see in Earthshots span the Landsat archive. The images from Landsats 1–3 used a sensor called the Multispectral Scanner (MSS). Its image resolution, the size of the smallest region on the Earth’s surface (known as a “pixel”) that can be observed by the instrument, was about 80 meters. On Landsats 4–5, the improved Thematic Mapper (TM) sensor provided a resolution of 30 meters. The Enhanced Thematic Mapper Plus (ETM+) onboard Landsat 7 and the Operational Land Imager (OLI) on Landsat 8 also provide 30-meter resolution. The images of Las Vegas, Nevada, show the difference between these two resolutions.
|Landsat 1||July 23, 1972||January 6 , 1978|
|January 22, 1975||February 25, 1982|
|March 5, 1978||March 31, 1983|
|July 16, 1982||June 15, 2001|
|Did not achieve orbit|
|April 15, 1999||Still operating|
|February 11, 2013||Still operating|
|September 27, 2021||In commissioning phase|
All of the colors in a satellite image, similar to images on a TV or computer monitor, are made up of a combination of red, green, and blue light, or RGB for short. The sensors capture these images in grayscale—each pixel is assigned a value of brightness on a scale from 0 to 255. Black is 0, and white is 255, and shades of gray are in between. These grayscale images are assigned the color red, green, or blue, which display the brightness of each of these colors. When we combine the three images, we get a false color image. With all of the possible combinations of red, green, and blue values, this provides for a display system capable of providing over a million different colors.
Each image shows a specific section of the electromagnetic spectrum, called a band. Landsats 1–3 collected data in four different bands. Landsat 5 collected seven different bands, and Landsat 7 collects eight different bands. The new Landsat 8 collects 11 different bands.
Three of the bands are combined to form an image, each band assigned as red, green, or blue (in that order) to produce natural color, false color, or color-infrared images as demonstrated in the Mt. Vernon, Washington, images.
Canada Centre for Remote Sensing, 2016, Tutorial—Fundamentals of Remote Sensing: Natural Resources Canada, accessed April 10, 2018, at http://www.nrcan.gc.ca/earth-sciences/geography-boundary/remote-sensing/fundamentals/1430.
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U.S. Geological Survey, 2016, Landsat—Earth observation satellites (ver. 1.1, August 2016): U.S. Geological Survey Fact Sheet 2015–3081, 4 p., http://dx.doi.org/10.3133/fs20153081.
U.S. Geological Survey, 2016, Landsat International Cooperators and Global Archive Consolidation (ver. 1.1, November 2016): U.S. Geological Survey Fact Sheet 2016–3018, 2 p., http://doi.dx.org/10.3133/fs20163018.
U.S. Geological Survey, 2018, Landsat Missions: USGS, accessed April 10, 2018, at http://landsat.usgs.gov.
U.S. Geological Survey, 2016, Tracking change over time (ver. 2.0, March 2016): U.S. Geological Survey General Information Product 133, variously paged, https://dx.doi.org/10.3133/gip133.