This series of maps shows the location of edited content in the world’s largest collaborative mapping project: OpenStreetMap.
The maps use OpenStreetMap data downloaded from GeoFabrik.de on December 12th, 2013. Each sub-region extract has been parsed and for each node (i.e., elements used in OpenStreetMap to represent any point feature), the coordinates, version, and last update values have been selected.
The first map was created by counting the number of nodes for each cell in a grid of 0.1 degrees of latitude per 0.1 degrees of longitude. The second map instead focuses on edits by summing the version numbers of all nodes in a cell (as this number is increased by one each time a node is modified), resulting in a count of all edits for the whole history of OpenStreetMap. The third map focuses on the age of content, and so records the latest update made to a node for each cell of the grid.
The first map offers a revealing picture of the presence of thick layers of content that annotate a few parts of the world, and a relative absence of content over much of the rest of the planet. The glowing centres of content in parts of North America, Europe, Oceania, and Japan, in many ways, parallels the visual intensity of lights in NASA’s Earth City Lights series.
The United States account for the largest total amount of content, collecting 21% of all nodes present in OpenStreetmap (OSM), followed by France, Canada, Germany and Russia, all counting more than 100 million nodes. These five countries alone collect 58% of the content, and high-income OEDC countries sum up to about 80% of OSM.
The Netherlands enjoy the highest density of content, with an average of over 1000 nodes per square kilometre, followed by Belgium with over 700 nodes per square kilometre, and Germany, the Czech Republic, Switzerland, and France, with about 400 nodes per square kilometre.
In contrast to the brightness of the Europe, the southern hemisphere is barely visible, as the amount of content available on OSM about that part of the world is far lower than in the northern hemisphere, with Africa and Latin America represented by less than 5% of the content. California alone accounts for almost as much content as the entirety of Africa.
Turkey and the western part of the Middle East are visible, but already fading into a less intense color. The emerging powers of Brazil, India, and China appear to be suffering from wide-spread content “blackout”, where only the largest urban centres are visible. Brazil accounts for fewer nodes than Switzerland, and China for even fewer. The same applies to most of the remaining parts of Africa, Asia, and Latin America. One of the oldest urbanized areas of the world, an amazing strip of lights that follows the course of the Nile, is barely visible. In fact, Egypt accounts for as many nodes as Iceland, despite being 10 times as big and accounting for 250 times the population.
Interestingly, content in parts of North Korea lights up the map: an unusual situation for a country not renowned for even appearing in most indices of online participation. This is most likely thanks to work done in 2011 by the OSM developers community. We see a similar situation in Newfoundland and Labrador: with large swaths of sparsely populated land characterised by relatively dense amounts of content. The Canadian case is likely a result of a detailed physical geography dataset that was bulk-uploaded to OpenStreetMap.
Several studies have been conducted on the quality of OSM’s coverage in these areas (e.g., see the paper by Haklay et al, 2010) where high-quality data from government agencies are also available for comparison. However, it has to be noted that these are the same countries where Open Data policies have spread, allowing lots of data to be uploaded to OSM. In fact, the visible distribution of content is not too different from the map of the GeoNames gazetteer project we published some months ago.
The second map below illustrates the number of edits made to OpenStreetMap. Unsurprisingly, the most content-dense areas are also the most heavily edited, because each new node included means one more edit made within the related area. However, statistical analysis suggests that the United States and Germany account for far more edits than would be expected given the related content in OSM, whereas content from Italy and Netherlands is far less edited than expected. In most parts of the rest of the world the number of edits is simply related to the number of objects in a given area.
The third and last map presents an illustration of the most and least recently updated areas in OSM, similarly to a map included in the recent Mapbox’s 2013 OpenStreetMap Data Report.
It is not surprising that most areas in Europe have seen at least one edit in the week before the data were collected. Similarly, it is evident how the most remote regions of the world have not been updated for years, from Siberia to the Australian Outback, from central Africa to the Amazon basin and northern Canada.
While most of the map shows a random mix of data, due to the volunteer-based nature of the projects, there are some evident areas of plain colour, which might indicate bulk uploading of new data and datasets from government agencies or companies. An examples can be found in Iraq, where most of the country has been updated between September and November 2013; in Australia, where large areas in South Australia have been recently updated, and the updates clearly follow the state borders with New South Wales and Victoria states; and in Estonia, which has also received recent edits for most of its territory.
OSM will turn 10 years old in a few months, and combining the findings obtained from these three maps, it is evident how it is a very good geographical representation of the most developed countries, and their urban environment. OSM also provide large amount of information about non-rural areas, although these are not as up-to-date and detailed as urban areas.
The quantity and the quality of the data make OSM one of the most powerful and exciting open-source projects that the Internet has facilitated in recent years, along with Linux and Wikipedia. Nonetheless, there is still a lot of work to do, and the development of the project in its second decade will probably depend on it attracting new volunteers among the new Internet users in Africa, Asia, Latin America, and the Middle East. Finally, OSM will be influenced by the relationships with those many companies which are currently based their mapping services on it, as well as the future spread of open data policies.
Apr 18, 2012
City Lights of the United States 2012
Old Night Vision Meets New
Korea and the Yellow Sea
City Lights of Australia, or Not
City Lights Illuminate the Nile
The Lights of London
Moon Phases Over the Persian Gulf
Auroras light up the Antarctic night
Marine Layer Clouds off the California Coast
Waves in Airglow
Auroras over North America
Gas Drilling, North Dakota
Monitoring the Arctic during Polar Darkness
Extensive Ice Fractures in the Beaufort Sea
Mustang Complex Fires in Idaho
Train Derailment and Fire, Lac Mégantic, Quebec
City Lights of South America’s Atlantic Coast
Night Lights 2012 – Flat map
Night Lights 2012 – The Black Marble
City Lights of Africa, Europe, and the Middle East
City Lights of Asia and Australia
City Lights of the Americas
South Asian Night Lights
Winter Storm across Central United States
Blackout in New Jersey and New York
Power Outages in Washington, DC Area
Overnight View of Hurricane Sandy
Sandy after Landfall
Gas Flares in Bahía de Campeche
Night View of Hurricane Isaac
Mount Tongariro Erupts
Night View of Fires in Siberia
City Lights, 2003
Earth’s City Lights 1994
Antarctic Ozone Hole
In the early 1980s, scientists began to realize that CFCs were creating a thin spot—a hole—in the ozone layer over Antarctica every spring. This series of satellite images shows the ozone hole on the day of its maximum depth each year from 1979 through 2010.
Water Level in Lake Powell
Combined with human demands, a multi-year drought in the Upper Colorado River Basin caused a dramatic drop in the Colorado River’s Lake Powell in the early part of the 2000s. The lake began to recover in the latter part of the decade, but as of 2012, it was still well below capacity.
The shoreline of Cape Cod provides a visual case study in the evolution and dynamic motion of barrier islands and spits.
Antarctic Sea Ice
Because of differences in geography and climate, Antarctica sea ice extent is larger than the Arctic’s in winter and smaller in summer. Since 1979, Antarctica’s sea ice has increased slightly, but year-to-year fluctuations are large.
Arctic Sea Ice
NASA satellites have monitored Arctic sea ice since 1978. Starting in 2002, they observed a sharp decline in sea ice extent.
The state of Rondônia in western Brazil is one of the most deforested parts of the Amazon. This series shows deforestation on the frontier in the northwestern part of the state between 2000 and 2012.
Mountaintop Mining, West Virginia
Based on data from the Landsat satellites, these natural-color images document the growth of the Hobet mine in Boone County, West Virginia, as it expands from ridge to ridge between 1984 and 2013.
Shrinking Aral Sea
A massive irrigation project has devastated the Aral Sea over the past 50 years. These images show the decline of the Southern Aral Sea in the past decade, as well as the first steps of recovery in the Northern Aral Sea.
Recovery at Mt. St. Helens
The devastation of the May 1980 eruption of Mt. St. Helens and the gradual recovery of the surrounding landscape is documented in this series of satellite images from 1979—2013.
Fire in Etosha National Park
Prescribed fires should prevent blazes from raging out of control in one of Namibia’s most prized wildlife preserves.
Green Seasons of Maine
Not many places on Earth have year-round greenery and four distinct seasons. The images in this series show the four seasons of Maine, the most forest-covered state in the U.S.A.
Columbia Glacier, Alaska
Since 1980, the volume of this glacier that spills into the Prince William Sound has shrunk by half. Climate change may have nudged the process along, but mechanical forces have played the largest role in the ice loss.
Drought Cycles in Australia
Droughts have taken a severe toll on croplands in Southeast Australia during the past decade.
Athabasca Oil Sands
The Athabasca Oil Sands are at once a source of oil, of economic growth, and of environmental concern. This series of images shows the growth of surface mines around the Athabasca River from 1984 to the present.
Burn Recovery in Yellowstone
In 1988, wildfires raced through Yellowstone National Park, consuming hundreds of thousands of acres. This series of Landsat images tracks the landscape’s slow recovery through 2011.
This collection features the strongest hurricane, cyclone, or typhoon (from any ocean) during each of the past 11 years, including storms both infamous and obscure.
Seasons of the Indus River
Fed by glaciers in the Himalayas and Karakorams mountains — and by monsoon rains — the Indus River experiences substantial fluctuations every year. Because the river irrigates 18 million hectares of farmland, the landscape changes along with the river.
Urbanization of Dubai
To expand the possibilities for beachfront tourist development, Dubai, part of the United Arab Emirates, undertook a massive engineering project to create hundreds of artificial islands along its Persian Gulf coastline.
The world is getting warmer, whatever the cause. According to an analysis by NASA scientists, the average global temperature has increased by about 0.8°Celsius (1.4° Fahrenheit) since 1880. Two-thirds of the warming has occurred since 1975.
Seasons of Lake Tahoe
Perhaps the most familiar change in our changing world is the annual swing of the seasons. This series of images shows the effects of the seasons on the Lake Tahoe region between 2009 and 2010.
Images of sunspots and UV brightness document the 11-year cycle of solar magnetic activity. The series spans 1999–2010, capturing the most recent solar maximum and minimum, as well as the emergence of solar cycle 24.
Larsen-B Ice Shelf
In early 2002, scientists monitoring daily satellite images of the Antarctic Peninsula watched in amazement as almost the entire Larsen B Ice Shelf splintered and collapsed in just over one month. They had never witnessed such a large area disintegrate so rapidly.
In the years following the Second Gulf War, Iraqi residents began reclaiming the country’s nearly decimated Mesopotamian marshes. This series of images documents the transformation of the fabled landscape between 2000 and 2010.
Yellow River Delta
Once free to wander up and down the coast of the North China Plain, the Yellow River Delta has been shaped by levees, canals, and jetties in recent decades.
El Niño, La Niña, and Rainfall
For many people, El Niño and La Niña mean floods or drought, but the events are actually a warming or cooling of the eastern Pacific Ocean that impacts rainfall. These sea surface temperature and rainfall anomaly images show the direct correlation between ocean temperatures and rainfall during El Niño and La Niña events.
Its very normal to arise question your inside, “why the forest is red and sky is blue in satellite images?” Chances are, you have a camera near you as you read this—in the smart phone in your pocket or on the tablet or computer you’re using to view this blog. Some of you might have a 35 mm film or digital camera nearby. And at some point this week, you probably looked through photos posted by friends or even strangers on the Internet. In our photo-saturated world, it’s natural to think of the images on the Earth Observatory as snapshots from space. But most aren’t.
Though they may look similar, photographs and satellite images are fundamentally different. A photograph is made when light is focused and captured on a light-sensitive surface (such as film or a CCD). A satellite image is created by combining measurements of the intensity of certain wavelengths of light, both visible and invisible to human eyes.
Why does the difference matter? When we see a photo where the colors are brightened or altered, we think of it as artful (at best) or manipulated (at worst). We also have that bias when we look at satellite images that don’t represent the Earth’s surface as we see it. “That forest is red,” we think, “so the image can’t possibly be real.”
In reality, a red forest is just as real as a dark green one. Satellites collect information beyond what human eyes can see, so images made from other wavelengths of light look unnatural to us. We call these images “false-color,” and to understand what they mean, it’s necessary to understand exactly what a satellite image is.
Infrared light renders the familiar unfamiliar. This infrared photograph shows the forests of Yellowstone National Park from Mount Sheridan. (Photograph courtesy National Park Service.)
Satellite instruments gather an array of information about the Earth. Some of it is visual; some of it is chemical (such as gases in the atmosphere); some of it is physical (sensing topography). In fact, remote sensing scientists and engineers are endlessly creative about what they can measure from space, developing satellites with a wide variety of toolsto tease information out of our planet. Some methods are active, bouncing light or radio waves off the Earth and measuring the energy returned; lidar and radar are good examples. The majority of instruments are passive; that is, they record light reflected or emitted by Earth’s surface.
These observations can be turned into data-based maps that measure everything from plant growth or cloudiness. But data can also become photo-like natural-color images or false color images. This article describes the process used to transform satellite measurements into images.
Seeing the Light
So what does a satellite imager measure to produce an image? It measures light that we see and light that we don’t see. Light is a form of energy—also known as electromagnetic radiation—that travels in waves. All light travels at the same speed, but the waves are not all the same. The distance between the top of each wave—the wavelength—is smaller for high-energy waves and longer for low-energy waves.
Visible light comes in wavelengths of 400 to 700 nanometers, with violet having the shortest wavelengths and red having the longest. Infrared light and radio waves have longer wavelengths and lower energy than visible light, while ultraviolet light, X-rays, and gamma rays have shorter wavelengths and higher energy.
Most of the electromagnetic radiation that matters for Earth-observing satellites comes from the Sun. When sunlight reaches Earth, the energy is absorbed, transmitted, or reflected. (Absorbed energy is later re-emittedas lower-energy radiation.) Every surface or object absorbs, emits, and reflects light uniquely depending on its chemical makeup. Chlorophyll in plants, for example, absorbs red and blue light, but reflects green and infrared; this is why leaves appear green. This unique absorption and reflection pattern is called a spectral signature.
Varied land surfaces have distinct spectral signatures. Fresh basalt lava and asphalt reflect different amounts of infrared light, even though they appear similar in visible light. (Photograph ©2012 Robert Simmon. Figure by Robert Simmon, using data from the USGS Digital Spectral Library.)
Like Earth’s surfaces, gases in the atmosphere also have unique spectral signatures, absorbing some wavelengths of electromagnetic radiation and emitting others. Gases also let a few wavelengths pass through unimpeded. Scientists call these “atmospheric windows” for specific wavelengths, and satellite sensors are often tuned to measure light through these windows.
Atmospheric windows are regions of the spectrum where most light penetrates through the atmosphere, allowing satellites to view the Earth’s surface. (Figure adapted fromCasey et al, 2012.)
Some satellite instruments also directly measure the energy emitted by objects. Everything gives off energy, usually in the form of heat (thermal infrared radiation). The hotter an object is, the shorter the peak wavelength it emits. At about 400°C (750° F)—the temperature of an electric stove burner set to high—the emitted light will begin to be visible. The colder an object is, the longer the peak wavelength it emits.
Turning Wavelength Data Into an Image
Satellite instruments carry many sensors that are each tuned to a narrow range, or “band,” of wavelengths (just red or green light, for instance). Viewing the output from just one band is a bit like looking at the world in shades of gray. The brightest spots are areas that reflect or emit a lot of that wavelength of light, and darker areas reflect or emit little (if any).
To make a satellite image, we choose three bands and represent each in tones of red, green, or blue. Because most visible colors can be created by combining red, green, and blue light, we then combine the red, green, and blue-scale images to get a full-color representation of the world.
A natural or “true-color” image combines actual measurements of red, green, and blue light. The result looks like the world as humans see it. (For tips on understanding true-color images, read How to Interpret a Satellite Image on the Earth Observatory.”)
A false-color image uses at least one non-visible wavelength, though that band is still represented in red, green, or blue. As a result, the colors in the final image may not be what you expect them to be. (For instance, grass isn’t always green.) Such false-color band combinations reveal unique aspects of the land or sky that might not be visible otherwise.
This series of Landsat images of southeastern Florida and the Northern Everglades illustrates why you might want to see the world in false color. (A related animation shows how the images were made.) The visible light image shows dark green forest, light green agriculture, brown wetlands, silver urban areas (the city of Miami), and turquoise offshore reefs and shallows. These colors are similar to what you would see from an airplane.
The second image shows the same scene in green, near infrared, and shortwave infrared light. In this false-color band combination, plant-covered land is bright green, water is black, and bare earth ranges from tan to pink. Urban areas are purple. Newly burned farmland is dark red, while older burns are lighter red. Much of the farmland in this area is used to grow sugar cane. Farmers burn the crop before harvest to remove leaves from the canes. Because burned land looks different in this kind of false-color image, it is possible to see how extensively farmers rely on fire in this region.
This false-color view also reveals how water flows through the Northern Everglades. Green islands punctuate the wetlands, which are black and blue. These are tree islands that are hard to distinguish in natural color. Their orientation aligns with the flow of the water, highlighting direction that is not obvious in the natural color image. It is also easier to see the extent of the wetlands against surrounding land, since water is dark in this view and plant-covered land is bright green.
The third image shows the scene in green, red, and near infrared light. Plants are dark red because they reflect infrared light strongly, and the infrared band is assigned to be red. Plants that are growing quickly reflect more infrared, so they are brighter red. That means that this type of false-color image can help us see how well plants are growing and how densely vegetated an area is. Water is black and blue, and urban areas—including Miami, Fort Lauderdale, and West Palm Beach—are silver.
Observing in Visible Light
Data visualizers and remote sensing scientists make true- or false-color images in order to show the features they are most interested in, and they select the wavelength bands most likely to highlight those features.
Blue light (450 to 490 nanometers) is among the few wavelengths that water reflects (the rest are absorbed). Hence, blue bands are useful for seeing water surface features and for spotting the sea- or lake floor in shallow waters. You can see that water reflects some blue light in the above image of Lake Issyk Kul, Kyrgyzstan. Water is lighter in the blue band than it is in either the red or green bands, though the lake is too deep for shallow features to be visible. Manmade creations like cities and roads also show up well in blue light. It is also the wavelength most scattered by particles and gas molecules in the atmosphere, which is why the sky is blue.
Green light (490 to 580 nanometers) is useful for monitoringphytoplankton in the ocean and plants on land. The chlorophyll in these organisms absorbs red and blue light, but reflects green light. Sediment in water also reflects green light, so a muddy or sandy body of water will look brighter because it is reflecting both blue and green light.
Red light (620 to 780 nanometers) can help distinguish minerals and soils that contain a high concentration of iron or iron oxides, making it valuable for studying geology. In the above image, for example, the exposed ground around Lake Issyk Kul varies in tone from pale tan to orange based on the mineral content of the soil. Since chlorophyll absorbs red light, this band is commonly used to monitor the growth and health of trees, grasses, shrubs, and crops. Red light can also help distinguish between different types of plants on a broad scale.
Observing in Infrared
Near infrared (NIR) light includes wavelengths between 700 and 1,100 nanometers. Water absorbs NIR, so these wavelengths are useful for discerning land-water boundaries that are not obvious in visible light. The below image shows the near infrared view of the Piqiang Fault, China. Stream beds and the wetland in the upper left corner are darker than the surrounding arid landscape because of their water content. (See a natural color view of the scene here.) Plants, on the other hand, reflect near infrared light strongly, and healthy plants reflect more than stressed plants. Finally, near infrared light can penetrate haze, so including this band can help discern the details in a smoky or hazy scene.
Shortwave infrared (SWIR) light includes wavelengths between 1,100 and 3,000 nanometers. Water absorbs shortwave infrared light in three regions: 1,400, 1,900, and 2,400 nanometers. The more water there is, even in soil, the darker the image will appear at these wavelengths. This means SWIR measurements can help scientists estimate how much water is present in plants and soil. Shortwave-infrared bands are also useful for distinguishing between cloud types (water clouds versus ice clouds) and between clouds, snow, and ice, all of which appear white in visible light. Newly burned land reflects strongly in SWIR bands, making them valuable for mapping fire damage. Active fires, lava flows, and other extremely hot features “glow” in the shortwave-infrared part of the spectrum. In the image below, different types of sandstone and limestone make up the mountains around China’s Piqiang Fault. Each rock type reflects shortwave infrared light differently, making it possible to map out geology by comparing reflected SWIR light. Enhancing the subtle differences between the 3 bands of reflected shortwave infrared light used to make this image gives each mineral a distinctive, bright color.
Midwave Infrared (MIR) ranges from 3,000 to 5,000 nanometers and is most often used to study emitted thermal radiation in the dark of night. Midwave infrared energy is also useful in measuring sea surface temperature, clouds, and fires. The images below contrast a visible-lightnighttime view of the Niger River Delta with the same view in midwave infrared; both images are from the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi-NPP satellite. The day-night band shows visible light—the lights of Port Harcourt and Benin City, bright gas flares, and moonlight reflected off clouds. The midwave infrared image shows emitted thermal radiation. The warmer ocean and river are pale, while the cold land and clouds are dark, and the hot gas flares are bright.
Infrared (IR) light—specifically between 6,000 to 7,000 nanometers—iscritical for observing water vapor in the atmosphere. Though water vapor makes up just 1 to 4 percent of the atmosphere, it is an important greenhouse gas. It is also the basis for clouds and rainfall. Water vapor absorbs and re-emits energy in this range, so infrared satellite observations can be used to track water vapor. Such observations are integral to weather observations and forecasts.
Mid-infrared (7µm) and thermal-infrared (12 µm) images showing water vapor (left), and temperature (right). The images are inverted to better show clouds: cold areas are light and warm areas are dark. (NASA/NOAA images by Robert Simmon, using data from theGOES Project Science Team.)
Thermal or longwave infrared (TIR or LWIR) light includes wavelengths between 8,000 and 15,000 nanometers. Most of the energy in this part of the spectrum is emitted (not reflected) by the Earth as heat, so it can be observed both day and night. Thermal infrared radiation can be used to gauge water and land surface temperatures; this makes it particularly useful for geothermal mapping and detection of heat sources like active fires, gas flares, and power plants. Scientists also use TIR to monitor crops. Actively growing plants cool the air above them by releasing water through evapotranspiration, so TIR light helps scientists assess how much water the plants are using.
How to Interpret Common False Color Images
Though there are many possible combinations of wavelength bands, the Earth Observatory typically selects one of four combinations based on the event or feature we want to illustrate. For instance, floods are best viewed in shortwave infrared, near infrared, and green light because muddy water blends with brown land in a natural color image. Shortwave infrared light highlights the difference between clouds, ice, and snow, all of which are white in visible light.
Our four most common false-color band combinations are:
- Near infrared (red), green (blue), red (green). This is a traditional band combination useful in seeing changes in plant health.
- Shortwave infrared (red), near infrared (green), and green (blue), often used to show floods or newly burned land.
- Blue (red), two different shortwave infrared bands (green and blue). We use this to differentiate between snow, ice, and clouds.
- Thermal infrared, usually shown in tones of gray to illustrate temperature.
Near infrared, red, green
One of our most frequently published combinations uses near infrared light as red, red light as green, and green light as blue. In this case, plants reflect near infrared and green light, while absorbing red. Since they reflect more near infrared than green, plant-covered land appears deep red. The signal from plants is so strong that red dominates the false-color view of Algeria below. Denser plant growth is darker red. This band combination is valuable for gauging plant health.
Cities and exposed ground are gray or tan, and clear water is black. In the image below, the water is muddy, and the sediment reflects light. This makes the water look blue. Images from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and from the early Landsats are often shown in this band combination because that’s what the instruments measured.
Shortwave infrared, near infrared, and green
The most common false-color band combination on the Earth Observatory uses the shortwave infrared (shown as red), the near infrared (green), and the green visible band (shown as blue).
Water absorbs all three wavelengths, so it is black in this band combination. In the below false-color image of Algeria, however, water is blue because it is full of sediment. Sediment reflects visible light, which is assigned to look blue in this band combination. This means that both sediment-laden water and saturated soil will appear blue. Because water and wet soil stand out in this band combination, it is valuable formonitoring floods. Saturated soil will also appear blue. Ice clouds, snow, and ice are bright blue, since ice reflects visible light and absorbs infrared. This helps distinguish water from snow and ice; it also distinguishes clouds made up mostly of liquid water or ice crystals.
Newly burned land reflects shortwave infrared light and appears red in this combination. Hot areas like lava flows or fires are also bright red or orange. Exposed, bare earth generally reflects shortwave infrared light and tends to have a red or pink tone. Urban areas are usually silver or purple, depending on the building material and how dense the area is.
Since plants reflect near infrared light very strongly, vegetated areas are bright green. The signal is so strong that green often dominates the scene. Even the sparse vegetation in Algeria’s desert landscape stands out as bright green spots in the above image.
Blue, shortwave infrared
Occasionally, the Earth Observatory will publish a band combination that assigns blue light to be red and two different shortwave infrared bands to green and blue. This band combination is especially valuable in distinguishing snow, ice, and clouds. Ice reflects more blue light than snow or ice clouds. Ice on the ground will be bright red in this false color, while snow is orange, and clouds range from white to dark peach.
The Earth Observatory also uses thermal infrared measurements to show land temperatures, fire areas, or volcanic flows, but most of the time, these are published as grayscale images. Occasionally, the thermal features of interest will be layered on top of a true-color or grayscale image, particularly in the case of a fire or volcano.
You can explore the way different band combinations highlight different features by using a browse tool called Worldview, which displays data from many different imagers, including Aqua and Terra MODIS. Click on “add layers” and then select one of the alternate band combinations (1-2-1, 3-6-7, or 7-2-1). The site also provides descriptions of common MODIS band combinations.
You can also explore false color imagery with Landsat. See a few examples with a description in the Landsat 7 Compositor, or watch thisanimation of the Florida Everglades in three different band combinations. You can also make your own Landsat images and experiment with band combinations by using software like Adobe Photoshop or ImageJ.Download data for free from the U.S. Geological Survey, then follow the instructions for Photoshop or ImageJ.
- Learn more
- NASA Take a tour of the electromagnetic spectrum.
- NASA’s Earth Observatory (2013, November 18) How to interpret a Satellite Image: Five Tips and Strategies.
- NASA’s Earth Observing System Our Changing Planet.
- Natural Resources Canada (2007, September 25) Fundamentals of Remote Sensing.