Related links to Spatialworlds
GeogSplace (a teaching blog for Year 12 geography)
Geogaction
Spatialworlds website
GeogSpace
Australian Geography Teachers' Association website
Tips and strategies to read satellite images
The Earth Observatory site hosts a rich, deep archive of more than 12,000 interpreted satellite images covering a wide range of topics and locations. The archive includes images of natural events as well as more diverse featured images
(November 18, 2013) on the NASA Earth Observatory site. For the full posting go to http://earthobservatory.nasa.gov/Features/ColorImage/
Additional articles and educational activities about interpreting satellite images are available on the NASA Earth Science Week web site, Mapping Our World.
Satellite images are like maps: they are full of useful and interesting information, provided you have a key. They can show us how much a city has changed, how well our crops are growing, where a fire is burning, or when a storm is coming. To unlock the rich information in a satellite image, you need to:
- Determining scale
- Looking for patterns, shapes, and textures
- Reading colors (including shadows)
- Finding north
1. Determining Scale
Some images from military or commercial satellites are detailed enough to show local features. Such satellites zoom in on small areas to collect fine details down to the scale of individual houses or cars. In the process, they usually sacrifice the big picture.
Earth science researchers typically want a wide-angle lens to see whole ecosystems or atmospheric fronts. As a result, NASA images are less detailed but cover a wider area, ranging from the landscape scale (185 kilometers across) to an entire hemisphere. The level of detail depends on the satellite’s spatial resolution (The width of each pixel is the satellite’s spatial resolution). Commercial satellites have a spatial resolution down to 50 centimeters per pixel.
Before beginning to interpret an image, it helps to know what
the scale is. Does the image cover 1 kilometer or 100? What level of
detail is shown? Images published on the Earth Observatory include a
scale.
You can learn different things at each scale. For example, when tracking a flood, a detailed, high-resolution view will show which homes and businesses are surrounded by water. The wider landscape view shows which parts of the county or metropolitan area are flooded and perhaps where the water is coming from. A broader view would show the entire region—the flooded river system or the mountain ranges and valleys that control the flow. A hemispheric view would show the movement of weather systems connected to the floods.
You can learn different things at each scale. For example, when tracking a flood, a detailed, high-resolution view will show which homes and businesses are surrounded by water. The wider landscape view shows which parts of the county or metropolitan area are flooded and perhaps where the water is coming from. A broader view would show the entire region—the flooded river system or the mountain ranges and valleys that control the flow. A hemispheric view would show the movement of weather systems connected to the floods.
2. Looking for patterns, shapes, and textures
Geographers are very good at finding patterns. This skill is useful in interpreting satellite imagery because distinctive patterns can be matched to external maps to identify key features.Bodies of water—rivers, lakes, and oceans—are often the simplest features to identify because they tend to have unique shapes and they show up on maps.
Other obvious patterns come from the way people use the land. Farms usually have geometric shapes—circles or rectangles—that stand out against the more random patterns seen in nature. When people cut down a forest, the clearing is often square or has a series of herring-bone lines that form along roads. A straight line anywhere in an image is almost certainly human-made, and may be a road, a canal, or some kind of boundary made visible by land use.
Straight lines and geometric shapes in this image of Reese, Michigan, are a result of human land use. Roads cut diagonally across the squares that define farm fields. (NASA Earth Observatory image by Jesse Allen and Robert Simmon, using ALI data from the NASA EO-1 team.)
3. Reading Colors
The colors in an image will depend on what kind of light the satellite instrument measured. True-color images use visible light—red, green and blue wavelengths—so the colors are similar to what a person would see from space. False-color images incorporate infrared light and may take on unexpected colors.
3.1 Water
Water absorbs light, so it is usually black or dark blue. Sediment reflects light and colors the water. When suspended sand or mud is dense, the water looks brown. As the sediment disperses, the water’s color changes to green and then blue. Shallow waters with sandy bottoms can lead to a similar effect.Sunlight reflecting off the surface of the water makes the water look gray, silver, or white. This phenomenon, known as sunglint, can highlight wave features or oil slicks, but it also masks the presence of sediment or phytoplankton.
Sunglint makes it possible to see current patterns on the ocean’s
surface around the Canary Islands. (NASA image courtesy Jeff Schmaltz LANCE/EOSDIS MODIS Rapid Response Team, GSFC.)
Frozen water—snow and ice—is white, gray, and sometimes slightly blue. Dirt or glacial debris can give snow and ice a tan color.
In some locations (high and mid latitudes), plant color depends on the season. Spring vegetation tends to be paler than dense summer vegetation. Fall vegetation can be red, orange, yellow, and tan; leafless and withered winter vegetation is brown. For these reasons, it is helpful to know when the image was collected.
3.2 Plants
Plants come in different shades of green, and those differences show up in the true-color view from space. Grasslands tend to be pale green, while forests are very dark green. Land used for agriculture is often much brighter in tone than natural vegetation.In some locations (high and mid latitudes), plant color depends on the season. Spring vegetation tends to be paler than dense summer vegetation. Fall vegetation can be red, orange, yellow, and tan; leafless and withered winter vegetation is brown. For these reasons, it is helpful to know when the image was collected.
The forests covering the Great Smoky Mountains of the Southeastern
United States change colors from brown to green to orange to brown as
the seasons progress. (NASA images courtesy Jeff Schmaltz LANCE/EOSDIS MODIS Rapid Response Team, GSFC.)
3.3 Bare ground
Bare or very lightly vegetated ground is usually some shade of brown or tan. The color depends on the mineral content of the soil. In some deserts such as the Australian Outback and the southwestern United States, exposed earth is red or pink because it contains iron oxides like hematite (Greek for blood-like). When the ground is white or very pale tan, especially in dried lakebeds, it is because of salt-, silicon-, or calcium-based minerals. Volcanic debris is brown, gray, or black. Newly burned land is also dark brown or black, but the burn scar fades to brown before disappearing over time.
3.4 Cities
Densely built areas are typically silver or gray from the concentration of concrete and other building materials. Some cities have a more brown or red tone depending on the materials used for rooftops.
The contrast between Warsaw’s modern and historic neighborhoods is easily visible by satellite. The new Stadion Narodowy is brilliant white. Śródmieście (Inner City) was rebuilt after World War II and most areas appear beige or gray. But some neighborhoods rebuilt with older-style buildings, such as the red tile and green copper roofs of Stare Miasto (Old Town). (Image courtesy NASA/USGS Landsat.)
3.5 Atmosphere
Clouds are white and gray, and they tend to have texture just as they do when viewed from the ground. They also cast dark shadows on the ground that mirror the shape of the cloud. Some high, thin clouds are detectable only by the shadow they cast.Smoke is often smoother than clouds and ranges in color from brown to gray. Smoke from oil fires is black. Haze is usually featureless and pale gray or a dingy white. Dense haze is opaque, but you can see through thinner haze. The color of smoke or haze usually reflects the amount of moisture and chemical pollutants, but it’s not always possible to tell the difference between haze and fog in a visual interpretation of a satellite image. White haze may be natural fog, but it may also be pollution.
Clouds, fog, haze and snow are sometimes difficult to distinguish
in satellite imagery, as in this MODIS image of the Himalaya from
November 1, 2013. (Image adapted from MODIS Worldview.)
Dust ranges in color, depending on its source. It is most often
slightly tan, but like soil, can be white, red, dark brown, and even
black due to different mineral content.
Volcanic plumes also vary in appearance, depending on the type of eruption. Plumes of steam and gas are white. Ash plumes are brown. Resuspended volcanic ash is also brown.
Volcanic plumes also vary in appearance, depending on the type of eruption. Plumes of steam and gas are white. Ash plumes are brown. Resuspended volcanic ash is also brown.
4. Finding North
When you get lost, the simplest way to figure out where you are is to find a familiar landmark and orient yourself with respect to it. The same technique applies to satellite images. If you know where north is, you can figure out if that mountain range is running north to south or east to west, or if a city is on the east side of the river or the west. These details can help you match the features to a map. On the Earth Observatory, most images are oriented so that north is up. All images include a north arrow.
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