Mosaics of entire continents or subcontinents became plausible when enough cloud-free Landsat images were acquired and reprocessed (to minimize tonal differences) The U.S. Department of Agriculture's Soil Conservation Service (SCS) put together the first Landsat "image" of the entire United States in 1974, as part of a NASA contract to prepare the mosaic in time for the U.S. Bicentennial year (1976). The idea for this was conceived and initiated by the writer (NMS). The contract work was monitored by his colleague, Arthur Anderson. Using 595 Band 5 black and white images, the result was the first higher resolution space portrait of the 48 States (Alaska and Hawaii were also done). The original photo version was many feet in dimension; what appears here has been greatly reduced in size, so that much detail present in the original is lost. Appearing here first is the eastern half of the United States; the version shown is somewhat degraded because the only copy available to the writer is the inside fly leaf of the book, Mission to Earth.
The western half (from the Rockies to the West Coast) appears better in part because of the sharper contrast in black and white (resulting from the dark appearance of vegetated surfaces [Band 5 was used] and the light tones in the deserts and lowlands):
The General Electric Company produced the first false color rendition for NASA and the National Geographic Society (NGS). We reproduce here a portion of the NGS Magazine's Landsat "map" of the U.S., which was part of its Bicentennial (1976) anniversary issue, in the natural color version.
In making this mosaic for NGC (there is also a false color red version), GE projected the MSS Band 7 through a green filter and Bands 4 and 5 through blue and red filters). It shows all of the western U.S. into the Great Plains beyond Denver and the Black Hills. Blackish areas on land are either water bodies (such as the Great Salt Lake) or vegetated mountain slopes. Obviously, relatively little detail in the landscape is visible at this scale (~1:11,000,000) but the impression of the overall geomorphology and geologic structures that set the western U.S. apart is striking (more so when the full size - 14 ft wide - is viewed). Compare this rendition with the equivalent area in the NOAA Advanced Very High Resolution Radiometer image presented on page 6-1.
We showed a colorful false color mosaic of the entire U.S. on page 6-1. Here is another one made from Landsat imagery that shows the U.S., Mexico, and part of Lower Canada in approximate natural color. The projection appears to a conical one, probably a modified Mercator. Look at this image and then try to pick out the major geographic subdivisions of the U.S. using the figure below as a guide.
Mosaics of each of the 50 states of the United States and of countries across the world can be found on the Internet. The best known, and most reliable, source is geology.com. Mosaics from that source of two U.S. states are shown here, followed by a comment:
The writer's comment is this: The makers of these mosaics, which use Landsat images as inputs, have arbitrarily selected a color scheme which overemphasizes greens and reds at the expense of other colors. In other words, they are not anywhere close to true color renditions. Green represents vegetation, red represents non-vegetation (blue is used for water). While "form" detail is fairly good, class detail is not. After awhile, this color scheme gets boring. Compare these to other mosaics in this Section, and draw your own conclusions.
To make that point even further, look at geology.com's rendition of cities, here Bangkok, Thailand. The urban areas are uniformly red, active vegetation is green, and the blue represents, in this case, rice fields that are kept submerged in water. /p>
These two Landsat TM images of the same general area show how the color assignments can provide either a more pleasing and/or a more informative rendition:
This critique is somewhat arbitrary. The geology.com version does disclose two important pieces of information: It better separates metropolitan from rural and it does reveal the condition of the rice paddies (which may be seasonal). Interpretations can be subjective.
Alaska and Mexico Mosaics
After the U.S. black and white mosaic was made, someone realized that the states of Hawaii and Alaska had been overlooked. An effort to correct this soon produced the missing states. Hawaii was easy. The Alaska mosaic was more problematic as at the time there was a missing strip of coverage and the western Aleutians were also not covered. Still, the mosaic shown here is imposing; a map of its main features is placed below it:
Notice on the map a number of symbols, e.g., OS. These refer to the generalized locations of different structural terranes - great blocks of crust that at different times in the distant past crashed into that part of the North American plate (both before and after its split from Pangaea) and thus built up the region by accretion. The concept of terranes is discussed in much more detail in Section 17 (second half). For now, we will just identify these terranes by name: Agm = Angayucham; Ch = Chugach; End = Endicott; Kyk = Koyukuk; NxF = Nixon Fort; OS = Overlap Sequence; Pn = Peninsular; Por = Porcupine; PW = Prince William; Sew = Seward; Tog = Togiak; Toz = Tozina; Wr = Warngellia; YT = Yukon/Tinana; Yuk = Yukon. Major faults are numbered.
On page 6-9 we saw a false color Landsat mosaic of all of Alaska except for the outer Aleutians. This is reproduced here:
A standard false color mosaic of Mexico, extending into parts of the southern United States, appears thusly:
Mosaics have now been produced for entire continents. North America has been constructed from Landsat images, and several images of the United States appear in this Tutorial. Using AVHRR data (see page 14-2), all of South America has been mosaicked; the Andes and the Amazon in particular stand out:
Color Mosaics of Europe: The Alps and Italy.
Since the advent of high resolution space imagery, beginning with Landsat, mosaics of entire continents have appeared, using various satellites as input. A search through the Internet shows that Europe has a number of high quality mosaics. Here are two (note that much of European Russia is included):
Below is a schematic map that shows most of the same area as in the Alps mosaic; this map defines the geologic/geomorphic segments of the Alps as well as for adjacent terrains. Try to match the main units to their topographic expression in the mosaic.
While we are focused on the Alps, compare this next scene, made by the MISR sensor on Terra, of a part of the Alps (fit it in to the above mosaic).
This next mosaic shows all of the Italian "boot", plus the islands of Sicily, Sardinia, and Corsica, and extends into the Alps:
Elsewhere in the Tutorial (page6-13), we showed a color mosaic of all of Africa. This is repeated here as another example of the now sophisticated practice of continental-scale mosaicking:
Mosaics of Iran, Pakistan, India, and Afghanistan
We look next at one of the most sensational mosaics yet made from Landsat images, assembled by R.D. Lawrence and R.O. Rogers of the Geology Department at Oregon State University.
The region displayed in this black and white, red band (Band 5) mosaic includes nearly all of Pakistan, part of eastern Iran and a bit of southern Afghanistan, and a smaller segment of India, extending over about 2,100 km (1,305 mi) from west to east. The writer (NMS) has sketched in the principal structural features on the locator map below.
The continuous mountain belt starting with the east-west Makran Ranges, then the north-trending Kirthar and Sulaiman Ranges in Pakistan, and the great bend of the Hindu Kush joining the north-west-trending western Himalayas, is the consequence of the monstrous, still continuing, collision of the Indian subcontinent during the last 130 million years as it drifted north, striking the underbelly of Asia along a vast subduction zone.
The region just to the north of the top center of this mosaic has became famous - and infamous - to Americans since September 11, 2001. The country of Afghanistan is in many ways similar to Pakistan, with desert, a group of central mountains, and mountains on its southeast border with Pakistan. Afghanistan lies within this 4 km resolution metsat AVHRR image of the broad region that includes Saudi Arabia, Iran, Pakistan, Afghanistan, and the several "Stans" such as Turkestan - countries once part of the Soviet Union
This next view zeroes in on the region that includes all of Afghanistan.
Compare the Metsat image with this mosaic made from Landsat images:
The mountain terrain in central Afghanistan is strongly depicted in this reprocessed single Landsat image:
Just to its northeast is this Landsat subimage that includes Kabul and several of the Afghan towns much in the news since the War on Terrorism:
We turn next to one of the most stunning ever produced: the entire continent of Australia. Landsat images were carefully reprocessed and joined. The writer has seen a large copy of the original, on the wall of the lobby of Building 33 at Goddard Space Flight Center. He asked John Everett of EarthSat Corp. in Rockville, MD (which produced this product) for permission to use in this Tutorial. Here it is, displayed somewhat larger than normal (but please note that the original gives a much better impression of details):
The Antarctic and the Arctic
The entire Antarctic continent and surrounding ice shelves has been imaged from sets of radar images acquired over six days in September of 1996 by the NASA Scatterometer (NSCAT), as shown below, with the South Pole appearing as a drawn-in black dot:
This whole continent, about the size of the U.S. and Canada combined, has been imaged in color by specially processing AVHRR scenes acquired by meteorological satellites. This mosaicked version, produced by Pat Chavez's image processiong group at the U.S. Geological Survey, is shown here in its annotated form:
It is a bit of a surprise to those (most of us) who know little by the Antarctic continent to learn that it has high mountains. The high point of the continent, the Vinson massif, at 4897 m (16067 ft) in the Ellesmere Range, has been climbed. Here it is in an ASTER image.
Turning to the other extreme, here is a very high resolution (2.4 m) of the U.S. Antarctic Research Base at McMurdo Sound (bottom center of the above mosaic) as imaged by the QuickBird satellite (see Overview):
The MODIS instrument on Terra (page 16-9) has obtained a view of the North Pole (in the image below the longitudinal lines converging at it are added artificially), even though it does not pass directly over it (in the image below it has looked slightly sidewards). The polar Arctic is not a land mass but a sea covered with ice. While the scene roughly resembles the ice sheet covering central Antarctica, the give-away that the Arctic has sea ice is the sets of leads (fractures) that result from ice cover breakup and movement.
The ability to produce mosaics from high resolution imagery such as Landsat opens a new dimension in the study of Earth's land masses at continental and regional scales.
Displays on your computer screen lose much of the detail evident at the resolution in the Building 33 wall version of Australia, but when we prepare a large space photomosaic, either at the scale (1:1,000,000), at which it is normally printed or at a moderately reduced scale (say, 1:2,500,000) to keep itís size manageable, we retain most of the original information. The chief users of mosaics, such as those shown above, are geographers, structural geologists, and geomorphologists, all of whom benefit from the panoramic vistas afforded by seeing local features in a regional context.
These mosaics also dovetail nicely with the lower resolution views of continents at the hemispherical scale provided by meteorological satellites, such as Nimbus (see page 14-4) and GOES (see page 14-7).