Exam Activity Answers¶
Contents
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ANSWERS TO “EXAM”¶
` <>`__Ex-1: The scene lies in the western half of the United States. California, Nevada, Oregon, Washington, Idaho, Utah, and parts of Montana, Wyoming, Colorado, and New Mexico are all in this image (look for faint black lines indicating borders). The lake with the purple upper half (which is caused by image color assignment to that part of the lake with considerable silt owing to blockage by the Union Pacific railroad causeway) is the Great Salt Lake.`BACK <OverviewActivity.html#Ex-1>`__
` <>`__Ex-2: Most of the scene, rendered in natural color, is filled by the northern half of California. The snow-covered areas are the High Sierra Mountains and the Trinity Alps. Within the Sierra is Lake Tahoe, and to its east is Pyramid Lake in Nevada near Reno. The central part of the image is occupied by the Sacramento segment of the Great Valley. The Coast Ranges run along the Pacific Ocean coastline.`BACK <OverviewActivity.html#Ex-2>`__
` <>`__Ex-3:The major urban areas, which show up here in the upper scene in various shades of bluish-white (indicating that vegetation is less common, since green is uncommon), are in the San Francisco Bay area. That area is presented in green in the lower image because in summertime the vegetation is in full growth (lush). The principal cities are San Francisco, Oakland, and San Jose (all large enough to have major sports teams). The bluish areas in the Pacific Ocean mark tongues and bands of sediment; in the inland S.F. Bay, these sediments are much denser (higher amounts) and show up as the browns typical of silt and clay. The areas rendered as dense green are heavily forested. The Great Valley shows the geometric shapes associated with crop fields. In the upper image, many are brownish because the scene was acquired in winter when such fields are fallow. The lower scene was imaged during the growing season when the crops are mature enough to show up as the greens associated with many vegetables.`BACK <OverviewActivity.html#Ex-3>`__
` <>`__Ex-4: This will be explained in much more detail later in the Tutorial. But here is a thumbnail answer: Sensors such as the scanners on the Landsat spacecraft acquire multiple duplicates of the scene, each at some narrow range of wavelengths. In a natural color image, three of the wavelength bands are in the blue, green, and red segments of the visible light spectrum. When each band, with the features present in various shades of gray, is projected through color filters (or color guns in a computer monitor) that are the same (blue B, green G, and red R respectively), onto color film, the natural color version results. In the false color version, the three images projected through B, G, and R filters are Green (B), Red (G), and near Infrared (R), this last being largely beyond the human eye’s capability to respond to. Now, healthy vegetation is very bright (light tones in a black and white print or nearly clear in a transparency) in the near IR. Since that band is projected through red, vegetation will appear red. Since no blue band is used in a false color composite, and since water has very dark tones in green, red, or infrared, the ocean will show up as dark. Cities are light blue. There is an eastern branch of the Coast Ranges in this scene; its colors include tan and brown, some of which is in a pattern controlled by drainage, so that a sense of relief (elevation variations) is imparted. When the two Landsat scenes with differing resolutions are both shown in small size, the eye cannot discern significant differences in detail (the 30 m resolution does not notably improve recognition of features that are also seen in the 79 m resolution version). But when the two scenes are enlarged to, say a color print about 3 feet in size, the sharpness and detail in the 30 m image is considerably enhanced. `BACK <OverviewActivity.html#Ex-4>`__
` <>`__Ex-5: Look at the upper peninsula where the City of San Francisco is located. You should note the Golden Gate Bridge and to its right and lower, the Bay Bridge.`BACK <OverviewActivity.html#Ex-5>`__
` <>`__Ex-6: Mount Diablo, the highest in the Bay Area at 3849 feet, is in the central upper part of the top image. The two towns that have undergone large population growth are San Ramon-Pleasanton and Livermore. The green and brown patterns, most bounded by straight dividers, are salt evaporation basins. The brown is many mud, with little water in the basin; the green is caused by an algae that grows profusely in saline conditions. The image was probably taken in early Spring; some of the fields in the Great Valley are still brown (fallow) but vegetation in the Bay Area is a healthy green, a consequence of the Winter rains. In the lower image, the two bridges in and out of San Francisco are now readily visible - this is an enlargement of 30 m resolution imagery, which now brings out details not seen in the small scale version above (linear features less than 30 m are commonly detectable for reasons covered later in this Tutorial).`BACK <OverviewActivity.html#Ex-6>`__
` <>`__Ex-7: In the photograph, which is probably at about 10 meter resolution, individual street patterns become evident over much of the city. Note how prominently the bridges now stand out.`BACK <OverviewActivity.html#Ex-7>`__
` <>`__Ex-8: The aerial photo shows much of the commercial part of San Francisco, including the downtown area (clustered around and north of Market Street) and piers (including the Ferry Building) on the Bay. The IKONOS image concentrates on the high rise portion of this part of the City. (You can fit the ground scene to the image by using the Bay Bridge, visible in both views, as a reference point.) Taken in 2000, there are many additional tall buildings since the aerial photo was taken. Most prominent is the famed TransAmerica (pyramid) building, opened in 1972, the tallest in S.F. (853 ft), and shown below. This says that the aerial photo is older than 1972. The buildings lean in the IKONOS image for one (or perhaps both) of two reasons: 1) this portion is part of a larger image and lies near the side, which causes a non-vertical aspect, and/or 2) the sensor on IKONOS was pointed slightly off vertical when the image was acquired.`BACK <OverviewActivity.html#Ex-8>`__
` <>`__Ex-9: The dark tones in the daytime thermal image relate both to vegetation (it generally creates a colder microclimate by evapotranspiration) and to water (often 10 to 20° or more cooler than the land). Both the ocean and bay are thus much cooler than the land (in processing the image, the coldest actual temperature features are rendered in the lowest gray levels [black]). Golden Gate Park (long rectangle), the Presidio near the Golden Gate Bridge, and some of the mountains (forest-covered) are displayed as dark because they are vegetated. In the nighttime HCMM thermal image, San Francisco Bay is actually warmer than the City and most of the land areas (water tends to experience only slight cool-downs from day to night, so that in the evening the land has cooled to lower temperatures than the water). The wisps in the Pacific Ocean are areas where warmer waters are circulating (perhaps some such water has upwelled from shallow depths). The very dark area in the upper right is the west slope of the Sierra Nevada mountains which are cooler because of higher elevations. The very bright band (warm) ringing the Sacramento Valley probably results from residual heat that is associated with storage of daytime heating of these grass-covered hills ringing the valley. The Valley itself is cooler in part because of vegetation and in part because cold air is denser and sinks from the higher terrain into this basin. (Note that the Salinas Valley inland from Monterrey Bay is also cool, for the same reason.)`BACK <OverviewActivity.html#Ex-9>`__
` <>`__Ex-10: These are both radar images, made during the SIR-C (third mission on the Space Shuttle, using the Shuttle Imaging Radar instrument) sensor. Mountains in this scene are identified by a pattern in which long parallel dark and light bands highlight the ridges, with some indentations associated with valleys cut into the ridges. In mountainous terrain, the mountains usually look like rugged topography (ridges and prominences) because the slope facing the side-looking radar beam is brightened and the slope away from the beam is dark, creating a “shaded relief” portrayal of the mountains present. The very bright (white) patches occur mostly in the downtown areas of the cities, owing to the presence of strongly reflecting surfaces such as tall buildings. On SIR-C, there were three radar bands (L-band and C-Band on the U.S. radar, and X-band on a German instrument) and these had different polarization modes (see page 8-7). Each band image and those of differing polarizations show differences from one another in their gray level patterns. Thus, any three of these multiple renditions can be combined into a color composite using the blue, green, and red photo filters or computer color outputs, which gives rise to a false color image that helps to separate various features.`BACK <OverviewActivity.html#Ex-10>`__