FAQ's about Remote Sensing

You can search and download aerial photography through the Earth Explorer site.  With this site you can view USGS archives in their entirety. 

In Section #1, (Select your dataset) pick the dataset by clicking the plus sign next to the product.  Select the dataset from the drop down list.

Under Section #2, (Enter your search criteria) select the area of interest.  You can choose this by typing in the Address/Place name, picking area on the map, or using Latitude and Longitude.

Finally, Section #3 (Search) run your search.  This will take you to a new page that will show you the results.  It may take awhile, and the page will continue to refresh with the results.

Once the search is complete, you will be able to view the footprints and download from the results you have searched.

Color infrared photography, often called false color photography because it renders the scene in other than the normal colors seen by the human eye, is widely used for interpretation of natural resources. Atmospheric haze does not interfere with the acquisition of the image, therefore is well suited to aerial photography. Because the film is high speed and subject to degrees of degradation in handling before exposure, the aerial photographs can vary in overall tone. This variability complicates the interpretation of color tones between photographs. However, some general guidelines can be given to aid the inexperienced interpreter.

The red tone of color infrared aerial photographs is almost always associated with live vegetation. Very intense reds indicate vegetation which is growing vigorously and is quite dense. An irrigated alfalfa field would be an example of such vegetation. An evergreen forest, which may be quite dense vegetatively, will not appear as a similar bright red because its level of growth activity is less, compared to irrigated alfalfa. Knowledge of the vigor and density of vegetation is important to the interpretation of the red colors on color infrared aerial photography.

As the vigor and density of vegetation decreases, the tones may change to light reds and pinks. If plant density becomes low enough the faint reds may be overcome by the tones of the soils on which the plants are growing. The ground areas in this case will appear in shades of white, blue, or green depending on the kind of soil and its moisture content. As plant vigor decreases, the vegetation will show as lighter shades of red and pink, various shades of greens, and possible tans. Dead vegetation, wheat stubble as an example, will often be shades of greens or tans.

Bare soils will appear as shades of white, blue, or green in most agricultural regions. In general, the more moist the soil the darker the shade of that particular soil color. Composition of the soil will affect the color tones shown on the photographs. Dry sand will appear white and, with more moisture, may be very light gray or possibly light tan. Clayey soils will generally be darker in color than sands and tend toward tans and bluegreens. Again, wetter clays will be darker shades of the same tones. Soils high in organic matter, like silts and loams will be even darker in color, and usually in shades of blues and greens. Wet organic soils can be very dark blue or green in the aerial photographs.

Man-made features will show in the tones that relate to the materials they are made of. Asphalt roads, for example, will be dark blue or black, gravel or dirt roads will show as lighter colors, depending on the soil materials involved in their composition, and concrete roads will appear light in tone, assuming clean concrete. The buildings and streets of towns can be considered in a similar manner, their color dependent on the material they are made of.

Water will appear as shades of blue, varying from nearly black to very pale blue. Clear, clean water will appear nearly black. As the amount of sediment increases, the color becomes increasingly lighter blue. Very shallow water will often appear as the material present in the bottom of the stream. For example, a very shallow stream with a sandy bottom will appear white due to the high level of reflection of the sand.

Degraded film will result in photographs which have an overall blue or green cast. When that occurs, the interpretation must consider what that overall cast will do to a "normal" rendition of the scene.

Black-and-white panchromatic (B/W) film primarily consists of a black-and-white negative material with a sensitivity range comparable to that of the human eye. It has good contrast and resolution with low graininess and a wide exposure range.

Black-and-white infrared (BIR) film, with some exceptions, is sensitive to the spectral region encompassing 0.4 micrometers to 0.9 micrometers. It is sometimes referred to as near-infrared film because it utilizes only a narrow portion of the total infrared spectrum (0.7 micrometers to 0.9 micrometers).

Natural color (also referred to as conventional or normal color) film contains three emulsion layers which are sensitive to blue, green, and red (the three primary colors of the visible spectrum). This film replicates colors as seen by the human eye. Color film is a valuable image interpretation tool because the human eye can discern a greater variety of color tones than gray tones.

A digital orthophoto quadrangle (DOQ) is a computer-generated image of an aerial photograph in which displacements caused by terrain relief and camera tilts have been removed. It combines the image characteristics of a photograph with the geometric qualities of a map.

The Landsat series of satellites began with the launch of ERTS-1 (Earth Resources Technology Satellite, later renamed Landsat 1) and continues to represent the worlds longest continuously acquired collection of space-based land remote sensing data. The Landsat Data Continuity Mission (LDCM) is planned for launch in 2011. More details about each mission can be found on the Landsat Missions Timeline Webpage.

Aerial photographs are downloadable at no cost through EarthExplorer or GloVis.

Scale refers to the relationship of distance on photographs or maps to the actual ground distance. It is a ratio that could represent any unit of measurement. For example, a scale of 1:40,000 means 1 inch on the photograph equals 40,000 inches on the ground, or 1 centimeter equals 40,000 centimeters on the ground.

Resolution refers to the ability to distinguish the smallest visible objects on a photograph. Resolution is a result of the combination of film type and the camera lens system.

The USGS Global Visualization tool will only display scenes from the USGS archive. For many locations outside the U.S., there may be scenes that were collected by the sensor but the data is not archived or distributed by the USGS EROS Data Center. These scenes may still be available from other International Ground Stations. Please note that this data is not a product of the USGS EROS Data Center, and therefore the prices, available formats, and/or processing options may vary according to the data provider.

EROS has digitized over 6.4 million frames of aerial film creating medium-resolution digital images (400dpi) and associated browse images for on-line viewing. Products can be downloaded at no cost through EarthExplorer or GloVis.

Both satellites and ground-based magnetometers are important for making measurements of the Earth's magnetic field. They are not redundant, but are, instead, complementary. After executing several orbits of the Earth, satellites can provide good geographical coverage for data collection. On the other hand, ground-based magnetometers are much less expensive than satellites, they are much easier to install and control than satellites, and, with an array of magnetometers, they can provide coverage from numerous locations simultaneously. Another consideration is that satellites orbit the Earth either inside or above the ionosphere, the electrically conducting part of the Earth's atmosphere. Since currents in the ionosphere contribute to the magnetic field, this means that the field measured by a satellite is somewhat different than the field measured at the surface. Finally, don't forget that it is at the surface of the Earth, where we live, that many of the effects of space weather are most important, so measurements from ground-based observatories will always play a critical role in space-weather studies.

Uses include the mapping of large forest fires from space, allowing rangers to see a much larger area than from the ground. Images allow for one to watch erupting volcanoes, track clouds to help predict the weather, monitor for dust storms, view the growth of a city, and track changes in farmland over several years or even decades.

Remote sensing also allows us to gather images of and map the ocean floor without needing to travel to the bottom of the ocean. Information is collected about the seafloor using a sonar system towed on a cable behind a ship. Instead of taking a picture using light to see, sonar "sees" using sound. By measuring the amount of time it takes sound to travel from the ship to the seafloor and back to the ship, and how strong the sounds bounces back, we can make a picture of the seafloor.

These remotely sensed images are often collected as digital files. This allows us to use computers to improve and analyze the images.

Go to http://www.usgs.gov/pubprod/aerial.html/ for examples of remotely sensed data and imagery.

The NAPP photography are acquired at 20,000 feet above mean terrain with a 6 inch focal length lens. The flight lines are quarter quad-centered on the 1:24,000-scale USGS maps. NAPP photographs have an approximate scale of 1:40,000, and are acquired on black-and-white panchromatic (B/W) or color infrared (CIR) film, depending on state or federal requirements.

The NHAP photography were acquired at 40,000 feet above mean terrain and flight lines were centered on the 1:24,000-scale USGS map series. Two different camera systems were used; a 6 inch focal length lens was used to acquire B/W film at an approximate scale of 1:80,000 and an 8.25 inch lens was used to acquire CIR film at an approximate scale of 1:58,000.

The special cameras are used to collect remotely sensed pictures of the Earth, and used to help us "sense" things about the Earth.

   Some examples are:

• cameras on satellites and airplanes take pictures of large areas of the Earth's surface, allowing us to see much more than we can standing on the ground
• sonar systems on ships can be used to create images of the ocean floor without needing to travel to the bottom of the ocean
• cameras on satellites can be used to make pictures of temperature changes in the oceans - an impossibly long task if we had to travel all over the ocean in a boat!

    Specific jobs that can be helped with remote sensing pictures:

• large forest fires can be mapped from space, allowing rangers to see a much larger area than from the ground
• tracking clouds to help predict the weather or watch erupting volcanos, and help watch for dust storms
• tracking the growth of a city and changes in farmland or forests over several years or even decades
• mapping the ocean bottom - mountains, volcanos, and canyons that are higher, bigger, and deeper than any on land exist on the ocean floor!

A GIS is a computer system capable of capturing, storing, analyzing, and displaying geographically referenced information; that is, data identified according to location. Practitioners also define a GIS as including the procedures, operating personnel, and spatial data that go into the system.

From July 1, 2009 until sometime after October 1, 2009, the USGS will be in the process of transitioning to new scanning and ordering systems. During this time customers will be unable to order scans of aerial declassified satellite photography. However, all other data in the archive, including the medium resolution digitized (400 dpi) aerial photography are available for free dowloads.

The National High Altitude Photography (NHAP) program was initiated in 1980 and coordinated by the U.S. Geological Survey (USGS) to acquire aerial photography of the 48 conterminous United States every 5 years. This interagency program was designed to eliminate duplicate efforts in various Government programs and to maximize the use of Government funds to build a uniform archive for multiple purposes. In 1987, the program name was changed to the National Aerial Photography Program (NAPP) in recognition of modifications in the user requirements and flight specifications.

The NAPP photography are acquired at 20,000 feet above mean terrain with a 6 inch focal length lens. The flight lines are quarter quad-centered on the 1:24,000-scale USGS maps. NAPP photographs have an approximate scale of 1:40,000, and are acquired on B/W or CIR film, depending on state or federal requirements.

The NHAP photography were acquired at 40,000 feet above mean terrain and flight lines were centered on the 1:24,000-scale USGS map series. Two different camera systems were used; a 6 inch focal length lens was used to acquire B/W film at an approximate scale of 1:80,000 and an 8.25 inch lens was used to acquire CIR film at an approximate scale of 1:58,000.

The National Hydrography Dataset (NHD) is the culmination of recent cooperative efforts of the U.S. Environmental Protection Agency (EPA) and U.S. Geological Survey (USGS). It combines the best of the EPA Reach File Version 3.0 (RF3) and USGS Digital Line Graph (DLG) hydrography files: hydrologic ordering, hydrologic navigation for modeling applications, and a unique identifier (reach code) for surface water features from RF3; and the spatial accuracy and comprehensiveness of DLG hydrography.