Remote Sensing Imagery
Remotely gathered data is available from a range of sources and data collection techniques and is often the only type of data that is not always easily found within the public domain. This is largely due to the fact that most of this data is acquired by equipment that is expensive to build and maintain. However, there are many types of basic imagery of high-quality that are readily available at largely subsidized costs, particularly within the United States.
Table of Contents
- Aerial Photography
- Satellite Imagery
Aerial photography has two uses that are of interest in class exercises: (1) collection of detailed measurements from aerial photos in the preparation of maps; and (2) to determine land-use, environmental conditions, and geologic information (outcrop locations, lineation, etc). It should be noted that aerial photographs are NOT maps. Maps are orthogonal representations of the earth's surface, meaning that they are directionally and geometrically accurate. Aerial photos often display a high degree of radial distortion that must be corrected. Most GIS packages have some mechanism or work flow for correcting this distortion (see georeferencing) or you make an effort to find corrected data online.
Types of Aerial Photography
- Black and White: Older and lower cost surveys are collected on black and white media and coverage over the U.S. is essentially complete. Multiple generations are ideal for comparing for recent change detection of the land surface.
- Color: More recent or higher cost aerial photo surveys are on color media and coverage over most key areas of the US are available but not always at zero cost.
- Infrared: Primary use is vegetation studies as vegetation is a very strong reflector of infrared radiation.
Sources of Aerial Photography
Within the United States, 1-3m resolution aerial photography is available from the U.S. Geological Survey and their business partner program and can be found online at the National Map Site.
Imagery ready to load generally comes in the form of Digital Orthophoto Quadrangle (DOQ) or Digital Orthophoto Quarter Quadrangle (DOQQ). These are available from many sources including the US Geological Survey DOQ online order site. There are also a range of commercial organizations that provide low-cost downloads of individual and regional DOQQ sets (e.g., Geocommunity).
Free sources of this data can be found at the Seamless Data Distribution System.
Satellite imagery is collected by a host of national and international government, and private agencies. Most of this data is protected by copyright and use access has to be negotiated by individual users or institutions. Free access is possible through collaboration with NASA and NASA funded institutions. Data products span the useful electro magnetic spectrum in a variety of resolutions and it falls upon the instructor to consider the utility of this data in the instructional environment. In many cases, free or low-cost data has a resolution cell size greater than 100m/pixel thus this type of imagery often provides a big-picture regional view but may not provide detailed insight to geologic features found on the ground. Nonetheless, this type of data can be very useful in multi-scale analysis and to help students gain an appreciation for the scale of features on the ground that can have a substantial influence on the bulk reflectivity collected during satellite flyovers.
Types of Satellite Digital Imagery
Remotely sensed satellite data comes in two basic types, passively collected data and actively collected data. Passive data collection focuses on acquiring intensities of electromagnetic radiation generated by the sun and reflected off the surface of the planet. Active data collection is largely restricted to devices that send and generate a pulse of energy to that is reflected back to the satellite to be recorded. Most of the readily available data is passively collected and is limited to energy not absorbed by the Earth's atmosphere. Satellite imagery based on passive reflectivity comes in 4 basic types, which are visible, infrared, multispectral, and hyperspectral.
The type and resolution of the data that is collected is generally keyed to the mission of the satellite. Visible data consists of pixels composed of color values of red, green, and blue to make three bands of data on a raster image. Infrared imagery usually consists of the images that include the visible channels as well as some range of the infra-red spectrum. Multispectral data include up to 7-12 channels of data and hyperspectral can be up to 50 bands or more of data collected over discrete bandwidths of the electromagnetic spectrum. How all of this data is used goes beyond the scope of this site, but it's worth keeping in mind that there are a range of available products and it may require a great deal of research to determine what type of data is useful in the context of the field based exercise.
As of the spring of 2007, NASA lists over 100 satellites providing imagery for viewing from various online image repositories. Images from Landsat-1, -2, -3, -4, -5, and -7 are by far the most common satellite sources used by geologists in conducting field based research. Landsat-5 and -7 are the only two satellites still in service where the instruments on board consist of a multi-spectral scanner (MSS) and an "Enhanced Thematic Mapper-plus" instrument, respectively. Specific details regarding these instruments and their imagery can be found at the GeoCover Tutorial Site hosted by NASA.
Sources of Satellite Digital Imagery
Much of the available data can be purchased from a number of commercial sources. One of the best ways to have wide ranging access to NASA related imagery is to collaborate with a NASA scientist. The Jet Propulsion laboratory offers a range of educator resources for individuals who participate in their Higher Education Faculty Program.
Other sources suitable for field mapping exercises include the NASA hosted GeoCover Site, which provides imagery through the MrSid format. The National Geologic Map Database provides a help page for working with MrSID images. Additional information on using imagery from the GeoCover site (a.k.a. Zulu) can be found at a GIS online tutorial hosted by the Rocky Mountain Mapping Center of the U.S. Geological Survey. The US Geological Survey provides a informative site dedicated to the Landsat program. Scenes can also be ordered from the USGS EROS site.