the USGS Earth Resources and Science Center (EROS) in Sioux Falls, South Dakota, which has an enormous archive of global satellite and airborne passive and active remote sensing data that is well organized, easily accessible, and mostly downloadable (http://eros.usgs.gov/find-data);
the USDA Aerial Photography Field Office (APFO), which has an archive of aerial high-resolution photography and imagery collected over the United States by a variety of USDA and USGS government agencies (http://www.fsa.usda.gov/programs-and-services/aerial-photography/index);
the NASA Distributed Active Archive Centers, which act as custodians of NASA earth science data and make it available to users (https://earthdata.nasa.gov/about/daacs);
NASA, which also serves NASA data through its Global Imagery Browse Services (https://earthdata.nasa.gov/);
NOAA’s Digital Coast, which archives and provides access to multiple lidar (both topographic and bathymetric) and multispectral airborne and satellite imagery data-sets collected primarily in the coastal areas of the United States (https://coast.noaa.gov/dataviewer/#/imagery/search/);
NOAA Climate Data Online, which provides access to NOAA’s archive of worldwide climate and weather data (https://www.ncdc.noaa.gov/cdo-web/); and
the Bureau of Land Management Aerial Photo Archive, a collection of aerial film at the National Operations Center in Denver, Colorado https://www.blm.gov/nstc/library/aerial/. Additionally, many states and local governments manage and serve image collections and make available their archives of both active and passive HVH-resolution imagery. Many of these image datasets can be found in ArcGIS Online.
Many national governments also collect and archive imagery. International sources of imagery are referenced in the relevant sections below.
Unmanned Aerial Systems
For the first time since the advent of remote sensing, technologies are available that allow anyone to collect imagery. UASs (sometimes called drones) have long been recognized for their potential as a means of accomplishing tasks that are too repetitive, inaccessible, or dangerous for manned aircraft. The military implications of such a device are obvious, and most development has occurred in the military as a result. However, in the last decade or so, the use of UASs for civilian purposes has grown tremendously. The proliferation of better and more effective software for processing imagery coupled with the miniaturization of sensors has allowed UASs to successfully collect remotely sensed data for a very large number of applications.
Today, virtually any sensor from cameras, video, multispectral and hyperspectral sensors, thermal imagers, radar, lidar, and others can be flown on a UAS. Some of these platforms are fixed-wing aircraft but many more are some type of helicopter, often with four, six, or eight propellers. These platforms can be small enough to fit in your hand or large enough to carry a substantial payload of sensors and equipment. Software for processing this data is available both commercially and in the public domain to create mosaicked images, thematic maps, topographic data, and other cartographic output. Esri has developed Drone2Map for the creation of professional imagery products from UAS-captured still imagery for visualization and analysis in ArcGIS.
UASs are making the collection of ultrahigh-resolution imagery a reality for small geographic study areas. The software allows users to process the data into custom products, often in the same day. For the first time, UASs are offering the promise of making remote sensing technology personal, in much the same way that PCs made computers personal.
Many companies now sell small UASs (sUASs) at reasonable costs that allow farmers, ranchers, environmentalists, researchers, utility companies, academics, and others to take advantage of this rapidly growing technology. Federal agencies from the United States and many other countries fly large UASs with heavy payloads for extended missions of both military importance and civilian usefulness. However, the growth in this technology is primarily in smaller UASs that fly short missions with small payloads. The list of applications for such remotely sensed data is endless and includes archaeology, engineering, wildlife habitat analysis, agricultural mapping, forest inventory, disaster monitoring, road and bridge inspection, and many, many more.
Perhaps the biggest stumbling block for the use of UASs today is the regulations surrounding their use. Many countries have modernized their regulations, clearly separating the use of manned versus unmanned systems, especially for sUASs. The United States has lagged behind in this adjustment and therefore is behind many countries in the use and development of this technology. New regulations for sUASs were recently published by the FAA (https://www.faa.gov/uas/media/Part_107_Summary.pdf).
Mapping Woody Debris in the Great Brook
Flows of the Great Brook in Vermont long caused problems for the residents of Plainfield. Over time, bank erosion resulting from natural and anthropogenic forces increased the amount of large woody debris in the stream. During extreme precipitation events, the debris moved downstream, collecting at the first bridge and forming an artificial dam, which diverted water out and over the stream bank causing tens to hundreds of thousands of dollars of damage to the bridge, roads, and surrounding homes. The damage occurred so often that it became economically unviable for the town to continue to make regular repairs. As a result, the town retained a consulting engineering team to evaluate bridge alternatives. Key to the development of alternatives was an estimate of the amount of woody debris predicted to move through the bridge during a storm. While only a dozen or so logs often caused the jam at the bridge it was unclear how much more woody debris there was moving downstream.
The town tried for several years to carry out woody debris inventories of the Great Brook, but the process was slow, cumbersome, costly, and dangerous. Remote sensing approaches, while compelling, were also not feasible because even the best commercial satellite imagery lacked the spatial and temporal resolution required, and imagery acquired through manned flights was far too costly. With funding from the US Department of Transportation and the Vermont Agency of Transportation, the University of Vermont’s (UVM) UAS Team began long-term monitoring of Great Brook starting in December 2014 with a goal of mapping and tracking the movement of woody debris through the 2015 spring flood season. The UVM UAS Team employed the senseFly eBee, a small, lightweight UAS specifically designed for mapping. The workflow for the eBee essentially consists of the operator using flight planning software to specify the flight area and flight parameters (e.g., desired ground resolution and maximum altitude), flight operations, and postprocessing. The eBee flies autonomously, following its preprogrammed flight path, acquiring imagery with the requisite overlap and at the appropriate angle for generating orthorectified imagery. Once the eBee is recovered the imagery is fed into photogrammetric software where it is orthorectified, making it suitable for using in GIS software.
Throughout spring 2015, the UVM UAS Team conducted multiple flights of a three-mile stretch of the Great Brook. Each time, the orthorectified imagery was brought into ArcGIS where technicians digitized the location of all of the woody debris, populated the attribute table with size information, and noted if it had moved or if the size had changed. A dry winter, combined with little in the way of spring participation, resulted in minimal changes to the woody debris conditions in the stream. Then, in July 2015, a highly localized storm dumped nearly half a foot of rain on the area during a Sunday evening. Floodwater moved rapidly down the Great Brook, trees piled up at the bridge, and the residents of Plainfield awoke to find a bridge that was in need of major repairs. The UVM UAS Team responded, collecting imagery of the damaged bridge along with the upstream area of