Aerial photography has many applications for rangeland science and natural resource management. Aerial photographs can serve as great base layers in a GIS and be used for land planning, assessment and monitoring, and creating soil maps. GIS users often use aerial photographs to digitize features on the land such as buildings, roads, utilities, fences, etc. Aerial photos can also provide a historical record for studying changes in land use, management, vegetation, or habitat.
One common type of aerial photograph is an orthophoto. A digital orthophoto is a computer-generated image of an aerial photograph in which the image displacement caused by terrain relief and camera tilt has been removed. The orthophoto combines the image characteristics of the original photograph with the georeferenced qualities of a map, allowing the orthophoto to be used in conjunction with other images or spatial data layers.
Historically, most aerial photography was captured on film, and scanned into a computer. Increasingly, however, aerial photographs are being acquired using digital sensors. Aerial photographs are typically either panchromatic (B &W), natural color, or color infrared. Each format has advantages and disadvantages for specific applications.
Natural color film has 3 layers: the top layer is sensitive to blue, middle layer is sensitive to blue and green, and bottom layer is sensitive to red and blue. Each layer acts like a band in digital photography, though is does not sample just from a narrow range like digital sensors do. It would be very difficult to quantify exact radiation from a portion of the spectrum. Natural color film has advantages over panchromatic because the light has been parceled into different groups which can allow you to learn more about features on the ground. One typically use of color photography is to map vegetation communities or land cover, tasks that benefit from color separation.
Using film for spectral remote sensing has many disadvantages compared with digital sensors. To be useful in a quantitative way, film prints will have to be scanned into a digital format. This process generally reduces the quality of the image and requires special equipment such as densitometers. Once digitized, the images can be brought into a GIS or a remote sensing software package. The suitability of using these images for spectral quantitative analysis is fairly limited because we cannot easily derive physical properties of the surface including reflectance. Unlike digital sensors which are highly calibrated, one cannot easily relate the digital number of a digitized image to measured quantities of radiation. If your remote sensing study is only looking at a single image, this may not be a problem. For example, you can run classification schemes based on the digital numbers. However, if your study requires comparing multiple images from multiple locations or dates, making the images comparable is very difficult. Variables like sun angle and shadowing are much harder to account for.
Before you use aerial photography, make sure you find out if it was originally acquired on film or digital sensors.
Unmanned aircraft systems are a type of aerial photography.
A number of satellite sensors now provide imagery at resolutions that rival aerial photography and with other features (e.g., data depth, clarity) that exceed most aerial photography. The availability of these high-resolution satellite products, is still limited though, due to high costs and licensing restrictions. Some of these satellite sensors are:
There are many sources for aerial photography, and in some instances, there is almost a hundred year record of aerial photographs. This wiki page currently deals only with U.S. federal government aerial photography sources and programs. Many states also have active aerial photography programs and archives. Additionally, there are many private companies that provide custom aerial photograph acquisition.
Listed below are aerial photography programs from the U.S. federal government. Imagery has been systematically acquired through several programs since the 1940’s. Most of the imagery has a spatial resolution of 1 or 2 meters.
NHAP was an interagency federal effort coordinated by the USGS which operated between 1980 and 1989. The goal of the program was to provide cloud free aerial images of the entire lower 48 states. The images were acquired at 40,000 ft. elevation.
NAPP is the successor of NHAP and operated between 1987 and 2004. The mission of the program was to cover the entire U.S. except Alaska every 5 years. The aerial photography was captured at an elevation of 20,000 ft.
The NAIP program started in 2003 and is still ongoing. The goal of the program is to photograph primarily agriculture areas during the growing season or “leaf on” period and provide orthophotos to government agencies. Their coverage includes the entire continental U.S.
The APFO houses a huge archive of aerial photography dating back to mid 1950’s. The images were originally used to support agriculture but today have many applications. The archive has images from many programs including the National High Altitude Program (NHAP), National Aerial Photography Program (NAPP), and Forest Service programs. The collection has images in many different resolutions and formats.
http://www.blm.gov/noc/st/en/business/aerial.html This is a very large archive dating back 25 years. They have black & white, as well as natural color and false color infrared image of BLM or adjacent land in the western U.S. All of the images were acquired on film. Contact them for ordering and prices.
Cost, acquisition procedures, and licensing of aerial photographs will vary depending on the source. For the most part, archived images collected as part of a U.S. government program are inexpensive or free and can be redistributed freely. Imagery from commercial providers can be more expensive and have licensing restrictions limiting its use and distribution.
Image format will also vary with the image source and provider.
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