Because we can classify almost everything surrounding us as an "Environmental Issue", is useful to select a "hot" and well known example from this area to illustrate the ability and flexibility of remote sensing in analyzing parts of our environment.

Wetlands are defined as "areas which are periodically or permanently inundated with water and which are typically characterized by vegetation that requires saturated soil for growth and reproduction.

Aerial photography
Is the most common method to accurately map specific wetland vegetation and boundary information used by federal, state, and local agencies. Photo analysis involves the identification and delineation of specific features such as tone, texture, shape, size, shadow height, and spatial relationship. Examination of the aerial photos stereoscopically enables the interpreter to observe the vertical and the horizontal spatial relationships of the subject features. It is important to select a film type, which will provide maximum contrast between different plant communities. It is important to collect the aerial photography the season that would take advantage of phenological differences among common wetland features.

Small scale photography
Small-scale photography has been utilized to map wetlands. The use of small-scale photography is primarily limited to the identification of plant communities with distinct spectral characteristics.

Example of small scale photography.

Medium- to Large-Scale photography
Specific locations and vegetation types are easily identifiable on large-scale photography. Very large-scale photographs are also useful in investigation of wetlands that are small, isolated, or narrow and linear.

Landsat MSS sensor
Landsat MSS has been investigated for its utility in wetland mapping and inventory updating of broad wetland communities. Some of the resource mapping problems encountered with the use of Landsat MSS data for wetlands originate from the low spatial re solution (80m) of the MSS sensor which hampers detection of wetlands smaller than 1.6 hectares, the inability to place boundaries with high reliability, and uncertain availability of multidate imagery (16-day repeat cycle and cloud free conditions require d) for use in discrimination of different vegetation types based on phenology.

Landsat TM sensor
Landsat TM has improved wetland-monitoring capabilities over the Landsat MSS system. In the comparison to the Landsat MSS, the TM sensor provides seven narrower spectral bands, better spatial resolution, improved radiometric sensitivity, and a higher number of quantization levels (i.e., digital numbers for TM=256 and for MSS=128). TM is still subject to a repeat collection cycle of 16 days, and use of the data is contingent upon near cloud-free conditions

Example of landsat TM sensor image.

SPOT sensors
Very few references were located in which SPOT imagery was evaluated for wetland mapping applications. SPOT Corporation has published brochures describing applications in which SPOT imagery was used for wetland monitoring and vegetation inventorying p rojects. One project involved the detailed mapping of a national wildlife refuge into 18 classes based on density, species composition, and other factors. SPOT also lists projects where submerged aquatic vegetation was located and mapped successfully, and another where a wetland area dominated by a particular species was mapped and correlated to vegetation index factors for assessing biomass distribution.

Example of SPOT sensor image (Same image as small scale photo)

Aircraft Multispectral Scanners (MSS)
Aircraft multispectral scanners (aircraft MSS) are multiband sensors that can have from 4 to 230 separate channels or bands in which the sensor collects information from various parts of the electromagnetic spectrum. These systems are designed to meas ure and record the radiant energy reflected and emitted from the ground, and has a wider range of spectral sensitivity than photographic systems.

Applications for aircraft MSS system include the following examples:

Radar applications
Radar can be useful for identifying broad wetland classes over large areas, particularly if the area is perpetually cloudy. The longer wavelength radar systems have the ability to penetrate clouds and herbaceous vegetation canopies. Disadvantages of u sing radar are the high cost, limited data availability, and the complexity of the imagery.In a study contacted by Place (1985), Seasat radar images were evaluated for their ability to improve wetland mapping when combined with conventional sources, i.e., and aerial photography.

Videography
Airborne Videography is not a replacement for aerial photography, but rather a substitute or complimentary data source when the quality and/or cost of an aerial survey can not be supported.

Advantages of video systems are:

Disadvantages include a lower resolving power than aerial photography and difficulties with calibration.

Imagery from these systems has been used successfully to detect and assess ecological conditions such as plant communities and species, insect pests, soil moisture, soil drainage and salinity, grass phytomass levels, and burned areas. High-resolution a irborne video data have also been utilized to update cartographic databases.

Microwave sensing
Microwaves are capable of penetrating the atmosphere under virtually all conditions. Depending on the wavelengths involved, microwave energy can "see through " haze, light rain and snow, clouds and smoke. Microwave reflections or emissions from earth materials bear no direct relationship to their counterparts in the visible or thermal portions of the spectrum. For example, surfaces that appear "rough" in the visible portion of the spectrum may be "smooth" as seen by microwaves. In general, ,microwave responses afford us a markedly different "view" of the environment-one far removed from the views experienced by sensing light or heat.

Questions? Comments? Contact: mouson@aol.com

Bibliography and related links: Links

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