Content area
Full Text
A clear view through the trees. That's what soil scientist Greg McCarty and USDA Forest Service ecologist Megan Lang have been working towards as they pioneer the merging of two remote sensing technologies to map forested wetlands in the Chesapeake Bay Watershed.
McCarty and Lang are based at the ARS Henry A. Wallace Beltsville Agricultural Research Center in Maryland, and they conduct research on the bay's Eastern Shore, which is located on the Delmarva Peninsula that juts between the bay and the Atlantic Ocean. Delmarva is an acronym for Delaware, Maryland, and Virginia-the three states that make up the peninsula.
The two scientists are the first to apply LiDAR (light detection and ranging) to forested wetland mapping. They are also among the few researchers to report combining the laser technology with a similar technology, a form of radar (radio detection and ranging) that relies on advanced technology called a "synthetic aperture radar" (SAR) sensor.
A Tale of Two Sensors
Both SAR and LiDAR are active sensors in that they transmit their own energy. Long-wavelength microwave energy is used by SAR, while shorter wavelength laser energy is used by LiDAR. Both sensors use the time it takes for the signals to travel to the wetlands and the strength of the returning signal to help detect the presence, extent, and, in some cases, depth of water.
SAR is currently collected by satellitemounted sensors, allowing for more frequent image collections over large areas. LiDAR sensors are usually flown on airplanes, so the imagery is collected much less frequently, but it has greater spatial resolution.
SAR is best for spotting short-term changes in water levels, such as flooding or short-lived pools of water, while LiDAR is better for producing highly accurate maps of flooding and understanding the surface-water flow paths in landscapes or recording changes over long time periods.
SAR sensors are not restricted by clouds or even most rainstorms. That is important when collecting data during rainy periods, when wetlands really show up. The imagery also makes it practical to monitor wetlands year-round, day or night.
Both LiDAR and SAR can literally see the wetlands through a forest canopy. Most bay-area wetlands are forested, as are half of the...