Coastal Services Center

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Mapping Techniques: Satellites and Aerial Photos

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Summary Satellite Imaging Airborne Hyperspectral Imaging (HSI)

Aerial Photography Light Detection and Ranging (LIDAR)

Summary

Satellites and aircraft equipped with sensors that send and receive light can provide information on near-shore benthic and shallow reef habitats. These sensors can provide descriptive information of seafloor features including corals, seagrass, sand, shellfish beds, and algae.

Summary view of airborne electro-optical nearshore characterization techniques.

Satellites are equipped with sensors that can gather data over vast areas. There are many different satellite sensors that provide different levels of detail. The resolution of some satellite images may be too low (on the order of several hundred meters) to discriminate different habitats. However, some satellites are equipped with high-resolution cameras that can distinguish objects as small as one meter square (about three feet square) in size.

A satellite image of nearshore coral reefs in the Philippines. Courtesy: NASA Jet Propulsion Laboratory

Airplanes are equipped with sensors that gather more detailed information than most satellite sensors. Digital scanners mounted below aircraft scan the ground beneath them and pick up characteristic light signatures from objects below. Different materials reflect different wavelengths of light, creating a unique pattern, called a spectrum. The spectrum provides a signature that can be used to identify benthic habitats such as live coral, sand, and algae. The spatial resolution of these systems can be controlled by altitude. They normally range from about 1-20 meters. Airborne Hyperspectral Imaging (HSI) is an example of this type of sensor used for benthic habitat mapping in coastal and nearshore areas.

Aerial photography can also provide detailed habitat mapping information for a relatively large area. It is a powerful tool for mapping the spatial extent and distribution of different benthic habitats including seagrass meadows, coral reefs, unconsolidated sediments, shellfish beds (that is, oysters and mussel), hard bottom areas, soft corals, macro-algal beds, and drift algal accumulations. Aerial photography is a primary data source for many benthic habitat mapping projects. It can detect subtle submerged features and resolve features smaller than 1 meter.

An aerial photograph showing mudflats and seagrass in Willapa Bay, Washington. Courtesy: NOAA Coastal Services Center

Another tool for mapping shallow water benthic environments is Light Detection and Ranging (LIDAR). This is used to collect topographic data along shorelines. The LIDAR sensor is mounted on an aircraft and emits a narrow, high frequency laser beam. When the laser beam hits the water, part of the energy is reflected off the surface and the rest travels through the water column and reflects off the seafloor. The sensor records the time it takes for the reflected signals from the surface and seafloor to return to the aircraft. The water depth is calculated from the time difference between the returns. LIDAR requires good water clarity and is an option for inshore, high-resolution, bathymetric mapping projects.

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