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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)

Satellite Imaging

Diagram of a satellite sensor. Courtesy: NOAA Coastal Services Center

Satellites provide a means for looking at a very large area of the world within a very short time period. Satellite sensors create pictures of the Earth from space using electromagnetic radiation covering a range of frequencies, from radio waves to gamma rays.

As electromagnetic radiation from the sun, or emitted from the satellite itself, hits objects on the Earth, and a portion of that radiation is reflected back to the satellite. Sensors on the satellite measure the wavelength and intensity of the reflected radiation. Different objects do not reflect radiation in the same way: clear water, for example, will reflect light differently than turbid water.

Advantages and Limitations

Satellite imaging is desirable because it can cover relatively large areas (spanning several kilometers) at relatively low cost. Although it is seldom possible to acquire satellite imagery under the appropriate conditions for effective benthic mapping (such as low-tide or calm sea state), satellite imaging has proven useful in tropical, clear water environments for coral mapping.

Some limitations are present in satellite imaging techniques. Shorelines are often difficult to detect because reflection through the water produces signatures similar to those received from adjacent land. In addition, cloud cover can block the reflected light, or the water may be too deep or too turbid. Sun glare from the sea surface also can cause interference, and the resolution of some satellite images may be too low (on the order of several hundred meters) to discriminate different habitats.

Landsat image of the coral reefs off the Florida Keys. Courtesy: NASA

New high-resolution satellites are equipped with cameras that can distinguish objects as small as one meter square, or about three feet, in size, but such data are not yet widely available due to security and licensing concerns.

Uses

Scientists can determine features such as the type of vegetation on the seafloor based on the signature pattern of the reflected signal. It is also possible for multiple images of a particular area to be created using light of different wavelengths (from different parts of the electromagnetic spectrum). This is called multispectral imaging (MSI), a technique that has proven useful for benthic habitat mapping in some shallow water marine environments. For example, broad-spectrum bands of visible light (red, green, and blue) have been used to differentiate habitat types like sand, seagrass, coral, and hard substrate in coral reef environments where airborne imagery was unavailable.

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