Many years ago, if you wanted a family portrait, it would be painted by an artist over many hours. Today, cameras can create a photo in seconds. Cartography has also changed. Cartographers use digital images to create maps with many more details that can be updated instantly.
Advanced technology has changed the way maps are made. The process of gathering data about Earth using instruments mounted on satellites, airplanes, or ships is called remote sensing. One form of remote sensing is detected with satellites. Features on Earth’s surface, such as rivers and forests, radiate warmth at slightly different frequencies. Landsat satellites record reflected wavelengths of energy from Earth’s surface. These include wavelengths of visible light and infrared radiation.
One example of a Landsat image is shown in the above picture. To obtain such images, each Landsat satellite is equipped with a moving mirror that scans Earth’s surface. This mirror has rows of detectors that measure the intensity of energy received from Earth. This information is then converted by computers into digital images that show landforms in great detail. Landsat 7, launched in 1999, maps 185 km at a time and scans the entire surface of Earth in 16 days. Landsat data are also used to study the movements of Earth’s plates, rivers, earthquakes, and pollution.
One satellite that uses radar to map features on the ocean floor is the TOPEX/Poseidon satellite. TOPEX stands for topography experiment and Poseidon (puh SY duhn) is the Greek angel of the sea. Radar uses high-frequency signals that are transmitted from the satellite to the surface of the ocean. A receiving device then picks up the returning echo as it is reflected off the water. The distance to the water’s surface is calculated using the known speed of light and the time it takes for the signal to be reflected.
Variations in time indicate the presence of certain features on the ocean floor. For instance, ocean water bulges over seafloor mountains and forms depressions over sea-floor valleys. These changes are reflected in satellite-to-sea measurements and result in images such as the one shown in the below picture, that shows ocean depths during a hurricane. Using TOPEX/Poseidon data, scientists were able to estimate global sea levels with an accuracy of just a few millimeters and could repeat these calculations a soften as every ten days. Scientists can also use this data and combine it with other existing data to create maps of ocean-floor features. The TOPEX/Poseidon satellite also has been used to study tidal changes and global ocean currents.