point on land or under-water that is accurately located in a geographic coordinate system. The data can be used to produce a digital surface model (DSM), which can then be processed to yield digital elevation models (DEMs), digital terrain models, contours, and three-dimensional feature data.
Interferometric synthetic aperture radar (InSAR) uses radar mounted on an airplane or satellite to measure the strength and round-trip travel time of a microwave signal to gather elevation data; these data can be used to produce DSMs and DEMs, among other products, as with lidar.
High-resolution swath bathymetry uses a transducer mounted on a ship to measure the round-trip travel time of sonar waves emitted in the water below the ship. The time elapsed between emission and reception of the signals allows the depth to the seafloor and other features to be determined.
Sensors mounted on satellite platforms measure various bands of the electromagnetic spectrum to produce images of land and water surfaces at wavelengths appropriate to determine vegetation type, general chemical composition, temperature, water content, and other aspects of a given land or water surface. Data from federally, commercially, or internationally owned and operated satellites are available either freely online or for purchase. The reader is referred to NRC (2008) for information on a variety of such satellite missions.
Global positioning system (GPS) technology measures a highly accurate three-dimensional position on Earth’s surface at a precise time relative to known positions of several satellites. Information can be used to determine rates of tectonic motion, including uplift rates. Differential GPS measurements have yielded detailed observations of glacier surface velocities and temporal variations in those velocities that are important in understanding the glacier dynamics.
The issue of “timing” or “age” of landforms and surface attributes is of great interest to Earth surface processes research. Accurate age data enable the history and rates of processes to be determined—for example, dating techniques are being used to study the rates of floodplain sedimentation and soil processes, faulting histories and paleoseismic recurrence intervals, and the topographic evolution of mountain ranges. A wide range of dating techniques exists; some of the most frequently used types are listed below:
Fallout-derived and short-lived isotopes are used for quantifying sediment transport, deposition, and chronology on time scales of days (for example, beryllium, lead, and cesium isotopes).