Using technology as an aid to the geomorphologist

Geomorphology requires characterization of the earth's surface at sufficient high resolution in 3 dimensions to explicitly represent landforms. Measuring change requires repeat survey, thus adding the 4th dimension. One exciting powerful tool that many geomorphologists have begun to use in their research is LiDAR (Light Detection and Ranging). LiDAR technology measures the earth's surface by using a scanning laser along with inertial navigation (IMU) and GPS to measure the landscape at high resolution (decimeter accuracy).

LiDAR can be used to create high-resolution imagery, reconstruct topography and allow geomorphologists to evaluate the landscape at a level beyond the capabilities of photography, satellite imagery, and topographic maps alone. Although it is not a replacement for being in the field and seeing things firsthand, it allows for an advanced level of analysis away from field sites as well as increasing efficiency and thoroughness in the field. An important capability associated with LiDAR data is processing to virtually "remove" vegetation and obstructions in order to see the bare earth underneath. This can allow for the identification of features such as faults or landslides otherwise invisible under the vegetative canopy. It can also help with field planning in order to know where to go to find various features. And, ecologists use the data to characterize the canopy structure and its relationship to the underlying topography.

Collecting airborne LiDAR for field study is both expensive (several hundred dollars per square kilometer) and involved. In order to collect the data, a low-flying aircraft scans a laser at pulse rates of 10s to 100s of kilohertz. Laser returns are collected within the aircraft, cataloguing the timing of return, the scanner orientation (IMU), and position (using GPS). There are also terrestrial LiDAR (TLS) units that allow for LiDAR data to be collected for a small area from a ground-based vantage. Many of the large LiDAR datasets such as the "B4" project flown along the southern San Andreas Fault in California are available online for free. One of the more dynamic free LiDAR websites is www.opentopography.org where many of the western United States airborne LiDAR datasets are available to download.

Once LiDAR data have been collected for an area of interest, geomorphologists can use the data products for research (See Figure 1). LiDAR data can be processed and made available in many different file formats, allowing for usage in computer programs like ArcGIS, Matlab, and the free program Google Earth. The applications of LiDAR for the geomorphologist are vast. LiDAR can be used to locate faults and measure offsets that could not be measured previously with satellite imagery alone (See Figure 2). The high resolution (<1m) of LiDAR topography allows for more detailed analysis at the sub-meter scale. Repeat LiDAR or comparison of LiDAR with aerial photography can be used to evaluate landscape change due to rivers and streams, landslides, coastal change, volcanic activity or earthquakes. LiDAR can be used to generate topographic maps, cross sections, geologic maps, and 3-D imagery.

One interesting study involving LiDAR and its applications to geomorphology is the work of Dr. Ramon Arrowsmith and Dr. Olaf Zielke (Arizona State University) studying offsets along the San Andreas Fault in California. Recently active fault breaks along the San Andreas Fault were extensively mapped using the B4 project LiDAR data, and troughs, ridges, sags, and offsets channels along the zone were assessed confidently (Arrowsmith and Zielke, 2009). Using the LiDAR data in Matlab, offset channel profiles can be back-slipped and a best fit determined, allowing for total offsets to be analytically calculated. This process has traditionally been done in the field, but LiDAR allows for many of these offsets to be analyzed in a matter of minutes.

• Arrowsmith, J R., and Zielke*, O., Tectonic geomorphology of the San Andreas Fault zone from high resolution topography: an example from the Cholame segment, Geomorphology; special issue on high resolution topography, doi:10.1016j.geomorph.2009.01.002, 2009.
• Christopher Crosby, M. S., A geoinformatics approach to LiDAR data distribution and processing with applications to geomorphology, Arizona State University, 119 pp., 2006. www.opentopography.org