Project 3: Acquiring Geographic Data
Topographic maps are maps that display large-scale detail and depict elevation. They come in many different forms including digital elevation models (DEMs). DEMs are “grids of elevation values that are arranged in south-north profiles” (DiBiase 2007). DEMS are raster representations of a terrain surface, and many different sources are available to produce DEMS (DiBiase 2007). One of the methods is LIDAR. LIDAR surveys have been conducted across many areas of the world, including the metropolitan region in which I live, Portland, Oregon.
What is LIDAR?
LIDAR is an acronym for Light Detection and Ranging, and is a remote sensing technique used by geographers to measure elevation and create high-resolution topographic maps. The International Lidar08 Mapping Forum defines LIDAR as “an airborne mapping technique which uses a laser to measure the distance between the aircraft and the ground.” (The International Lidar08 Mapping Forum 2007). By measuring the distance between the aircraft and the ground, an extremely accurate map of the terrain can be produced. LIDAR is also useful because it has the ability to determine topography through vegetation canopies and it can even determine underwater terrain! LIDAR is also referred to as Laser Altimetry, Airborne Laser Mapping, or Airborne Laserscanning.
There are three generic types of LIDAR including range finders, Differential Absorption LIDAR (DIAL), and Doppler LIDAR (USGS 2007). Range finders are the simplest types of LIDAR, and are used to measure distance from an object to the LIDAR instrument (USGS 2007). DIAL is used to measure chemical concentrations in the atmosphere by using two different laser wavelengths. One is absorbed by the molecule of interest while the other is not. The difference in intensity of the return signals is used to deduce the concentration of the molecule being investigated (USGS 2007). Finally, Doppler LIDAR is used to measure the velocity of a target. When the laser transmitted from the LIDAR instrument hits a moving target, the wavelength is changed slightly. This is known as a Doppler shift. Doppler LIDAR records the Doppler shift and uses this to determine velocity of atmospheric targets such as aerosol particles within the wind (USGS 2007).
LIDAR has a variety of uses. Police officers use range finder LIDAR to enforce speed limits. LIDAR used by the police forces is able to actually calculate the speed, which is different from radar, which only can measure a car’s Doppler shift to interpolate the speed of a car (Veil 2007). The LIDAR that police use observes the changing amounts of time it takes to “see” reflected pulses of the lasers that are emitted in a highly focused beam of light from the LIDAR gun to the cars (Veil 2007). This amount of time is then calculated in order to determine the speed of the vehicle. LIDAR is also used by geologists to measure seismic faults, by meteorologists to measure atmospheric winds, and by oceanographers to measure phytoplankton fluorescence. In addition, it has been used by car manufacturers in testing Adaptive Cruise Control and by NASA in its Mars rovers. The above examples are only a sampling of the varied uses of LIDAR currently employed.
Geographers employ LIDAR to produce maps of terrain, using the range finding method of LIDAR. This form of LIDAR measures the distance from the Earth’s surface back to the plane or helicopter. A laser pulse is generated from the plane, travels down to the surface, and then bounces back up to the plane (Harding 2000). The time the pulse takes to travel is converted into distance using calculations based upon the speed of light (Harding 2000). The laser scanner (or laser altimeter) is mounted photogrammetrically in the bottom of the airplane or helicopter along with an Inertial Navigation System (INS) and an Airborne Global Positioning System (GPS) (Spencer and Gross Inc. 2007).
Figure 1: Laser-scanning from an airplane to produce LIDAR images. Obtained from http://www.sbgmaps.com. Used here for educational purposes only.
The laser scanners can fire thousands of pulses per second along a narrow corridor that can be adjusted by a rotating mirror. The GPS locates the airplane’s position along the flight path (using both satellites and a known location on the ground), while the INS measures the orientation of the plane as well as the laser scanner mirror (Harding 2000). Together, all of this information combines into a map of the terrain. It also immediately geo-references the data, reducing turn around time for the production of the DEMs. One company specializing in LIDAR notes that “with a laser altimeter system composed of these components the absolute coordinates of surface spots can be determined with vertical and horizontal errors of less than 10 cm (4 in) and 20 cm (10 in), respectively” (GeoLas Consulting 2007).
Advantages and Disadvantages of LIDAR DEMs
USGS DEMs are produced by "interpolation from Digital Line Graph (DLG) hypsography and hydrography layers" (Dibiase 2007). DEMs using LIDAR have finer spatial resolution than the USGS DEM. This resolution aids in the production of maps with terrain elevation that are more accurate than those created by USGS DEM. In addition, LIDAR data is already in a digital form and geo-referenced, so it can be used immediately by GIS operators (GeoLas Consulting 2007). The figures below show a portion of a 7.5 minute USGS topographic quadrangle and the USGS DEM and LIDAR DEM of that portion. The figures below show that the finer spatial resoultion of the LIDAR DEM produce maps with finer precision and accuracy.
Figure 2: The above maps show the differences between USGS DEMs and LIDAR DEMS. Obtained from http://pugetsoundlidar.ess.washington.edu/example2.htm. Used here for educational purposes only.
Another advantage of LIDAR DEMs is that LIDAR can penetrate vegetation canopies and even underwater. USGS DEMs cannot judge topography beneath vegetation canopies or underwater. LIDAR surveys do not need property owner permission and can be completed in weather conditions (such as rain or cloudy skies) that would limit the use of aerial photography and photogrammetry (used to create USGS DEMs).
Figure 3: The above figure demonstrates a LIDAR survey beneath the vegetation canopy. Obtained from http://www.geolas.com/Pages/laser.html. Used here for educational purposes only.
The main disadvantage of using LIDAR DEMs over USGS DEMs is cost. Although LIDAR DEMs are more cost effective during production than USGS DEMs, USGS DEMs are taxpayer funded and generally free for the GIS user. LIDAR DEMs need an aircraft and trained personnel in LIDAR technology. So although it is cheaper to produce, if you need a DEM as a GIS operator, USGS DEMs are often cheaper to obtain.
Another disadvantage of LIDAR DEMs is that they are geo-referenced based upon GPS. GPS units are prone to certain errors, known as User Equivalent Range Errors, which could dilute the precision of the data obtained from the LIDAR system (DiBiase 2007). Finally, LIDAR cannot accurately map stream channels, shorelines, or ridgelines that often are visible on photographic images (North Carolina Cooperating Technical State 2007). Contours based upon downward-flowing streams are not available solely from LIDAR, and have to be manually entered (NCCTS 2007). USGS DEMs use DLG hypsographic and hydrographic data, and therefore can provide a better depiction of stream channels than LIDAR DEMs.
Case Study
As a professional archaeologist, I was intrigued to see how LIDAR could be used to aid in cultural studies of the past. In November 2000 at a NATO sponsored workshop, research was presented on a LIDAR survey of Yorkshire, UK that “revealed evidence for the earthwork survival of a Roman fort that had previously been though to have been completely leveled by plowing” (English Heritage 2007). This prompted the use of LIDAR in archaeological surveys in the United Kingdom (English Heritage 2007). In the course of my research, I discovered that the majority of LIDAR surveys conducted specifically for archaeological studies have occurred in the United Kingdom. An interesting and significant case study was when LIDAR was used to map the physical and cultural landscape around Stonehenge.
Figure 4: Stonehenge, Wiltshire County, England. Obtained from http://www.english-heritage.org.uk/server/show/nav.16465. Used here for educational purposes only.
Stonehenge is one of the world’s most renowned archaeological and cultural sites. It is comprised of the well-known stone circle surrounded by a ceremonial landscape comprised of more than 300 burial mounds as well as Stonehenge Avenue, the Cursus, Woodhenge, and Durrington Walls (English Heritage 2007). In 1986, Stonehenge was inducted as a UNESCO World Heritage Site (UNESCO 2007). Altogether, the Stonehenge World Heritage site comprises 2,600 hectares (approximately 6,400 acres). As part of the requirements to produce a management plan for the site, English Heritage and the National Trust commissioned a LIDAR survey of Stonehenge and the surrounding landscape. LIDAR was used because there was hope that it would produce a detailed terrain model for the entire site and also test LIDAR’s ability to record archaeological features that have been leveled by years of plowing (English Heritage 2007).
The LIDAR survey of Stonehenge was a success. A detailed LIDAR DEM was produced for the Stonehenge region. Known archaeological features were seen in greater clarity than previously mapped, and additional features were seen that were unknown prior to the LIDAR survey. The LIDAR survey at Stonehenge, one of the most studied archaeological sites in the world, shows that LIDAR is an effective tool to examine an archaeological landscape, even if it has been the subject of extensive plowing and/or archaeological excavations. The figure below shows the results of the LIDAR survey.
Figure 5: Stonehenge LiDAR data, processed into a 3D surface model ready for analysis. Major archaeological features are annotated. Obtained from http://www.stonehengelaserscan.org/landscape.html. Used here for educational purposes only.
The LIDAR survey was also used to create an amazing 3-D flyover of the Stonehenge region. This video is used as both a research tool by archaeologists and as a way to show the public the cultural landscape surrounding Stonehenge (http://www.vimeo.com/387367/).
The benefits of LIDAR for archaeologists are incredible. LIDAR is able to map the terrain around an archaeological site with a high degree of accuracy and precision. In addition, there is a potential during a LIDAR survey for previously undetected cultural features to be found. This enables not only a better understanding of a study area, but it also saves time and money excavating in search of underground features that a LIDAR survey potentially could pick up.
CONCLUSION
The use of LIDAR in surveys has proven to be a fast and extremely accurate way of generating DEMs. While the cost of LIDAR DEMs relative to USGS DEMs is high, LIDAR produces DEMs quicker and with better resolution. In addition, LIDAR surveys can happen in weather conditions that would prohibit photogrammetry, and also can determine topography beneath vegetation canopies. The ability of LIDAR to quickly deliver highly detailed terrain maps that are already geo-referenced outways the cost of the surveys, especially for particular projects, such as the Stonehenge case study. As technology becomes cheaper, LIDAR technology will continue to expand and will occupy a larger portion of the marketplace in producing high quality digital maps.