Topographic Mapping

Subtitle

LIDAR

Figure 3: A laser scanner emits pulses of laser light and records return time and intensity.  Return time tells the distance between the aircraft and the surface. GPS satellites constantly track the latitude, longitude, and altitude of the aircraft. Using these xyz coordinates, the angle of the sensor, and the distance to the surface based on laser return time, a computer is able to determine the surface elevation. This image shows a whiskbroom scanner, which swings back and forth perpendicular to the flight direction to increase in the swath width. The readings will be in a zigzag pattern.

Image credit: U.S. Forest Service  https://www.fs.fed.us/pnw/olympia/silv/lidar/

Lidar is similar to radar, but it uses visible or infrared light, allowing the use of low-divergence laser beams due to the shorter wavelength. A laser is mounted on an aircraft or satellite and pulses as it flies (Figure 3). The return time and return intensity of each pulse is recorded. Return intensity gives information about surface type because different surfaces have different reflection characteristics. Return time tells the distance between the aircraft and the surface. GPS satellites constantly track the latitude, longitude, and altitude of the aircraft. Using these xyz coordinates, the angle of the sensor, and the distance to the surface based on laser return time, a computer is able to determine the surface elevation. The sensor can have an accuracy around 5cm, but the limiting factor is the GPS accuracy, reducing total accuracy to 15-25cm over non-forested surfaces. Accuracy is reduced in forested areas because some pulses will bounce off the canopy while others will hit the ground, making it more difficult to determine ground elevation. Still, LIDAR is more accurate than radar at measuring point elevations, particularly in forested areas because radar beams have higher divergence and thus do not penetrate the canopy as easily. The lower beam divergence of LIDAR makes it good for high-accuracy coverage of a small area, but it is more difficult to map large areas at high resolution due to the smaller field of view and the limitation that accuracy decreases with speed because the distance between pulses increases, so LIDAR is usually flown on slow aircraft and is rarely flown on Earth-orbiting satellites. Another limitation is that LIDAR uses visible and infrared wavelengths that do not penetrate clouds as easily as radar microwaves, so it cannot be used by high-altitude aircraft or satellites when there are clouds below.

Applications
LIDAR is great for collecting high-resolution topographic data for small areas. Broader mapping is more expensive and less practical because LIDAR is usually flown on aircraft instead of satellites, as mentioned above. That said, NASA's Ice, Cloud, and Land Elevation Satellite (ICESat) used a 1064nm and 532nm laser to map ground and ice topography, in addition to clouds, but the resolution was not great. For higher resolution mapping, aircraft are used. LIDAR is very useful for geologic mapping, especially when detail is important. For example, an active fault scarp was identified in 1996 on Bainbridge Island near Seattle, using airborne LIDAR. Other remote sensing techniques, such as radar and visible imagery would not have been able to detect the fault due to the thick canopy cover, which was only penetrable by thin laser beams. Identification of this scarp was extremely important so that geologists could locate and study its rupture patterns in order to determine the risk to the Seattle area. Shallow-water bathymetry (up to 50m depth) is another important use of LIDAR because green light penetrates water more easily than microwaves. The USGS  Experimental Advanced Airborne Research Lidar (EAARL) uses 530nm light to create high-resolution maps of coastal areas and rivers. This can be extremely helpful for sea and river navigation, geology, and tracking changes in coral reefs. The moon, Mars, and asteroids are also well-suited for LIDAR mapping because they lack clouds, and a satellite's orbital velocity at a given altitude is much lower than it would be above Earth due to their lower gravity, so it is possible for the satellite to travel low and also slowly. Some examples of space missions using LIDAR are Clementine, the Mars Global Surveyor, and NEAR Shoemaker, which respectively mapped the moon, Mars, and the asteroid Eros.