LiDar in VirginiaWhat is LiDAR?
LiDAR, which stands for Light Detection and Ranging, is a method of remote sensing that uses lasers to measure the distance between two objects. Precise, three-dimensional models of the Earth’s surface can be generated from these measurements.
Airborne LiDAR is collected using a pulsed laser that originates from an airplane or helicopter. A sensor on the aircraft measures the time it takes for the laser to return after reflecting off a target, in this case, the surface of the earth. The laser travels at a known speed. By measuring the time it takes for the laser to return to the aircraft, the precise distance between the aircraft and the ground surface can be calculated. The LiDAR sensor also includes a GPS receiver to detect the location and orientation of the aircraft when the laser pulses and the sensor receives data.
The collected LiDAR data are converted into millions of discrete points, called a point cloud. The elevation of the return will vary, depending upon the shape and size of the objects the LIDAR laser pulse is reflected from on the ground surface. Using specialized computer software, these points can be classified into various types of returns: tall trees, brush, grass, asphalt surfaces, buildings, etc. Geoscientists are generally only concerned with how the ground surface appears, without vegetation. Thus, only ground surface, or bare-earth returns, are used and vegetation and building returns are excluded. This bare-earth point cloud is then used to generate a digital elevation model (DEM) of the earth’s surface.How do geoscientists use LiDAR?
LiDAR helps geologists see the surface of the Earth with greater precision and accuracy than was previously possible. Many geologic landforms visable on LiDAR derived DEMs cannot be seen using traditional aerial photography, like that found on Google Earth, or even by walking on the ground!
LiDAR DEMs can be used to show the surface elevation, but geoscientists make derivative maps to help visualize the landscape. Two of the most common maps are hillshade (or shaded-relief) and slope maps. Hillshade maps portray the landscape in 3-D. Slope maps show variations in the steepness of the topography. When symbolized from dark-to-light, dark colors represent steep areas and the light colors represent gently sloping to flat areas. Those slope maps or “slope shades,” can show the terrain in high-detail.
Older DEMs were generated by the U.S. Geological Survey (USGS) with a pixel resolution of either 30-meters or 10-meters. This resolution is coarse and does not show the detail of geologic landforms very well. DEMs derived frm the newly acquired LiDAR for Virginia are available as rasters with 1-meter resolution, a significant improvement. Below is an example of the older, low-resolution, 10-meter USGS DEM data for McAfee Knob, a well-known overlook on the Appalachian Trail in Roanoke County. Compare this to the high-resolution, 1-meter LiDAR data that is now available.
10-meter Digital Elevation Model
from older USGS dataset
1-meter LiDAR derived hillshade raster
1-meter LiDAR derived slopeshade raster
Download a sample of LiDAR data for McAfee Knob here! You will need Google Earth loaded on your computer to view the data.Where are LiDAR data available in Virginia?
LiDAR data for about two-thirds of the Commonwealth have been collected and processed, and are available for download from the USGS. Please keep in mind that LiDAR data sets contain millions to billions of points and can be several gigabytes in size. LiDAR data are often found in LAS or LAZ file format, an industry-standard binary format used for storing 3-D point cloud data. A user must have some type of Geographic Information System (GIS) software to work with files. The first step is often transforming the LAS file into a different format. At DGMR, we use ESRI ArcGIS products to convert the LiDAR point cloud into the raster images you see above, but there are several freeware programs that can be used to view and convert LIDAR data.
LiDAR Coverage Map of VirginiaWhere can I find LiDAR data?
LiDAR is available to the public through the U.S. Geological Survey – 3-D Elevation Program (3DEP). Existing LIDAR 1-meter datasets in the United States are available as either LAS files or IMG tiles. You can directly download LIDAR data from several websites including the National Map, the USGS FTP website and VGIN Clearinghouse.
The National Map Viewer is useful for downloading data for a small area, like a neighborhood or town. You will need GIS software to convert the data into a hillshade or slope map.
LIDAR data can be downloaded from the USGS FTP site. This is useful if you need a dataset for a large area, like a county or region. The data are available by project area as LAS or IMG files.
The VGIN Clearinghouse shows the spatial extents and status of LiDAR acquisition projects in Virginia from 2010-2016.
How does DGMR use LiDAR?
DGMR geologists regularly use LiDAR data to map the distribution of bedrock formations and surficial deposits, locate natural geologic features such as faults and sinkholes, identify areas affected by human activity such as historic mining, and assess the impact of landslides and other geologic hazards. A recent project in Petersburg National Battlefield used LiDAR data to identify Civil War earthworks and fort structures!
Follow the links below to see some LiDAR images of common geologic features throughout the Commonwealth:
Have more questions? Ask a geologist!