RASTERTIN

What is LiDAR? LiDAR = Light Detection And Ranging Active form of remote sensing measuring distance to target surfaces using narrow beams of near-infrared light (e.g., 1064 nm) For geomatics applications, LiDAR systems are primarily operated from airborne platforms. ACTIVE PASSIVE

R = Range or distance c = Speed of light (299,792 km/sec) tp = the time that the pulse was emitted t = the time the pulse energy was observed back at the origin Divide by 2 to compensate for round-trip distance VIDEO LINK

DISCRETE POINTCONTINUOUS WAVEFORM The returned pulse is classified into one or more discrete returns x, y, z and intensity Returns are recorded when the return energy exceeds the systems predefined threshold An intensity threshold is used to distinguish a true return from noise Alternatively, the returned energy may be stored as a waveform A waveform is a description of the entire return intensity or behavior, for each pulse, as a function of time Multiple targets within individual pulses from the upper canopy to ground surface to characterize vertical forest structure

AIRBORNE LIDAR TECHNOLOGY Laser Source Emits pulses of laser energy with a typical duration of a few nanoseconds and that repeats several thousands of times per second (kHz) in what is called pulse repetition frequency (PRF). Scanning Mechanism An oscillating mirror that steers the laser beam side-to-side perpendicular to the line of flight. GNSS Reference Station Global Navigation Satellite System – used along with data about IMU to records laser source altitude, location, and speed Swath width? Across-track resolution? Along-track resolution?

TIME VERSUS INTENSITY PLOT The laser energy spreads in a conical fashion as it propagates through the atmosphere Most commercial airborne LiDAR units are characterized by beam divergences that produce footprints between cm from their typical operational altitudes, and thus are considered “small footprint” systems. When the pulse exits the sensor, it generally has a nearly Gaussian profile. As the light interacts with the trees or the ground, some of the energy is reflected back towards the sensor, modifying the waveform shape according to the geometric properties of the target.

SCANNING LIDAR 80,000 to 160,000 pulses per second The LiDAR beam is about the size of a dinner plate Inter-pulse distance is 10cm (relatively constant) Density of point cloud can vary widely (e.g., the lower and slower the plane/helicopter flies, the denser the point cloud)

ARCHAEOLOGY EXAMPLE FIRST RETURN DIGITAL SURFACE MODEL Canopy Open areas BARE EARTH DIGITAL ELEVATION MODEL Built structures under canopy cover ‘Caana’ pyramid Causeways Terraces

ESTIMATION OF SURFACE HEIGHTS A DEM is typically fitted to the returns classified as ground The DEM is subtracted from non-ground returns to calculate surface height relative to ground surface INTERPOLATION OF VEGETATION HITS INTERPOLATION OF GROUND HITS = 50 CM GRID OF CANOPY HEIGHT

SPATIAL INTERPOLATION NATURAL NEIGHBOR  SPLINING ALGORITHMS  INVERSE WEIGHTING 

BAINBRIDGE ISLAND EXAMPLE

LiDAR TYPES PROFILING LIDAR Earliest LiDAR instruments Popular in the ‘80s Used for creating a transect profile SMALL-FOOTPRINT/SCANNING LIDAR Commercially available Typically record at high sampling densities Accuracy ~15cm vertically and ~40cm horizontally Captured via fixed-wing plane or helicopter

LiDAR TYPES GROUND-BASED LIDAR Instrument is mounted on tripod, truck, or tower Commercially available Scans a plane or a hemisphere Most often full-waveform 1.5M points/scan LARGE-FOOTPRINT LIDAR Same principles as small-footprint LiDAR, but the beam diameters are larger More smoothed response surface