Airborne Laser Scanning:Remote Sensing with LiDAR

• Laser remote sensing background• Basic components of an ALS/LIDAR system• Two distinct families of ALS systems

• Waveform• Discrete Return (small footprint)

• Working with the data, processing requirements• Current applications and developing applications• Advancing ALS / LiDAR technology

ALS / LIDAR OUTLINE

Airborne Laser Scanning• ALS/LiDAR is an active

remote sensing technology that measures distance with reflected laser light. LiDAR: LIght Detection and Ranging or Laser Imaging Detection and Ranging)

• 1st developed in 1960 by Hughes Aircraft inc.

• Modern computers and DGPS make it practical.

• Typically used in very accurate mapping of topography.

• New technologies and applications are currently being developed.

ALS systems take advantage of two of the unique properties of laser light:

1. The laser is monochromatic. It is one specific wavelength of light. The wavelength of light is determined by the lasing material used.Advantage: We know how specific wavelengths interact with the atmosphere and with materials.

2. The light is very directional. A laser has a very narrow beam which remains concentrated over long distances. A flashlight (or Radar) on the other hand, releases energy in many directions, and the energy is weakened by diffusion.Advantage: The beam maintains its strength over long distances.3mrad divergence = 3 m at 1 km and 15m at 5 km.

Why use a Laser?

Radians• A full angle is 2π radians, so there

are 360° per 2π radians, equal to 180°/π or 57.29° / radian.

• A milliradian (mrad) is 1/1000 radian.

• 3 mrad = 3*0.05729°.

ALS mapping concepts• The position of the aircraft is

known (from DGPS and IMU-Inertial Measurrment Unit).

• Measures distance to surfaces by timing the outgoing laser pulse and the corresponding return (s).

• Distance = time*(speed of light)/2

• By keeping track of the angle at which the laser was fired: you can calculate the X, Y, Z position of each “return”.

• Requires extremely accurate timing and a very fast computer.

Basic ALS System Components• Laser (usually NIR –1064nm)

mounted in an aircraft.• Scanning assembly –

precisely controlled rotating mirror.

• Receiver for recording reflected energy “Returns”.

• Aircraft location system incorporating Differential GPS and Inertial Navigation System(INS, = IMU).

• A very fast computer to synchronize and control the whole operation.

Two Distinct Families of ALS SystemsWaveform systems (a.k.a. large-footprint)

Discrete-return systems (a.k.a. small-footprint or topographic)

Records the COMPLETE range of the energy pulse (intensity) reflected by surfaces in the vertical dimension. Samples transects in the horizontal (X,Y) plane.Waveform systems designed to capture vegetation information are not widely available. Waveform systems include SHOALS, SLICER, LVIS, ICESat.

SAMPLES the returned energy from each outgoing laser pulse in the vertical plane (Z) (if the return reflection is strong enough).

Most commercial lidar systems are discrete return, many different types are available.

Waveform LIDARThis is the “waveform” graph of energy intensity

Notice that it follows the “shape” of the tree biomass

Waveform LIDAR: SLICER

Waveform systems have been used to accurately measure (r2 >.90) vertical distribution of tropical and temperate forests (Lefsky 2001)

Spaceborne waveform LIDAR systems.ICEsat launched in 2003 for survey of topography and polar ice. VCL (Vegetation Canopy LIDAR) was designed to sample 5 separate widely spaced tracks with 25m footprint waveforms. Has been cancelled.Do not provide “wall to wall” mapping.

Spaceborne LIDAR: ICEsat and VCL

Discrete-return LIDAR• Records data as X,Y,Z

points.• Spatial resolution is

expressed in terms of “post spacing” which is the avg. horizontal distance between points.

• Returns are “triggered” if the laser reflects from a surface large enough to exceed a pre-set energy threshold.

• Minimum vertical distance between returns is ~ 5 meters.

• New capability to record the intensity of point returns.

Discrete Return Data: Millions of X,Y,Z points

Area is approximately: 1 X 0.75mi. includes ~ 440,000 returns

From Point Clouds to 3-D Surface Models• Points are used to create

3D surface models for applications.

• Triangular Irregular Networks (TIN)s are used to classify the points and to develop Digital Elevation Models (DEM)s.

• Points must be classified before use: “bare earth” points hit the “ground”; other point categories include tree canopy and buildings.

• Correct identification of “bare earth” is critical for any lidar mapping application.

LIDAR Surfacing

Discrete return lidar data requires several complex processing steps to develop useable products. These steps are usually divided into two phases: Pre-processing is the preparation of the raw data, correcting for errors induced by flight geometry, removing overlaps between flight lines, surfacing, and accuracy assessment of the raw point locations.

Post-processing is the development of usable information from the point clouds including development of TINs, DEMs, and other products like canopy height maps.

Pre-processing is always done by the lidar contractor.

Working with Commercial LIDAR

• Average reported point location accuracies for a hard flat reflective surface (e.g. airport runway):

2-15 cm vertical25-100 cm horizontal

Discrete Return LIDAR Accuracy

• Reported measurement accuracies: (forestry research, best cases)

Height prediction r2 = 0.93 (Means 2000)

Volume prediction r2 = 0.92 (Maclean and Krabill 1992)Comparing field measurements to laser measurements

• Timing system accuracy is critical: 1 nanosecond = 30cm

• Accuracy is also affected by errors in GPS, surfacing and data analysis.

• Measurements of object height requires accurate surfacing to provide accurate base height.

Applications for Discrete-Return LIDARDigital Elevation Model (DEM)

4m DEM of PREF Subset

Geomorphology

Mapping geomorphic features (Puget Sound LiDAR Consortium)

Geology

Mapping Faults (Puget Sound LiDAR Consortium)

Other ApplicationsMeasuring volume change in open pit mines.

Utilities use lidar to map power transmission line curvature and clearance.

• Can be used to measure forest characteristics including stand height, biomass distribution, volume.

• DEMs are used in planning for erosion and engineering projects including roads.

• Mapping of forest stand structure characteristics and fire fuels conditions is under development.

Discrete Return LIDAR for Forestry

LIDAR Mapping of Forest Structure

LIDAR Captures 3D Structure Information

LIDAR Products for Forest Management

• Systems now available record 50,000 pps. Expect to see 100,000 pps systems soon.

• Automatic merging of multi-spectral imagery with LIDAR is being developed now.

• Laser induced fluorescence and measurement of changes in laser coherence and polarization will provide more information about the surfaces that the laser light interacts with.

• Look for multi-spectral LiDAR using multiple lasers in different frequencies recording intensity to spectrally identify surface materials.

The Future of ALSALS/LIDAR technology is evolving rapidly, the next 10 years will bring many new capabilities and applications.

New ALS Technologies

• Expect higher data density: current systems operate at 15,000hz;

• New systems operate at 50,000hz,• Next generation will be 100,000hz +

This is an example of a new application that merges lidar and multi-spectral imagery to automatically classify features.

1. ALS/LIDAR is an active remote sensing technology designed to take advantage of the unique properties of laser light.

2. Two basic families of lidar systems exist: waveform and discrete-return.

3. Commercially available lidar is discrete-return, it is used in for a number of applications.

4. Accuracy of ALS depends on a number of highly complex interacting factors.

5. Complex processing steps are required to use lidar and accurate surfacing is critical for applications measuring object heights.

6. Development of lidar technology is occurring at a rapid pace, new applications will be developed.

Concepts to Remember About ALS