electromagnetic energy that illuminate a given spot on the earth for an instant (less than 1/100,000 of a second). The energy emitted can be of ultraviolet through near-infrared wavelengths (250 nm to 10 μm), which are much shorter than those of radar pulses. The pulses of light then bounce back and are recaptured by the lidar instrument where the durations of their paths are recorded and analyzed to extract elevation information. The number of returns per unit area for discrete return lidar can be much higher than the number of pulses sent earthward, because each pulse can have multiple (typically three to five) returns.
There are two types of airborne lidar: topographic and bathymetric. Topographic lidar uses an infrared laser to measure elevations across the surface of the earth. Bathymetric lidar employs green laser light to penetrate water and measure the depth of water bodies. In topographic lidar, pulses of light encounter porous objects, such as vegetation, which will have multiple returns. For example, as shown in figure 3.10, a selected single pulse from this discrete return airborne lidar system has three returns from branches and a fourth return (the final return) from the ground. DTMs are generated from the last returns, DSMs from the first returns (buildings must be removed using specialized algorithms), and DHMs from the difference between the digital surface model and the digital terrain model. Lidar returns collectively form a lidar “point cloud” consisting of millions to billions of points that each contain the point’s latitude, longitude, and elevation.
Figure 3.10. Illustration of the returns from a topographic lidar system. Source: Dr. Maggi Kelly
Lidar point density is measured by the average number of pulses sent downward from the aircraft per square meter of ground. As of this writing, “high density” airborne lidar is generally considered to have a point density of greater than eight points per square meter. In vegetated terrain, only a fraction of the pulses of light sent earthward by the lidar system penetrate all the way to the ground, and the number of ground returns decreases as the thickness of the vegetated canopy increases. The lack of ground returns in thickly vegetated areas can lead to inaccuracy in the digital terrain models derived from a lidar dataset. For this reason, the effective resolution of the digital terrain model and the digital height model depend on the point density of the lidar data. The higher the point density, the more ground returns and the higher the resolution of the derived DHM and DTM. It is recommended that lidar data be collected at a point density of at least eight pulses per square meter in project areas with dense forests. Eight pulses per square meter is the minimum point density that meets the US Geological Survey’s (USGS) quality level 1 lidar data specification.2 Figure 3.11 compares hillshades derived from a digital terrain models at USGS quality level 1 versus USGS quality level 2 lidar data, illustrating the enhanced detail and resolution gained by collecting lidar data at higher density.
Figure 3.11. Comparison of a hillshade derived from 1.2 pulses/m2 lidar to one derived from eight pulses/m2 lidar. Source: Quantum Geospatial, Inc.
There are two common types of airborne topographic lidar: discrete return and waveform. Discrete return lidar provides elevation values at the peak intensity of each return. Typically, a maximum of between three and five returns is possible where there is vegetation, but only one return will occur in open areas. Each of the multiple returns is stored as a point in the point cloud, with its associated latitude, longitude, and elevation.
Full waveform lidar—which is mostly still in the R&D phase—provides the entire “waveform” graph associated with a lidar pulse. Because it records the entire waveform of a lidar pulse’s returns and not just three to five discrete peaks, waveform lidar requires 30 to 50 times the amount of data storage as discrete return lidar.
Historically, lidar systems have been able to transmit energy in only one wavelength. However, recent advancements in lidar technology allow for transmitting energy in multiple wavelengths, making multispectral lidar images possible (Teledyne Optech Titan system). Additionally, new technologies such as Geiger-mode (Harris) and Single Photon (SigmaSpace/Hexagon) have been introduced that significantly improve the rate of data collection and resulting point density by increasing the sensitivity of the lidar sensors.
Lenses
Objects emit or reflect electromagnetic energy at all angles. The angles between an object and an imaging surface change as the imaging surface moves closer to or farther from the object. The purpose of a lens in a camera or in an eyeball is to focus the electromagnetic energy being emitted or reflected from the objects being imaged onto the imaging surface. By moving the lens back and forth relative to the imaging surface, we can affect the angle of electromagnetic energy entering and exiting the lens, and thereby bring the objects of interest into focus.
Most remote sensing systems capture electromagnetic energy emitted or reflected from objects at a great distance from the sensor (i.e., at an effectively infinite distance), from hundreds of feet for a sensor in an aircraft to hundreds of miles for a sensor in a satellite. Because these distances approach infinity relative to the focal length, the lenses have a fixed focus.
The combination of the sensor’s lens and the resolution of the imaging surface will determine the amount of detail the sensor is able to capture in each image—its resolving power. The resolution of a digital image is determined by the format size of the digital array of the imaging surface.
Openings
The purpose of a sensor opening is to manage the photons of electromagnetic energy reaching the imaging surface. Too large an opening results in the imagery being saturated with photons, overexposing the imaging surface. Too small an opening results in not enough photons captured to create an image.
Our irises manage the amount of light reaching our retinas by expanding and shrinking to let more or less light onto our retinas. In a camera, the diameter of the opening that allows electromagnetic energy to reach the imaging surface is called the aperture, and the speed at which it opens and closes is called the shutter speed. Together, aperture and shutter speed control the exposure of the imaging surface to electromagnetic energy. In a digital camera, the CCD array is read and cleared after each exposure.
Bodies
Remotely sensed imagery can be used for visualization—to obtain a relative concept of the relationship of objects to one another—or to measure distances, areas, and volumes. For either visualization or measurement, the geometry of the lenses, opening, and imagery surface within the camera body must be known. In addition, for measurement the location and rotation of the imagery surface when the image is captured must also be known.
Sensor Summary
While remote sensor components share similarities with our eyes and consumer cameras, they differ in the following fundamental ways:
Imaging surfaces must be absolutely flat to minimize any geometric distortion.
The energy sensed may be passively received by the sensor from another source (commonly the sun) or actively created by the sensor and then received back by the sensor.
Because
