Applications
Topography
In surveying, laser distance measurement for large-scale mapping applications is revolutionizing digital elevation data collection. This technique is an alternative to other data collection sources such as the Digital Terrain Model (DTM). It can be used as a data source for contour processes and contour generation for digital orthophotos.
A lidar system emits pulses of light that are reflected by the terrain and other objects of a certain height. Photons from the reflected pulses are transformed into electrical impulses and interpreted by a high-speed data recorder. Since the formula for the speed of light is well known, the time intervals between emission and reception can be easily calculated. These intervals are transformed into distance helped by the positional information obtained from the aircraft/terrain GPS receivers and the onboard inertial measurement unit (IMU), which constantly records the altitude of the aircraft.
Lidar systems record position (x, y) and elevation (z) data at predefined intervals. The resulting data gives rise to a very dense network of points, typically at intervals of 1 to 3 meters. The most sophisticated systems provide data not only from the first return but also from subsequent returns, which provide heights of both the terrain and its vegetation. Vegetation heights can provide the starting basis for analyzing applications of different vegetation types or height separation.
A significant advantage of this technology, over others, is that data can be acquired in atmospheric conditions where conventional aerial photography cannot. For example, data collection can be done from an airplane during a night flight or in conditions of reduced visibility, such as those that occur in foggy or cloudy weather.
Standard photogrammetric products derived from lidar data include contour and elevation models for orthophotos. To obtain accurate contours, post-processing of the initial data is required. Since lidar data is obtained on elevated objects (e.g. buildings), sophisticated algorithms are used to remove points relative to these objects. Due to the high density of points, very few, if any, breaklines are required to accurately represent the terrain. However, the presence of the lidar system and the use of post-processing software, validation procedures must be incorporated into the process to ensure that the final contours are representative of the terrain. The end user should also consider that lidar-derived contours will have a different appearance than those compiled using conventional photogrammetric techniques. Due to the point density obtained, lidar-derived contours, although highly accurate, will tend to have a more broken appearance.
Post-processing and 3D verification are also recommended when using lidar data to generate digital orthophotos. Although the vertical precision requirements for generating an orthophoto are less strict than for contour generation, the data should be verified for bulk errors. Points on buildings are not necessarily required to be removed. In fact, buildings modeled with lidar data will be "rectified" in their true position (true orthophoto) and radial distortions caused by tilting of buildings eliminated. This improvement is somewhat affected by the fact that the edges of buildings may tend to look rounded; this depending on the location of the points relative to the edge of the building.
With post-processing the following data can be obtained:.
• - Extraction of ground level.
• - Extraction of buildings.
• - Extraction of trees and forest masses.
• - Terrain purification tools.
• - Creation of three-dimensional vectors.
• - Building squaring tool.
• - Editing tool.
• - Cropping images.
The precision of the data obtained using the lidar technique depends on:
• - Pulse rate.
• - The flight height.
• - The diameter of the laser beam (depending on the system).
• - The quality of GPS / IMU data and post processing procedures.
Accuracy of 1 meter in the position coordinates and about 15 cm in the height coordinate can be achieved, if the conditions under which the measurements are carried out are optimal. However, for any large-scale application that requires high precision, the data obtained will have to be compared with other techniques. Usually the points obtained (with their three dimensional coordinates) are superimposed on digital images. To achieve this, digital photogrammetric stations are used.
Most lidar systems and applications work with the same format, the LAS format, whose specification has been developed by the American Society for Photogrammetry & Remote Sensing (ASPRS), and which has become a de facto standard for working with lidar data.
LAS is a public file format that allows the exchange of files containing three-dimensional point cloud information. The LAS format is a binary file that maintains all the information from the lidar system and preserves it according to the nature of the data and the capture system.
[2].
Speed detection
It is the technology used by police laser guns to determine the speed of vehicles traveling in road traffic. It differs from radar in that instead of using radio waves, a beam of pulsating laser light is used in the infrared band, whose pulsation frequency is 33 MHz and whose wavelength is 904 nm.
The advantages of lidar over radar are several:
• - It's much faster. Under normal circumstances it can obtain the vehicle speed in only 3 tenths of a second.
• - Since it emits a beam of laser light, the beam does not diverge as much and is much narrower than that of radar, which disperses and bounces off the environment. The laser beam forms a very narrow cone. At about 500 meters it has a width of approximately 2.5 meters in diameter, so you can point the gun at a specific vehicle and determine its speed even if there are more cars circulating around it. It can, therefore, be used in heavy traffic by targeting the vehicles chosen. Furthermore, due to this way of working and its speed, detection using detectors installed in vehicles illuminated by the beam is quite ineffective, since when the detector alerts of the presence of the laser it is too late, because the gun has already registered its speed.
• - It is easier to handle, transport and maintain.
• - It is more economical than a radar.
• - It can work, like radar, at night, in the rain, from bridges, in parked vehicles, in automatic or manual mode, etc.
• - The only limitation of the lidar laser is that it always has to be static. The radar can be used while moving, but the lidar laser cannot move while measuring.
adaptive optics
Adaptive optics is a technique that allows you to correct the most important disturbances that astronomical images suffer due to the Earth's atmosphere. With this system it is possible to obtain sharper images, or as astronomers explain, better spatial resolution. The difference introduced by this technique is comparable to that between looking at an object located at the bottom of a pool with or without water.
Of its importance for astronomical research speaks the fact that all telescopes or observatories with telescopes larger than 4 meters have developed or are developing adaptive optics systems appropriate to their needs.
The possibilities that adaptive optics offer to astronomy are spectacular. Eliminating disturbances produced by the atmosphere is essentially equivalent to observing from space.
Atmospheric disturbances cause a loss in sharpness or spatial resolution. This loss translates, on the one hand, into a diminished ability to resolve objects, that is, to carry out detailed studies of their morphology. On the other hand, it also influences the ability to detect weak objects, since the image is dispersed into larger points of light.
The improvement introduced by adaptive optics can be quantified using the relationship between the size of the telescope and the size of the best image it can obtain. The detection power of a telescope increases with the diameter of its primary mirror and decreases with the size of the image it forms of a point object (hence the importance of image quality in a telescope). Therefore, the difference with the same 10-meter mirror, between being able to focus images of 0.4 seconds of arc (possible on a night of excellent visibility) and an image of 0.04 seconds of arc, which should be possible with an adaptive optics system, would be equivalent to having a primary mirror of 100 meters. Hence, as we said at the beginning, most of the important observatories and telescopes either already have their own adaptive optics system or are working on it.
Adaptive optics is a technology that allows us to determine and correct a large part of the aberrations with which the wave front of the observed objects arrives. The wave front is the geometric envelope of all the light rays that came out at the same time from a luminous object. When the origin of the light is a point, the wave front is spherical; but if it is far enough away, as in the case of stars, that front is practically flat.
In an adaptive optics system, the wavefront, disturbed by the atmosphere, is first analyzed by a wavefront sensor, which determines its aberrations. This information is passed to the phase reconstructor"), which calculates the corrections that must be made and the deformations that the deformable mirror must adopt to compensate for the original aberrations of the wavefront.
With the "sensing" of the wave front, the aim is to measure the aberrations introduced by the column of atmosphere that the light from the astronomical object passes through. Normally, the objects to be studied are very faint, so the measurement of wavefront perturbations must be carried out with a bright star close to the object of interest so that the light from this reference star passes through approximately the same column of atmosphere as the object. However, it is not always possible to find stars close enough to the astronomical object of interest and bright enough to be used to measure the wavefront.
Forest management
In firefighting, the availability of an accurate model of the type of fuel present at each point on the terrain is essential to be able to accurately predict the behavior of the fire and thus be able to make decisions about the attack techniques to use or the resources necessary to fight the fire.[3].
Thanks to lidar, it is possible to generate an accurate map of fuel patterns based on the vertical information captured by lidar measurements. In addition, it is possible to further improve precision by combining the data captured by the lidar with data obtained by other means, such as multispectral images.[4].
Taking into account the height values provided by the lidar and the vertical distribution of fuels, captured by the relative position in different height intervals of groups of measurements within the point cloud, it is possible to determine both the amount of biomass present and its type.[5].
Geology and soil science
The appearance of lidar technology has represented a great advance in the study of the earth. Thanks to the high-resolution digital elevation models obtained through this technique, it allows its application in various fields of geology.
The possibility of obtaining detailed models of topographic structures: river channels, terraces, among others, has promoted and facilitated the study of physical and chemical processes of the Earth's surface; interference of atmospheric agents, characterization and genesis of relief forms, erosion and weathering processes….
This technique has managed to become a leading tool for detecting faults, monitoring them and studying them. With 3D digital models, it allows us to obtain the before and after of a plate movement, being able to make precise measurements, key to understanding how these natural phenomena occur.
Among other geological applications, it is worth highlighting the monitoring of glaciers (to evaluate the retreat of glaciers and its relationship with changes in the hydrological cycle), analysis of coastal change, movement of tectonic plates, volcanic eruptions, landslides...
rock mechanics
Lidar is widely used in rock mechanics for the characterization of rock masses and the detection of changes in slopes. Some of the rock mass properties that can be extracted from 3-D point clouds acquired using LiDAR are:
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- Orientation of discontinuities[6][7][8].
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- Spacing of discontinuities and RQD[9][10].
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- Opening of discontinuities.
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- Persistence of discontinuities[10][11].
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- Roughness of discontinuities[10].
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Some of these properties have been used in the characterization of rock masses through the RMR. Furthermore, since the orientation of discontinuities can be determined from LiDAR data, it is also possible to obtain the geomechanical quality of rock slopes using SMR. Finally, the comparison of different 3D point clouds of a slope acquired at different times allows researchers to study the changes produced in the scene during the studied time interval due to the development of rock falls or other gravitational processes.
Other applications
In atmospheric physics, through the use of lidar instruments it is possible to measure densities of certain constituents of the atmosphere (aerosols, clouds, potassium, sodium, oxygen and molecular nitrogen, etc.). With the most advanced technology it is possible to calculate temperature profiles or measure the wind structure.
As automakers and technology players scramble to develop autonomous vehicles, lidar has become a highly sought-after technology.[12] The downside to these sensors is their price (they can cost tens of thousands of dollars). New developments aim to have a lidar sensor on a chip smaller than a grain of rice. This development does not seek to replace current sensors, but rather to coexist with them, since while a lidar sensor provides long-range precision, solid-state lidars would be for detection at short distances, focusing on details that sometimes go unnoticed.[13] Recent studies have shown that the use of lidar together with artificial intelligence techniques allow us to offer solutions to the challenges of autonomous driving.[14][15].