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Application of LiDAR in Surveying and Civil Engineering

Introduction & LiDAR History

LiDAR technology, which stands for Light Detection and Ranging, uses light to map virtually any kind of physical environment. Extremely accurate images can be rendered due to LiDAR technology’s use of very precise laser beams. It is a Remote Sensing technique, using either ground-based (Terrestrial Laser Scanning (TLS)) or airborne systems (Airborne Laser Scanning (ALS)), and is also referred to as Airborne Laser Swath Mapping (ALSM); in some military contexts it is known as LaDAR (Laser detection and ranging). In its broadest sense LiDAR refers to a much wider spectrum of techniques than can be addressed in this note, and can be used from static or moving platforms including aircraft and vehicle mounted scanners. LiDAR had its origins in the military, although some of the first uses for laser beams tested by the military utilised high-power beams in attempts to destroy missiles etc, the modern concept of LiDAR grew

from a somewhat less destructive idea. Following on from the concepts behind radar, Airborne Laser (or LiDAR) Bathymetry (ALB) grew out of efforts in the mid 1960s to use the newly invented laser to find submarines. Tests were carried out in the 1970s and by the 1980s there were operating systems in the US, Canada and Australia. Current models of bathymetric LiDAR, specifically the US Geological Survey SHOALS system (Scanning Hydrographical Operational Airborne LiDAR Survey), can map topography above and below the surface of the water, down to a depth that is roughly equivalent to three times the visible depth. While bathymetric LiDAR was being developed, the concept was also extended to topographic LiDAR for measuring surfaces on land. One of the key reasons behind the delay in implementation lies in the nature of the LiDAR technique. The core constituent of the LiDAR system only measures relative position by recording the time taken for a single laser pulse to be fired from a sensor array, strike the surface of an object below and be detected as a reflected

Technology: LiDAR

Sonjoy Deb, B.Tech, Civil Associtate Editor

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signal. It is only possible to calculate the actual location on the ground by knowing the exact position of the sensor array at the time that it fires and records the beam.

To fully understand both the positive and negative aspects of using mobile LiDAR technology for the purposes of surveying and mapping, one must first understand both the positive and negative aspects of traditional surveying. Research indicates that some methods of traditional surveying are deficient in certain aspects where LiDAR excels. In some cases LiDAR is capable of replacing traditional surveying. There are many benefits to using LiDAR technology versus traditional surveying methods. Some of these include worker safety, the improved quality of generated maps and measured data accuracy, economical sustainability, and environmental sustainability. Like any form of technology, there are some downsides to LiDAR technology. These include issues with lines of sight, weak GPS signal strength, initial expenses of LiDAR systems, as well as a slight lack of human involvement in the surveying process. Refer Figure 1 for LiDAR Basic Process.

LiDAR presents an appealing survey method with impressive capabilities, several considerations and limitations attend this method.

Once collected, processed, and cleaned of errors, the terrestrial LiDAR data arrive to the end user as ‘first return,’ which includes the elevation of structures and vegetation, or ‘bare earth.’ The survey data typically appear as XYZ coordinate point data in ASCII text files in the horizontal and vertical reference of choice. Due to the high density of these surveys, the files commonly contain tens of millions of data points, consume gigabytes of storage, and can present challenges for the end user wishing to visualize and analyze the data.

Typically, the point data is converted into a surface, or Digital Elevation Model (DEM). The DEM, comprised of a grid, can be fine or coarse depending on the application and density of the data. Having flexibility for the end user to create DEM’s of various scales is beneficial. Once a surface is created, analysis can be performed including cross sections, contour analysis, and volume calculations. If there are data collected over the same area at different times, the surfaces can be compared and change analysis can be performed.

Cost of LiDAR Survey

The costs for LiDAR surveys largely depend on the size of the subject area and mobilization costs. Terrestrial LiDAR surveys, per square mile, cost substantially less than hydrographic surveys. This cost difference likely results from the lower cost of the terrestrial laser systems as well of as the pressures of marketplace competition. While hydrographic surveys cost more, the widespread application of the data presents opportunities for cost sharing. The same opportunities apply to mobilization costs for large-scale surveys.

Overall, there are many benefits of LiDAR mapping technology. To begin, traditional mapping methods can become expensive due to the length of time that it takes to collect data. LiDAR cuts the time down and therefore can potentially save hundreds of thousands of dollars for a company over the years. Next, LiDAR technology saves vast amounts money when roadways are surveyed. Also, many full time employees can be eliminated, along with several production vehicles, with the usage of LiDAR technology. The elimination of workers and vehicles saves a company money. Lastly, the use of LiDAR technology saves time, as compared to traditional mapping techniques. The implementation of LiDAR technology can save consumers both time and money.

Although cost benefits of LiDAR surveying techniques are the main advantage of LiDAR technology, several other profits present themselves. To begin, much less work needs to be done in the surveying process after the implementation of LiDAR technology. For example, many full-time employees and surveying vehicles are necessary in order to survey land

Figure 1: Basic Lidar Scan

LiDAR in Surveying

Survey methods using Light Detection and Ranging (LiDAR) have become more common for both terrestrial and hydrographic surveys. This rapid survey method provides unprecedented detail over large, even regional areas, and has demonstrated great potential for a variety of uses by coastal engineers and scientists. Until recently, the analysis of LiDAR data presented a great number of challenges, and some types of analysis were impractical without expensive high-end computer workstations and software. However, recent advances in standard desktop computer hardware and software now facilitate the analysis of LiDAR data and provide a new level of analytical capabilities and information. Information from this analysis ultimately allows researchers and managers to gain a better understanding of coastal processes, make better decisions, and save costs.

LiDAR systems employ an aircraft mounted laser system that measures the reflection of laser pulses emanating in a swath over the ground or sea floor. Advances in laser technology facilitate extraordinarily detailed survey data at amazing speeds of up to 100,000 pulses per second for terrestrial systems and 3000 pulses per second for bathymetric systems. These systems can survey over 20 square miles per hour. Depending on the application, point-based bathymetric survey data can collect and provide data in two to five meter spacing. While

Technology: LiDAR

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using today’s typical methods. After the implementation of LiDAR, the number of each can be reduced by over half. A reduction in the number of workers and vehicles needed in order to survey an area of land also means that these workers and vehicles can be either used for surveying elsewhere, or can be eliminated completely, saving the company money.

Economic Sustainability of LiDAR

It is impossible to define one chief definition of sustainability, especially in today’s world where environmental and economic issues of current stability and future sustainability are being taken more seriously than ever before. In general, sustainability is commonly thought of as the ability to endure over time. LiDAR possesses many sustainable features which mesh together in a way that makes LiDAR appear to be an incredibly sus-tainable advancement in areas of economics, ethics, and in environmental matters.

Environmental Sustainability of LiDAR

Mobile LiDAR also provides many overlooked environmental benefits compared to traditional surveying. One might assume that less paper and hand-drafting equipment is consumed when LiDAR is used, but that is not all. The United States Department of Agriculture (USDA) performed a study on LiDAR’s effects on the environment and found virtually no dangers whatsoever. This USDA study concluded that, “LIDAR is unobtrusive and environmentally friendly. Apart from the need to validate the LIDAR with ground truthing, it is not necessary to send pervasive ground crews to conduct intense survey operations”

Principles of Airborne LiDAR

In basic terms airborne LiDAR consists of an active laser beam being transmitted in pulses from a fixed wing or rotary aircraft and the returning reflection being measured. The precise location of the sensor array is known due to a combination of Global Positioning System (GPS) and the Inertial Measure-ment Unit (IMU) in the aircraft ( Refer Figure 1). Using the principle of measuring distance through the time taken for a pulse of light to reach the target and return it is possible to record the location of points on the ground with a very high degree of accuracy, typically 100–150mm in both plan and height. The majority of laser sensors operate by sending out a laser beam that scans across the ground surface by means of a mirror (rotating or oscillating depending on the sensor) or alternatively by a fibre optic scanner. Whatever is the means of emitting the beam, the calculations that enable the creation of Digital Terrain Models (DTMs) etc are based on the returning (reflected) pulse to the sensor. In general, most airborne LiDAR uses eye-safe lasers within wavelengths in the infrared (IR) range; those systems on the current market range from 900nm to 1,550nm. The exception to this is bathymetric LiDAR, which uses a twin beam system; the green beam penetrates the water

and detects the seabed, while the infrared beam detects land and sea surface. Airborne LiDAR, therefore, provides the ability to collect very large quantities of high precision three-dimensional measurements in a short time. This facilitates very detailed analysis of a single site, or data capture of entire landscapes. It does not necessarily provide any information about the point being recorded in the way that multi-spectral data can, nor does it give any inherent information about the nature of the feature being recorded (though see below for full waveform LiDAR). It should also be noted that unlike some remote sensing tools LiDAR is an active sensor in that it sends out a beam and as such it is possible to use it at night or in circumstances when passive sensors would not work. It should be noted, however, when planning a survey that flying at night means that the aircrew are less able to see whether there are clouds present that may affect the quality of the survey, until after the data have been processed. Refer Figure 4,5 and 6 for application of LiDAR survey and output files.

For many early-generation sensors only a small number of return echoes were collected from each pulse – often just the first and last return, with occasionally an additional one or two in between. The first and last returns were considered the most important: the first being equivalent to the Digital Surface Model (DSM) and the last being used as a means to help calculate a Digital Terrain Model (DTM). Within the last few years the latest development of LiDAR sensors has expanded and now, instead of just recording between two and four returns, the new full waveform system digitises the entire analogue echo waveform for each emitted laser beam (Refer Figure 2).

The most useful product of LiDAR for archaeologists is the three-dimensional model of the ground, the DTM, because of the information it can provide in woodland; in non-wooded areas the DSM is preferable because of the absence of smoothing effects. The DTM still requires careful manipulation using specialist software, to facilitate analysis and interpretation of the archaeological features, discussed further below ( Refer Figure 3). Refer Figure 4 for generic LiDAR tile showing the intensity of the returned signal.

Advantages of LiDAR in Topographic Mapping

In the past decade, because of the advantages of LiDAR, it has largely displaced Photogrammetry as the process for

Figure 2: Full waveform LiDAR – The image shows how the full waveform of the LiDAR pulse is recorded over various ground surfaces.

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development of large scale topographic maps. The LiDAR advantages in topographic surveying may be as found below:-

- LiDAR sensors can be operated in any weather - LiDAR sensors are not affected by low sun angles - which

would prevent useful photos - LiDAR can actually operate at night - LiDAR offers greater efficiency, faster results, and can

cover more ground than photogrammetry - Rural and remote areas are easier and quicker to survey

with LiDAR because each point has geo-referenced location and elevation – no orthorectification of image – no network of photo panels required

- Photogrammetry needs to be able to see the ground to create contours

- LiDAR returns come from every object illuminated – the lowest being the ground – wherever the sunlight hits LiDAR will return XYZ

- LiDAR creates a 3D model directly from the returns - Photogrammetry requires the incremental comparison of

a pair of stereoscopic photographs – indirect and much more labor intensive

- Photogrammetry requires contrast to see ground surfaces desert, wetlands, beaches, coasts are difficult or impossible

- LiDAR results come directly from the returned signal contrast not required

Conclusion

Although LiDAR is a relatively well-established technique it has only been used for archaeological research since

the turn of the 21st century. Because it primarily measures three-dimensional data it is only really effective for recording features that exhibit some form of surface topographic expression. The exception to this generality is intensity data that can be used to analyse the reflectivity of the surface being hit by the laser and thus in certain circumstances may aid interpretation in a similar way to crop marks on traditional aerial photographs. The accuracy and resolution of the LiDAR data are heavily dependant on the method of capture and the levels of processing before it reaches the end user. One of the key factors that affect the viability of LiDAR is the land use of the area to be surveyed. Because LiDAR primarily records three-dimensional data, and therefore requires a topographic surface expression to the features to be surveyed, the better the earthwork survival, the better the results. While LiDAR will work in most landscapes it provides an unequalled means of recording archaeological earthworks within wooded areas. The best results are obtained from mature broadleaf canopy with little understorey vegetation, whereas significant areas of dense, young woodland regeneration or unthinned conifer plantation will greatly restrict the potential of the survey and may prevent it from being a viable option.

“In the end, the only definite conclusion that can be made at this time pertaining to LiDAR technology is that LiDAR is here to stay. It may not be right for every state’s department of transportation and every privately owned surveying company to make a rapid switch towards using LiDAR technology, but it should be seriously considered by any company which prefers to be up to date with the most cutting-edge technology.”

Reference

- http://www.english-heritage.org.uk/publications/light-fantastic/light-fantastic.pdf

- Irish, Jennifer L., Lillycrop, W. Jeff, and Parson, Larry E., 2004. Accuracy of Sand Volumes As a Function of Survey Density in Proceedings of the 25th Conference on Coastal Engineering, American Society of Civil Engineers, New York, NY.

- Optech Inc., 2006, http://www.optech.ca/. - Tenix LADS, Inc., 2006, http://www.tenix.com/Main.asp?ID=30. - U.S. Army Corps of Engineers, 2002. Chapter 18 - Coastal

Engineering Surveys in Engineering Manual EM 1110-2-1003.- http://www.ncgisconference.com/2013/documents/pdfs/

Rankin_Thu_130.pdf- http://136.142.82.187/eng12/Chair/data/papers/3064.pdf

Figure 3: Generic LiDAR tile showing heights differentiated by colour shading

Figure 4: Generic LiDAR tile showing the intensity of the returned signal

Figure 5: Mobile Lidar Data Figure 6: Railway Mobile Lidar

Technology: LiDAR