Using LiDAR for Planning and Designing

Introduction 

Light Detection and Ranging (LiDAR) has grown to become a vital technology used in remote sensing technology from aerial or land-based platforms. Environmentalists have used LiDAR extensively in resource mapping for conservation efforts in the past. However, one of the promising applications of LiDAR is planning and designing of engineering projects of varying magnitudes and complexities. Some of the outstanding drawbacks of using traditional data acquisition methods include the amount of time spent collecting the data and the reduction of the data during processing. Such issues are significant in engineering projects due to the corresponding cost and safety implications. Using LiDAR from aerial platforms allows engineers to acquire and process data quickly with high levels of accuracy as compared to the traditional methods. LiDAR involves the use of directing single laser pulses on targets and performing an analysis on time taken to receive the reflected pulses (Finkl, 2013). The data collected helps in the determination of elevation details of an area. Additionally, engineers can use the global positioning system (GPS) and digital photography with the LiDAR elevation details to present accurate and comprehensive information about the desired location for a project. The current paper focuses on the LiDAR data generated from aerial sources although several types of data exist.

Literature Review

Aerial-based LiDAR

The aerial-based platforms using unmanned aerial vehicles or light aircraft enable the engineers to survey larger areas than the mobile and ground-based systems, and this is important in designing large-scale projects such as infrastructures and drainage canals. However, engineers focusing on small-scale projects can benefit from the land-based systems that offer high-definition and high-quality data of site-specific locations that are typically small. The LiDAR data obtained from the aerial platform are useful in producing digital terrain models and digital surface models in remote sensing (Steinebach, Guhathakurta, & Hagen, 2009). The suitability of LiDAR in planning and designing depends on data qualities such as precision and accuracy of the generated LiDAR datasets. The data generated from aerial sources also depend on factors such as density and rate of sampling, post processing levels, aerial flight precision, and the execution and type of ground controls used on the elevations. According to Olsen (2013), the use of mobile LiDAR platforms has revolutionized surveying and decision-making processes because it allows the surveyors to determine the exact features to assess. Design documentation is crucial when using LiDAR, and the engineers must indicate the relevant metadata to show that the information presented in the LiDAR form adequate supports the objectives of the planning process. Some of the metadata indicated in the LiDAR forms include point density, average point spacing, flight elevation, and quality level. The LiDAR forms should also indicate the geographic projection units, date of scans or flights, and geographic projection approaches used in gathering and processing the data.

Quality level specifications based on the quality levels from the National Enhancement Elevation Assessment

Nonetheless, the suitability of using aerial-based LiDAR in planning and designing include obstructions by buildings and vegetative cover during the data collection, on-site changes after the completion of data collection, and water surface presence at the site of data collection. The data gathered using LiDAR scans on surfaces covered by water are often considered invalid and unreliable although the laser pulses can penetrate the water. Thus, the engineers conducting the land surveys must use other methods to collect data on the inundated surfaces. Additionally, the engineers must put into consideration various changes on land surfaces in the period after the collection of the data using the LiDAR scans. Current aerial photography is one of the methods that can help the engineers in identifying changes in landforms. Engineers can also use LiDAR to survey land under tree canopies as well as surfaces covered by snow depending on the wavelength used in the pulsating laser. Thus, LiDAR allows engineers to survey land surfaces in various weather conditions. Usually, the data collected from LiDAR scans are presented in the form of variable density points that can be processed, used, and displayed as grid data. Factors such as the size of the grid and the original point density can be used in omitting or distorting some of the elevations and locations that are shown on the grid. The engineers can use techniques such as contour vectoring and flow accumulation to assist in ensuring consistency in the links between flow paths.

Applying Aerial LiDAR in Planning and Designing

Aerial LiDAR presents the advantage of conducting surveys over a wide range of terrains and locations. In urban planning, LiDAR presents the advantage of allowing engineers to create three-dimensional models of infrastructure and buildings using high-density images (Pinto, 2014). The costs of projects involving the use of LiDAR for data collection vary depending on factors such as time, deliverables, location, point density, and boundary attributes. However, engineers can benefit from properly verified LiDAR datasets for planning and final designing purposes. The engineers must assess the LiDAR metadata to determine the level of appropriateness and accuracy of the data for engineering works such as channel hydraulics, hydrology, stage-storage computations, and structural layouts among others.

Various standards for the minimum quality levels exist for different design practices and uses. For example, Snyder (2012) notes that the National Enhanced Elevation Assessment quality levels identified the minimum values that would create improved elevation data, and the engineers should adhere to such guidelines. Additionally, the engineers should understand how elements such as contours, digital elevation models (DEM), data thinning, data sampling, and triangulated irregular network (TIN) influence the raw data. An important consideration during the planning and designing process is the fact that LiDAR data can turn inaccurate for various design uses depending on how the data is processed. For instance, LiDAR data obtained from a four-meter digital elevation model may not adhere to the accuracy standards although a one-meter digital elevation model spacing of the same LiDAR data could.

During the designing process, the engineers should conduct ground truthing or field verification when using LiDAR. Approaches used in conducting field verifications vary depending on the objectives of the planning and designing activity. However, the first step commonly employed in field verification is the establishment of temporary control points and benchmarks on site. The control points used in the survey must be indicated in the data and projections applied in the LiDAR reporting. The second step is conducting additional surveys, preferably in twenty or more locations, outside the targeted area to determine the changes in landforms that could affect the survey area. For example, the engineers could evaluate active erosions as well as the positions and elevations of the surrounding areas. Additionally, the engineers should assess the vegetative cover and exposed surfaces to identify whether inaccuracies caused by the vegetation arise in the LiDAR set. The engineers in the surveying exercise should use checkpoints at surfaces with uniform gradient for a radius that exceeds the grid spacing of the digital elevation models. The engineers should consider placing some of the survey checkpoints on permanent or hard surfaces such as bridge decks or concrete pads.

The third step is checking to ascertain the accuracy of the LiDAR data in representing the project site as well as the correct identification of the project location. The engineers need to apply elevation adjustments in situations where the elevations meet the required accuracy and precision but are marked with steady elevation or location shifts between the LiDAR surface and surveyed elevations. However, the discontinued application of LiDAR data is recommended on sites where the accuracies and precisions are unacceptable, or the discrepancies are irreconcilable. The fourth step is collecting survey data using the appropriate alternative methods for any surfaces at the survey site submerged under water or that were inundated when the LiDAR data collection occurred. Additionally, the engineers should focus on the land surfaces undergoing active changes such as erosions during the survey as well as the grade breaks that may not be fully incorporated in the LiDAR data. Such site-specific data is crucial in replacing or supplementing the LiDAR data to enhance its appropriateness for a particular design. The engineers should then collect survey data at property boundaries, right-of-ways, and geologic features. Finally, the engineers should set markings or flags to delineate the proposed construction area and design for comparisons of the layout elevations of the site with the LiDAR surface elevations. This helps in evaluating and determining whether the differences identified between the two elevations are tolerable for specific projects.

Personal Reflections

The use of LiDAR in planning and designing for engineering purposes is an important breakthrough for complex and large-scale projects because it allows engineers to expedite data collection without compromising on accuracy. The automation of the LiDAR system implies that surveying using this method requires minimum input from human operators, and this helps in reducing errors during data collection. Furthermore, the use of active illumination sensors allows the application of LiDAR in data collection during the day and night, and this is a significant advancement over the traditional photogrammetric methods. LiDAR is a dynamic system that can be combined with other methods of collecting data to enhance the accuracy of the data at a location. For example, integrating LiDAR with technologies such as the global positioning system is vital in planning and designing complex engineering projects such as urban planning, survey assessments, and landscape engineering among others. The LiDAR systems vary to produce different levels of optimizations that create diversified datasets.

Conclusions

The Light Detection and Ranging (LiDAR) approach in surveying has become a crucial remote sensing technology that allows engineers to engage in projects of diverse magnitudes and complexities. The application of LiDAR in complex engineering planning and designing relies on the precision and accuracy of the datasets generated from the aerial scans over target areas. The engineers must ensure that the datasets are reliable to avoid costly errors that could affect major projects. Consequently, factors such as ground controls, aerial flight precision, sampling density and rates, and post-processing levels are crucial when using the LiDAR technology. Additionally, the engineers must adhere to the standards stipulated by the United States Geological Survey, State agencies, and Federal agencies, and the National Digital Elevation Program to help in improving infrastructure safety and support conservation management efforts.

References

Finkl, C. W. (2013). Coastal hazards . Dordrecht: Springer.

Olsen, M. J. (2013). Guidelines for the use of mobile LIDAR in transportation applications . Washington DC: Transportation Research Board.

Pinto, N. N. (2014). Technologies for urban and spatial planning: Virtual cities and territories . Hershey: Information Science Reference.

Snyder, G. I. (2012). National Enhanced Elevation Assessment at a glance. Retrieved from https://pubs.usgs.gov/fs/2012/3088/pdf/fs2012-3088.pdf

Steinebach, G., Guhathakurta, S., & Hagen, H. (2009). Visualizing sustainable planning . Berlin: Springer.