keywords: laser scanning, point cloud, spatial model, orthoimages
What is a laser scanning?
Laser scanning is a measurement technique that uses a laser beam to create accurate 3D surfaces. The scanner, emitting laser pulses, very quickly determines (even several million points per second) X, Y, Z coordinates of a large number of points, in any coordinate system, determines them by determining horizontal directions, vertical angles and distances with respect to the center of the scanner system (polar method).
Scanning result
The point cloud is the primary product of scanning, it reaches up to several hundred million measured points in size. Knowing the coordinates of each of these points, we are able to describe the geometry of the object or its structure.
Another product of the scanner are photographs, which are optional during the measurement, but they allow us to color a given point cloud, in a way that is natural and reflects reality. In addition, they help to discern where the scanner was and what was in its field of view when the measurement was taken at any given time, which is useful during postprocessing.
The above products allow visual analysis of the object under study – measuring the geometry. The collected data allows you to determine, for example, distances between objects, deformations from the designed reference plane. The latter will find application in the study of deformation, verticality of walls and any concrete structures.
The results of post-processing, i.e. further computer processing of the resulting data, of our products, are, for example:
Spatial models – the creation of triangle meshes on the basis of the point cloud, creating models of entire buildings or the elevations themselves, a given floor, etc. Thanks to the density of point clouds, we are able to reproduce even the most complex objects, for example, water dams and their infrastructure, as their certain surface features.
Vertical sections and projections – they are created by cutting models with a cutting plane at a given specified height, so we are able to create projections of a given floor of a building, the elevation of a building, examine the verticality of a chimney, etc.
Orthoimages of walls, floors, vaults and decorative elements – orthogonal projections of the model on the object. Thanks to the possibility of obtaining accuracy of 1-2 mm, no other method of measurement can guarantee sufficiently high accuracy of the smallest details. Scanners are integrated with various sensors, which in combination gives the possibility of analysis based on color compositions. Thus, we are able to visualize, for example, inscriptions on tombstones, any gouges, as well as cracks and scratches, and even the degree of dirt and degradation.
Of course, point clouds are such a huge carrier of all kinds of information that many more different products of the resulting studies can be mentioned.
[1] [3][4]
Scanning options
Before taking a measurement, you must first determine the resolution, that is, the density of the point cloud. It should be selected appropriately to the distance from the scanned object, with objects close by too high resolution – greater than the measurement error of the scanner distance will be associated with noise on the cloud (noise will be defined as any points registered by the measuring device, which strongly deviate from the measured objects) . And with objects far away, cloud density can help capture the object more clearly. These settings have a huge impact on measurement time. A big advantage of the scanners is the ability to be remotely controlled, which makes it possible to work in difficult conditions.
Limitations
Type of material – can scatter the laser beam
Reflection angle – the laser may slip
Type of surface – e.g., glass scatters the laser beam
Noise – unwanted points e.g. moving objects Obstructed objects – the scanner sees only what the human eye sees, objects obscured by a cabinet, for example, will not be scanned. This rule also applies to objects high up, e.g. roofs, here drones with a mounted LIDAR scanner come to the rescue
Weather conditions – any precipitation will cause noise [2]
Examples of applications
Architecture and construction:
- Inventory of buildings and monuments
- Monitoring the progress of construction work
Geodesy:
- Topographic mapping
- Measurements of tunnels and bridges
Industry:
- Deformation monitoring
- Measurement of heap volume
Other:
- Investigating construction accidents and disasters
- Creating relistic models for games
What we gain
High accuracy and precision of several millimeters for 3D models that faithfully reproduce reality in virtual form. The information is collected quickly on permanent disks, the measurement is performed by one person and the first results are available almost immediately after the scanning is completed. With the large number of buildings and large-area studies, measurement by traditional means would prove to be enormously time-consuming. The whole process is performed without any interference with the object in question, which is important for particularly delicate objects. Scanning makes it possible to collect data from a distance, making it possible to measure in difficult conditions. The final resulting data can be easily integrated into GIS systems as well as further processed using BIM technology.
[1] http://www.geomatyka.eu/publikacje/isbn9788393460960/isbn9788393460960.pdf
Aleksandra Cholewińska
