3D laser scanning
3D laser scanning is a new technique. Several names are currently given to it:
- 3D laser scanning ; the most used term for terrestrial applications,
- Laser scanner survey ; common term for airborne and terrestrial surveys,
- High density survey ; term introduced by Leica Geosystems,
- Laserometry ; term sometimes used in the industrial field.
The main principles of 3D laser scanning
The laser scanner surveys are comparable to 3D photographs of the measured object. Each scanned point on site is
immediately known under X, Y, Z coordinates.
From a land surveyor's point of view, we note a change in philosophy compared to classic topography: we don't survey the characteristic points of the object but measure the object in its entirety.
The accuracy of such a system is illustrated by the standard deviation on each measured point. However, the accuracy of the modeled result can be better than the accuracy of the raw data and this thanks to the very high density of information.
This technology is interesting if we're looking for high resolution information.
Definition of a 3D laser scanner system
For the user, a 3D laser scanner system is an instrument which measures 3D points (XYZ) on an area of an object:
- in an automatic and systematic manner,
- at a significant survey rhythm (hundreds to hundred-thousands of points per second),
- which allows obtaining 3D coordinates in real time.
A scanner can possibly add and for each point:
- a function value of the reflectivity of the object.
- RGB values obtained via a digital camera.
3D laser scanners can be used:
- In a fix position (quality control in industry).
- In a mobile position (terrestrial scanner used in topography).
- Airborne with a browsing system on board.
Definition from:
3D SCANNING INSTRUMENTS
Wolfgang BOEHLER, Andreas MARBS
i3mainz, Institute for Spatial Information and Surveying Technology, FH Mainz, University of Applied Sciences,
The important factors of a laser scanner system
- The associated software for post-processing.
- The accuracy of the system (expressed in general by the standard deviation on each point).
- The size of the laser spotlight depending on the distance.
- The range of the instrument: shorter distance and longer distance.
- The speed of the scan (expressed in points/second, from 1,000 to 500,000 depending on the systems).
- The angular field.
- The work environment (temperature, humidity, radioactivity…).
- Easy to use (with regard to the weight for example).
- The electric supply solution.
- The danger of the laser system for eyes (some systems need marking of the construction site during the measurements).
- The manufacturer.
The Leica ScanStation 2 system
TPLM-3D is equipped with the ScanStation 2 system from Leica Geosystems.
The system consists of a laser distance meter without a reflector (measurement by propagation time of a laser pulse), a vertical rotary mirror deflecting the laser beam into space (270°). The body of the scanner is mounted on a horizontal encoder which manages the horizontal rotation (360°). The main innovation compared to former systems is a very high rhythm of capture (up to 50 000 points/second) and the integration of a bi-axial compensator which opens up new accuracy horizons on wide-ranging construction sites.
With a wide range (up to 300 m at 90% of reflectivity and 150 m at 18% of reflectivity), the ScanStation 2 laser scanner system allows to rapidly survey millions of points (6 minutes for 1 million points), without any contact with the object.
The ScanStation 2 laser scanner allows scanning on 360° in horizontal and 270° in vertical.
The main characteristics of the high density laser scanner survey are:
- the direct acquisition of tridimensional data on the land,
- the very high density of information (up to one point every 2 mm on the measured object),
- the accuracy of the measurements (5 mm at 50 m, 12 mm at 150 m on each of the points),
- the survey of inaccessible details in safety condition.
The point clouds can be used the same day with the client in the field (distance, surface, angle, and volume controls for example).
The Leica HDS 6100 system
TPLM-3D has invested in this next generation scanner to carry out surveys where the availability of the construction
site – or intervention window- was highly reduced (planned stops for example).
Unlike the ScanStation 2 system, this system is based on the measurement of the phase shift between the emitted laser wave and the reflected wave. A horizontal cylinder in permanent rotation deflects the beam on the vertical plan, as the body of the device operates a rotation in the horizontal plan. Its minimum and maximum ranges are respectively of 1 m and 79 m.
The field of the shot of the HDS6100 is 360° in horizontal and 310° in vertical (which is higher than the one on ScanStation 2).
The HDS6100 is moreover perfectly adapted to surveys in the nuclear environment , because it is from the IP 50 class and can be very easily vinyled. Its congestion is moreover highly reduced.
The main characteristics of the laser scanner survey with this system are:
- the acquisition rapidity of the data up to 500,000 points / second
- the information density is of about one point every 5 mm
- the direct acquisition of tridimensional data on the field
- the accuracy of the measurements (6 mm at 25 m, 10 mm at 50 m on each of the points).
The post-processing of laser scanner data
The extraction of surfaces, edges, volumes, lines, and points with specific software, allows us to model the objects in 3 dimensions. The extraction of 2D data is still possible (sections, plan views).
The whole of the plans and models are delivered in the usual CAD computer formats.
