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Laser Scanners in Terrestrial Surveying

Dr. Satyaprakash
[email protected]

Introduction
Light Amplification by Stimulated Emission of Radiation (LASER) is a device that creates a uniform and coherent light (www.pcmag.com) or that emit highly amplified and coherent radiation of one or more discrete frequencies (www.Answers.com).

Lasers have become a common place, be it the pointers we use during presentations, the home DVD players or it is the reflector-less total station, a surveyor uses. This technique has been used in the surveying industry to perform laser scanning by aiming millions of beams at an object by which one can calculate the distance and direction of an infinite number of points and discern and maps the surface of nearly any kind of object (Professional Surveyor).

This technique was first introduced into LiDAR and then it was stripped of some components and terrestrial LiDAR mapping was created. Presently it is known by different names such as, 3D laser scanning, High Definition Surveying (a Leica GeoSystems trademark) or LiDAR on a stick. This is a powerful technology that uses advanced laser technology to obtain measurements at many thousands of points per second. The measurements captured through 3D Laser scanning can be used to generate high-quality visual deliverables. 3D Laser scanning has many applications in surveying where precise three dimensional relationships are required. However, there are four criteria for gauging where its practicality and efficiency can be best applied and exploited required level of detail accessibility safety and traffic/ business disruptions.

With the technology becoming more accessible, the benefits of such fast data acquisition have been realized and surveyors have started looking towards 3D scanning and Spatial Imaging as a new tool for the future and to widen their business opportunities. There has been many case studies world over, where this technique has been used successfully in conjunction with the conventional surveying methods. Some of the case studies were published in the Professional Surveyor Magazine (February 2007).

Types
The two types of 3D scanners are contact and non-contact. Contact 3D scanners probe the subject through physical touch. It is used mostly in manufacturing and can be very precise, although they have not found much use in Surveying. As the name suggests, non-contact scanners does the measurements without coming in contact with the objects. These scanners can be further divided into two main categories, active scanners and passive scanners. The active scanners emit some kind of radiation or light and detect its reflection in order to probe an object or environment. Possible types of emissions used include light, ultrasound or x-ray. Following different techniques of measurement are used in active scanners, Time-of-flight Triangulation Conoscopic Holography Structured light and Modulated light.

In the case of passive scanners, they do not emit any kind of radiation themselves, but instead rely on detecting reflected ambient radiation. Most scanners of this type detect visible light because it is a readily available ambient radiation. Other types of radiation, such as infrared could also be used. Passive methods can be very cheap, because in most cases they do not need particular hardware. The different techniques employed are, Stereoscopic Silhouette and Image Based Modeling.

How does it work
Three most popular techniques used in surveying are – laser triangulation, time of flight and phase shift. These laser scanning techniques are typically used independently but can also be used in combination to create a more versatile scanning system.

  • Laser triangulation is accomplished by projecting a laser line or point onto an object and then capturing its reflection with a sensor located at a known distance from the laser’s source. The resulting reflection angle can be interpreted to yield 3D measurements of the part.

  • Fig. 1 Laser Triangulation
  • Time of flight laser scanners emit a pulse of laser light that is reflected off of the object to be scanned. The resulting reflection is detected with a sensor and the time that elapses between emission and detection yields the distance to the object since the speed of the laser light is precisely known.

  • Fig. 2 Time of flight
  • Phase shift laser scanners work by comparing the phase shift in the reflected laser light to a standard phase, which is also captured for comparison. This is similar to time of flight detection except that the phase of the reflected laser light further refines the distance detection, similar to the vernier scale on a caliper.


Fig. 3 Phase Shift

Among the different techniques available, “Time of Flight” is the most used technique for the terrestrial surveying which is based on the principle of sending out a laser pulse and observing the time taken to reflect from an object and return to the instrument. Advanced electronics are used to compute the range to the target. The distance range is combined with angle encoder measurements to provide the three-dimensional location of a point. This is somewhat similar to the Direct-Reflex (DR) technique used in Total Station, however, the difference is the speed of measurement.

The ability to accurately position objects at over 1000 times the speed of a total station allows a 3D scanner to quickly produce large amounts of survey data. This data, commonly referred to as a “point cloud”, can provide a three dimensional shape, or visualization, of the feature being measured.

For surveyors who are more familiar with measuring discrete points to identify a feature, the sheer amount of data that can be produced from 3D scanning may seem overwhelming. However, a 3D scanner is still providing 3D positional information in a similar way to a total station. The main difference is that the speed of measurement allows a 3D scanner to provide more measurements in a shorter amount of time. This ability allows users to either significantly reduce field time or to collect a denser amount of points, which results in more accurate detail of the survey site.

Among the different laser scanners available for terrestrial surveying, products from two companies, viz., Leica and Trimble have been listed.