10 любых вопросов на английском
For several years now, terrestrial laser scanning has become an additional surveying technique in geodesy. Recent developments have improved several aspects of terrestrial laser scanners, e.g. the data acquisition rate, accuracy, and range. Since such instruments are relatively new and constructed by manufacturers who do not have advanced experience in surveying instruments, investigations are needed to assess the quality of the instrumental characteristics and the acquired data. In this way, manufacturers will understand the needs of geodesists and in turn enable geodesists to provide the necessary support in the development of improvements. This thesis has three objectives* the calibration and investigation of a terrestrial laser scanner, the post-processing of point clouds acquired by laser scanners, and applications of terrestrial laser scanning.
Terrestrial laser scanning can be viewed in a similar way. The technology is relatively new, but the laser scanner is on its way to becoming integrated in the geodesists’ world. The performance is impressive regarding the data acquisition rate and the accuracy is in the range of centimeters or less. The first generation of terrestrial laser scanners were developed several years ago by companies* who did not understand the needs of geodesists, but recent developments have ignited the interests of geodesists in these instruments. Laser scanners have now come into die focal point of geodesy and have a high potential for complementing the geodesists’ instruments. The investigation and calibration from the point of geodesists have started. Within the last few years* studies have been initiated by independent institutions for a better understanding of the performance, the potentials and the limitations of terrestrial laser scanners.
Terrestrial laser scanning is a promising technique and has the potential to be adapted as an equal surveying technique. Laser scanners are now in the second to third generation of development and fulfill several geodesists’ needs. Investigation regarding potentials and limitations have been launched some years ago.
Independent examinations from different institutions have established a comprehensive knowledge regarding properties, influencing parameters, performance and limitations. The collaboration between geodesists and manufacturers results in improvements, which ease the use of laser scanners and significantly improve the accuracy.
The calibration and investigation of the laser scanner ^Imager 5003r of Zoller+Trohlich was carried out regarding several aspects, such as:
distance precision and accuracy
angle precision and accuracy
The results have shown that laser scanners offer a high potential regarding high accuracy within the millimeter-scale. The most critical factor is thereby defined by the distance measurement system. Since the distance measurement is based on prismless measurements, the signal-to-noise ratio influences the precision significantly. The signal-to-noise ratio is generally affected by three parameters: the range, the reflectivity of the object and the angle of incidence. This thesis showed that the noise related to the distance can be reduced by an adequate filtering technique, which takes into account the orientation of the noise along the measurement direction.
Systematic effects produced by methodological errors may also affect the data significantly. The investigation regarding the wobble of the vertical axis and the errors of the collimation axis showed that there are indeed systematic effects, especially for short ranges below ten meters. Furthermore, the verification of the laser scanner system and its performance, i.e. measurement noise and instrumental errors, over time are recommended either with a system calibration or with a component calibration.
The two discussed applications regarding static laser scanning showed the potential for applications in the field of engineering geodesy. Point clouds defining objects in a high point resolution achieved results from the whole object and also in only discrete points. The flow directions of a road section or the displacements on a tunnel face after excavation establish a new method of interpretation, especially in the related engineering fields of urban water management and engineering geology. The accuracy in the millimeter range, achievable by the laser scanning data, has established laser scanning as an alternative to traditional surveying techniques, i.e. tachymetry, GPS, levelling, photogrammetry.
The third example involves a kinematic application. Kinematic applications are of greatest interest in tunnel applications, railways and roads. The surveying in three dimensions, including intensity information, in a kinematic way significantly increases the performance of surveying objects. An example of kinematic laser scanning in a test tunnel showed that laser scanners can provide a sufficient absolute 3D accuracy.
Concerning deformation monitoring during and after tunnel excavations, laser scanning seems to offer an alternative to static convergence measurements. The limiting factor is not the accuracy of the laser scanner, but the accuracy of the required 3D trajectory acquired by additional sensors for an absolute positioning fixing, e.g, total station, inclinometers, GPS,. INS. The calculation of the 3D trajectory becomes an important aspect. However, laser scanners also offer the possibility of capturing die environment quickly and precisely in kinematic applications.
Terrestrial laser scanning is a part of geodesy and offers a high potential for fast, nearly continuous and precise data capturing. The improvements regarding accuracy, range, sampling interval and the implementations of inclinometers, levels and digital cameras define a powerful surveying instrument. Sensor fusion between GPS and total station in recent years have allowed for the development of an all-in-one instrument in the near future that will consist of a GPS-based scanning total station, thus combining the advantages of each individual instrument in one. The lack of staking out, surveying discrete points and other tools have yet to be resolved. Furthermore, the possibility of operating laser scanners in harsh climatic conditions, e.g. low and high temperatures, humidity (rain or snow), and the reliability in the operation of laser scanners will help to extend the field of applications.
The first steps for combining a laser scanner with a total station were carried out at ETH Zurich. A laser scanner, LMS200 of Sick (Germany), is mounted on a total station to benefit from the advantages of both instruments.
Hie acceptance is not only dependent on improvements on file side of hardware, but also a standardization is recommended to assess and compare the different types of terrestrial laser scanners. An international guide including information regarding standardized parameters for distance accuracy, angle accuracy, 3D accuracy of single points, 3D accuracy of objects, e.g. spheres, would help users to distinguish between
available laser scanner systems. Since laser scanning also means the processing of point clouds, the data transfer and exchange between laser scanners and software packages is essential and should be simplified.
Furthermore, the processing of point clouds has to be supported by fully-automatic algorithms to reduce the time for post-processing work. The limiting time factor is not only the data acquisition, but also the processing of the huge numbers of point clouds.
What had become an additional surveying technique in geodesy in the last several years?
Whyare investigations needed to assess the quality of the instrumental characteristics and the acquired data?
Will manufacturers understand the needs of geodesists and enable themto provide the necessary support in the development of improvements in this way?
The technology is relatively new, isn't it?
Who invented the first generation of terrestrial laser scanners?
Why have studies been initiated by independent institutions within the last few years?
Have the results shown the laser scanners to have a high or low potential regarding high accuracy within the millimeter-scale?
What may systematic effects produced by methodological errors affect?
What has established laser scanning data as an alternative to traditional surveying techniques?
What will the possibility of operating laser scanners in harsh climatic conditions (low and high temperatures, humidity) help to extend?