As some of the long-standing LIDAR News readers may recall, the Oregon State University (OSU) geomatics program conducted a brief online survey of laser scanning professionals in September, 2010, both on Gene Roes blog and at Laser Scanning Forums UK. The survey was not designed to be rigorous, but rather to get a feel for the industrys use and understanding of TLS – terrestrial laser scanning inclination sensors and applications.
One interesting qualitative result of the survey was a question of nomenclature: What do we call the built-in device that measures how far out of level the laser scanner has been setup? While the survey called the device a level compensator, it became clear that other possible names exist, some better than others:
Inertial Measurement Unit (IMU)
For my graduate studies, I decided to use the name inclination sensor since it seemed to be more generic. This is important because different equipment manufacturers take different approaches to using inclination sensors; for example, some scanners assume that the setup is out of level and record inclination data very efficiently, applying corrections during post-processing, while other scanners require that the operator set up the instrument very close to level in order to use the inclination sensors.
The results show that 81% of the 48 respondents indicated that their scanners were equipped with an inclination sensor and 69% of the 48 respondents used it all or most of the time. The survey results also generally indicated an informed understanding of the scanners inclination sensor and its uses.
Many TLS professionals who use inclination sensors stated that they rely on their inclination data to provide additional control, reducing field time required to set up additional targets, thereby simplifying scan registration to an existing coordinate system – georeferencing.
The most common complaint regarding inclination sensors was that their use significantly increased scan time for certain scanner models, which explains why some users choose not to always have them enabled. Other reasons generally entailed an inverted or unstable (ship deck, heavy equipment, hot asphalt) setup, or the suspicion of lowered accuracy and unnecessary redundancy.
Even while using target registration, at OSU we have found inclination data very useful for either validating or discovering errors in the control coordinates. By plotting the periodic inclination sensor results around a radial chart, we can also find abnormalities in the setup (nearby pedestrians, passing trucks, unstable platforms, etc.) over the course of the scan that might explain problems matching scans. The additional control provided by using inclination sensors can also improve and reduce the processing time and field time required for a tight registration.
Overall, I consider the user survey a great success, and I appreciate everyones involvement in the endeavor. For more information on the survey results and the additional studies I performed regarding inclination sensors and coordinate transformations, you can read my Masters thesis at http://hdl.handle.net/1957/21759. I am preparing two manuscripts from this work for publication in the ASCE Journal of Surveying Engineering.