ScanPhase

Distance measurement:

The scanning systems mainly capture the physical position of a target object, represented as a series of points (forming a "point cloud"), usually in Cartesian coordinates (XYZ). This is achieved by comparing the pulse of light emitted and returned, and determining the value of the objective object t in relation to the position of the scanning instrument.

The scanner calculates the position by measuring the angle of the scanner assembly (scanner head and reflector) and the travel time of the light (measured directly as in flight time scanners, or indirectly, as in light-based scanners). in phase).

Color and intensity:

The scanner also records a measure of the return energy (represented as an intensity value) from the surface, which is a function of the characteristics of the target surface and ambient light conditions. Most scanners have the ability to determine the color of each point by using a camera (can be incorporated or separated), which is represented by the scale of values ​​RGB (red, green, blue) commonly used. Because scanners are optical systems, only what the scanner can "see" is captured, so scanners can not go through walls or other obstacles (these create "shadows" in the cloud of points where data is not captured) . In fact, the integrity of the data depends on the environmental conditions during the acquisition, as well as the intensity and color data, which vary according to the lighting conditions.

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Point clouds:

The measurement values ​​are represented by a file format that expresses the position, intensity and color of each individual point in the point cloud. Ultimately, the data can be encoded in a variety of point cloud file formats (ASCII, PTS, LAS, E57, etc.), which use some variation in the XYZ / RGB position intensity color scheme . Several hardware and software providers have a proprietary point cloud that can be easily converted according to the customer's needs. It is often prudent for customers to specify deliverable file formats based on the end use of data in negotiation with service providers, especially when data will be integrated with existing information management systems. Customers may consider using a standard file format that is not proprietary, such as the ASTM E57 file format for the exchange of 3D image data to ensure compliance with project needs.

Accuracy of the measurement

Several factors affect the accuracy of the point cloud data, including instrument capabilities and calibration, and quality control measures. Environmental conditions that affect the integrity of the data include the reflectivity of the surface, the angle between the scanner and the target (angle of incidence), and the range to the target object (the laser beam diverges with distance, so the measurements furthest from the instrument are less precise). 3D scanning service providers are experts in controlling these sources of error, so it is crucial to establish functional performance requirements before field acquisition so that the optimal instrument, the scanning position (s) and the Acquisition times can be negotiated with the service provider to achieve the highest quality deliverables. However, a series of general rules that customers must know to ensure the proper development of functional performance requirements are applied.

Mobile / aerial systems:

The mobile and aerial scanning systems, where the scanner is mounted on a moving vehicle during the measurement process, capture additional data about the movement of the vehicle to compensate for the movement. A global navigation satellite system (GNSS) detects the position and speed of the scanner, an inertial measurement unit (IMU) detects the attitude speed and acceleration of the scanner. This information is stored and processed during the data acquisition process, and then the processing. The software generates a point cloud file with the adjusted physical positions, which allows the project teams to capture data very quickly from the physical environment without having that mobilize the team several times. Additional data, such as environmental conditions during the data capture time, as well as calibration and service provider data, may also be linked to the data set. Aerial and mobile scanning is fast becoming the standard method for creating digital city models, documenting road networks, tunnels or power lines, and other infrastructure measurement initiatives.