How to Design the Monitoring Network for TotaLite Sensor
Reference points
Reference points are used considered points that are fixed and are not expected to be displaced, usually located outside the zone of influence of the monitored structure. They are installed on stable foundations, and are used as a means of compensation, see Compensation Methods.
Slope Distances
The slope distance is the distance between the target (prism heart) and the sensor (the pinhole aperture). This is used to convert angles on the image to displacements in millimetres, and to accurately compensate for sensor motions when using reference points. Often sub-mm accurate measurement of slope distance is not required, as it concerns a percentage error of the total distance. Note that any error in your slope distance measurement will result in a similar error in the computed displacement. For example, an error of 1 cm in slope distance estimation for a monitoring point at 20 m, is equal to 0.05% which is equivalent to an error of 0.0025 mm error for a 5 mm displacement. Hence, with slope distance typically within the range of 5 to 70 metres, an accuracy of a few centimeters in slope distance will suffice. Below we will give some ways to measure the slope distance:
- The best way to measure the slope distance is using a total station and determine coordinates for the prisms heart and the pinhole aperture. The pinhole aperture is located 1 cm inwards the centre of the front glass and ca. 1.5 cm inwards of the housing. When measuring on the TotaLite housing front on both sides of the pinhole (see image below), you can define the centre position (in 2D). Applying a depth offset of 1.5 cm and you can define 3D coordinates of the pinhole aperture.
- A more quick and dirty method is to use a laser distometer. The accuracy of most devices is within limits but pointing them accurately is a challenge. Put the distometer on top of the TotaLite sensor and aim it at the prism (or a colleague holding something in front of the prism that is easier to aim on).
Baseline
The baseline is the foundational reference measurement for the TotaLite Sensor monitoring system. It captures the initial position of all targets when monitoring begins, establishing the "zero point" against which all future displacements are measured. Creating a quality baseline is crucial as it directly impacts the accuracy of all subsequent measurements.
During baseline creation, the system records:
- The precise angular position of each target prism
- The relationship between targets and reference points
- Initial slope distances to all targets
- Ambient conditions affecting measurements
Once established, the baseline serves as the permanent reference frame for the entire monitoring project.
Target Pairs (inter-target distance)
Target pairs are user-defined relationships between two monitoring points that allow for relative displacement measurement. By pairing targets, you can:
- Monitor differential movement between two points on a structure
- Calculate convergence in tunnels or excavations
- Analyze structural behaviour by comparing movement patterns
Each target pair calculation is performed as (Target 2 - Target 1), showing how the second target moves relative to the first. This is particularly valuable for applications like:
- Tunnel diameter changes (convergence/divergence)
- Differential settlement between structure elements
- Relative displacement across joints or cracks
Target pairs provide insight into structural behaviour beyond what individual target measurements alone can reveal, offering a more comprehensive understanding of deformation patterns.
Measurement Components for Target-Pairs (inter-target distance)
The platform presents three components for each target pair, all displayed as changes relative to a baseline (i.e., in a static scenario all values remain zero).
Inter-Target Distance
Since TotaLite measures angles only, the slope distances from the device to each target are provided by the user at setup and assumed to remain constant. The angle between the two targets is then used to compute the 3D distance between them. Because this calculation depends solely on the angle between the targets, any rotation of the TotaLite device itself does not influence the result.
Vertical Distance
The internal tilt sensor is applied to derive the height difference between the two targets. This component shows the upward (positive value) or downward (negative value) movement of Target 2 with respect to Target 1. Note that an upward movement of Target 2, or a downward movement with equal amplitude of Target 1 produce the same result — the two cannot be distinguished.
Horizontal Distance
The internal tilt sensor is again used to transform raw measurements into level-compensated data. The difference in azimuth between the two targets is then converted into a metric displacement (millimeters) using the horizontal distance from TotaLite to Target 1. This component represents lateral movement of Target 1 relative to Target 2.