Establishing a Datum
All measurements are taken with reference to a datum - getting it right is essential!
It is critical that the location of the reference sensor and the source of the datum is known at the design stage of the project. The ASP relies on one or more sensors to be at a known elevation – either fixed or derived.
Fixed Datum
The ASP reference sensor(s) are placed in a location assumed to be fixed. This is most typically achieved by placing the reference sensor beyond the zone of influence of the monitored works.
Alternatively, a fixed reference position can be created within the zone of influence by using deep foundation support, such as a pile or deep datum.
When using a fixed position within the zone of expected settlement, special consideration should be given to protecting the ASP from shearing.
It is important to continue monitoring references assumed to be fixed, either by level survey or other means to confirm a fixed elevation.
Derived Datum
Derived datums are calculated by monitoring the position of the reference sensor(s) with another monitoring system such as a robotic total station or In-Place Extensometer. Any measured vertical displacement of the reference sensor(s) is applied to the measurement sensor as detailed in the Data Conversion section.
It is important to consider the measurement performance of the secondary system to ensure it meets the requirements of the project. Any reduction in accuracy or monitoring frequency of the secondary system will impact the performance of all sensors referenced to that system.
ASP-to-ASP Derived Datum
A derived datum can also be achieved by overlapping two or more ASPs. The measured elevation of measurement sensors of an ASP with a known reference position can be transferred to reference nodes of another "floating" ASP by mounting on a common concrete slab or by strapping the two systems together, ensuring the measurement sensors are aligned with the intended reference nodes.
Where three or more ASPs are linked in a string as described above, it is recommended that a fixed datum is present at both ends of the profile.