GNSS CORS Local Monitoring Survey Minimum Requirements
Prepared by A.Woods in consultation with B.Twilley
There is a requirement to monitor GNSS CORS for local displacement which may impact the integrity of the permanent site. Jurisdictions are encouraged to undertake monitoring surveys (horizontal and vertical) on a regular basis and update site log files with derived vectors (dx, dy, dz) from the GNSS to each of the surrounding reference marks. These surveyed vectors are listed under section 5 in the station log file (including a date of survey field).
Local monitoring survey results should be precise to 1 mm. If differences in survey results approach 2 mm there is most likely a need to investigate further. Perhaps, repeat the most recent survey or look to conduct monitoring surveys more often.
The frequency of conducting local monitoring surveys:
* On site installation
- Conduct a direct survey from the GNSS pillar to each of the reference marks and establish the height offset from the pillar BM to the pillar plate.
- Conduct an indirect survey from each of the reference marks to the GNSS antenna and monitoring pins.
* Six (6) months after installation
- Indirect survey to monitor for short term displacement
* Twelve (12) months from the last survey
* Every two (2) years from then on.
The frequency of conducting local monitoring surveys is site specific. Jurisdictions may be extremely confident in the stability of some sites, for instance sites built directly into large granite rock outcrops. Therefore, after confirmation of site stability six (6) months after installation, jurisdictions may decide to go straight into a two (2) year survey cycle or even a three (3) year survey cycle. However, jurisdictions may be concerned about the stability of other sites, such as those built on less stable surfaces or in places where seasonal variation may effect the site. In this instance, surveys may be scheduled on a more frequent basis.
The accessibility of the site should also be considered. Some sites may be difficult to access or be a long way from survey offices. In this case it may be more appropriate to hold off on conducting local monitoring surveys until the next maintenance visit. The survey could be included in a visit to replace batteries or other equipment.
The GNSS time series should provide an indication of any significant or abrupt change in site conditions. GNSS time series analysis may suggest a need to conduct a local monitoring survey or at least visit a site to identify any local disturbances.
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