- 24th November 2017
- Posted by: Sophie Bunker
- Category: Projects
The Environment Agency commissioned us to look at coastal flood risk across the Isles of Scilly. A number of uncertainties became apparent within the existing survey data. These included the historic use of the 19th Century levelling network, errors in the level datum between islands, error in the Ordnance Survey (OS) Geoid Model (GM) OSGM02 correction surface, and inconsistency with the European Sea Level Service record for the St Mary’s tide gauge.
The survey was therefore required to confirm OSGM15 levels on the Isles of Scilly, across the four islands of St Mary’s, Bryher, Tresco and St Martin’s. The project had to establish survey control through a combination of checking OS benchmarks and existing survey control. The topographical survey included surveying predefined flood defences, crest points, culverts and LiDAR check surveys.
Logistically, even getting to and from the islands was a challenge. The summer months were long gone, winter was just around the corner, and vehicle access was restricted. The last passenger boat of the season was leaving St Mary’s at the start of November, so time was critical.
A plan was made. The surveyors would first get onto the main island of St Mary’s using the passenger ferry from Penzance. The equipment was couriered to the guest house where they were going to stay. A local tourist boat operator would take the survey team out to the other islands each day, and collect them later in the afternoon. Local taxis would help transport the surveyors around each island. All being well, the survey team would be back on the return passenger boat from St Mary’s to Penzance a few days before the service finished for the season.
The weather would play a big part in getting the survey completed on time, and also had a potential impact on the quality of the survey results. Especially relevant to the digital levelling that was being observed between all the survey control. Thankfully, the wind didn’t blow too strong and the weather was generally kind. The survey was completed on time.
The plan to get both surveyors and equipment back off the mainland was the same as before, but in reverse. Simple? No, unfortunately not. The same courier company that had very efficiently delivered the equipment onto St Mary’s now said they could not collect it from the same place. Initial panic set in. However, some quick thinking came up with an alternative of getting the equipment onto the freight ferry leaving later that day. The courier would then collect it when the equipment was unloaded in Penzance.
The survey incorporated various integrated surveying techniques to provide a robust solution, with redundant observations and self-checking built into the solution. Static Global Navigation Satellite System (GNSS) observations were recorded over a period of in excess of 48hrs. This was computed through both unconstrained and constrained processing methods to provide certainty in the results. Along with an OS Net Active GNSS station on St Mary’s to fix additional survey control across the islands, the observations were used to provide redundancy within the computations. All the survey controls and the specified OS bench marks were levelled digitally using closed loops, again providing self-checking within the observations.
Total Station observations were based on the control stations established and any Real-Time Kinematic (RTK) GNSS observations taken. They used the nearest station option, ensuring that all the results were being computed from controls located on the island. This was important to counter any possible effects of tidal loading on the survey results.