We recently caught up with two of our Inverness surveyors to find out what challenges they face in their remote corner of Scotland. They mentioned mapping the changes at a hydro scheme and I thought it might be an idea to find out how we updated our OS MasterMap database to show the Glendoe Hydro Scheme, Scotland’s largest recent civil engineering project. Craig and Dave faced a technical challenge in finding the best way to map the new and changed topographical features.
The Glendoe Hydro Scheme is located in the hills above Loch Ness near Fort Augustus and although a significant part of the project is underground, many new and changed features needed to be incorporated into our OS MasterMap database. These included the dam wall, the reservoir, all of the access and service roads, changes to water courses and their associated walls and sluices, and changes to the extents of vegetation and other surface features.
The area was originally surveyed using photogrammetry and then published at a scale of 1:10000. Peter Todd, Senior Production Manager at Ordnance Survey said, “Photogrammetry would be the normal approach to revising a large area of change in a remote location, but our surveyors chose to work on the ground for a number of reasons: the development was classified as a prestige site so we needed to update our data before the official opening with Queen Elizabeth II; with unpredictable weather in the area we couldn’t guarantee we could fly over and take the imagery in time; and the reservoir would not be filled with water until just before the opening, so we would need to survey the edge of the water by ground methods anyway.”
Our surveyors use OS Net, the country’s most comprehensive GPS correction network with 90 base stations across Great Britain. OS Net ensures centimetre level accuracy for data-collection operations ensuring Ordnance Survey’s 5000 daily changes can be relied on. The surveyors’ car transmits a signal back to our Southampton head office creating a virtual reference station. A radio signal then transmits the data to the surveyors’ mobile GPS receiver. Doing this depends on maintaining a reliable mobile phone connection to the hub in Southampton and a radio link between the receiver and the repeater in the car. Mobile phone coverage can be a problem in remote areas of Scotland and it proved to be the case on this task. For parts of the site Craig and Dave needed to find an alternative to their normal approach.
They fixed approximately 12 local base stations in the parts of the site where there was no mobile phone signal. Observations were taken over a period of about 15 minutes at each station and the results processed using Leica Geo Office software by one of our GPS technical advisers who was also working in the area at the time. These local base stations were expected to provide sufficient control for RTK methods using two receivers in the areas without a mobile phone signal, but the surveyors found that there were still areas where they could not receive the correction data broadcast from the local base by radio. In these situations the local base network was supplemented by a series of stations that were fixed as simple RTK detail points. These points were used with a two-receiver solution to capture the remaining data. This innovative solution was acceptable considering the accuracies expected and the type of detail being surveyed.
The Glendoe Hydro Scheme generates 100MW of energy; enough to power every home in a city the size of Glasgow. Glendoe’s 600 m head (the drop from the reservoir to the turbine) is the highest of any hydro electric scheme in the UK. The 905 metre-long dam, a rock-filled embankment 35 metres above the valley floor, is hidden from view from all of the current houses and public roads in the area.