Knole: a measured survey for the National Trust
As part of the National Trust’s project to conserve and restore Knole in Kent, MOLA’s Geomatics team has been undertaking a measured survey of the showrooms and several new spaces. Catherine Drew, who has been working on the survey, explains what they have been up to…
Before the survey team started work in the house we created a ‘control network’ around the outside of the building. This was done by establishing a series of ‘fixed points’ by drilling survey nails into solid garden features and then using a total station and a set of prisms to measure angles and distances between them, a process called traversing. Using a differential GNSS kit, which locates positions using satellites in a similar way to navigational GPS equipment used in cars, we took readings over several of the points.
We combined all this data to create an accurate network of points, each with 3D coordinates on the Ordnance Survey British National Grid. We then extended the network into the house, so that all the survey work in the rooms could be related to the OS grid.
To start work in a room, we set up the total station over one of our control points and take an orientation reading to a prism which has been set up over a second point. This process enables the total station to work out its orientation and means that all data subsequently recorded will be aligned to the OS grid. Once set up, we put the total station into ‘reflectorless’ mode, which enables us to use the laser beam to take measurements on almost any surface without needing to use a prism. As well as recording the location of walls we also record windows, doors, fireplaces, and wood panelling. Each point we measure is given an individual feature code like ‘East Fireplace’ so that when the data is processed we know exactly what every line represents.
This data is then used to produce a series of wall elevations and floor plans, which, along with a photographic record, will be used by those carrying out future conservation and restoration works at Knole.
The team has had to overcome an assortment of challenges associated with working on such a complex and unique project. The first issue was how to stop the tripods that hold the total stations from sliding on the waxed floorboards. We came up with an ingenious system involving wooden feet for the tripod legs, which sat on non-slip rubber matting and were linked together by lightweight metal chains (hand crafted by Geomatician, Neville Constantine).
Another dilemma was trying to keep permanent control points in the rooms, as we have had to be very careful not to damage the house, so have been unable to leave permanent marks. As the house is open to the public this is made doubly difficult, as any form of non-permanent mark is in danger of being walked off by visitors. To overcome this we have had to use features which already exist in the house, for example floor board nails or knots in wood. We have been very diligent about creating witness sketches of these markers as it is difficult to remember exactly which floorboard nail was used! We also discovered that the floorboards can be very bouncy, especially with so many visitors walking on them. Working in a publicly accessible house has its advantages too; the visitors are very keen to question us on what we are doing and we have thoroughly enjoyed talking to them!
2013 is almost over - where has the year gone?! The year may have flown by but we certainly have lots to show for it.
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