The Geodetic monitoring system
of the 17th Century Ship Vasa.
Picture 1. The Vasa.
The Vasa sank on her maiden voyage in 1628. She sailed about a mile on a fine August day with a light wind on her beam. Then a slightly stronger gust filled the sails, causing the ship to capsize and sink in the middle of Stockholm’s harbour.
Picture 2. The Design.
Even today, our knowledge about the design of the Vasa is limited and we do not know all the facts about the disaster. 17th century shipbuilders did not produce construction drawings or calculations of stability. The art of shipbuilding was more like a well-kept secret.
The Vasa was also something of an experiment. The ship was unusually heavily constructed and had more large guns than previous Swedish ships.
Picture 3. 333 years on the bottom.
Of course, attempts were made to try and salvage the ship. In the 1660:s most of the guns were recovered, but slowly the ship was forgotten.
Picture 4. Anders Franzén discovers the
Vasa.
In 1956, Anders Franzén found the Vasa after searching for five years. Franzen’s equipment was a rowing boat, a grapnel and a homemade core sampler. A few days later the navy diver Per Edvin Fälting confirmed the find.
Picture 5. Salvaging the Vasa.
After much discussion, the Vasa was salvaged using conventional methods. Heavy steel cables were laid under the hull and attached to submergible pontoons, which were gradually able to lift the hull. In 1961, after 333 years on the seabed the Vasa broke the surface.
Picture 6. A wreck, but afloat.
After being salvaged the Vasa spent just a short time in the open, first afloat and then dry-docked. During this very intensive period, work on the ship was done on many different levels.
Picture 7. Singing in the rain.
The archaeologists registered about 14000 finds in just five months, digging through mud and silt aboard the Vasa. They worked through the summer in rubber suits, as the ship was sprayed with water to avoid shrinking and damage to her timbers.
Picture 8. Preservation.
How do you preserve 1000 tons of oak, 14000 wooden components, 500 figure sculptures and 12000 small objects?
The method adopted was immersion in polyethylene glycol and water, or spraying with a mixture of polyethylene glycol and water, and by developing the method at the same time. The general idea was to displace the seawater in the wood to prevent shrinkage and splitting. The spraying of the hull continued for 17 years.
Picture 9. Building a ship.
When the ship was salvaged, there was a lot of concern about the strength of the hull.
The ballast was still in the hold and large amounts of mud and muck remained aboard. Most of the iron fastenings were corroded, so the ship depended on her wooden treenails for holding the hull together. The divers managed to strengthen the hull quite a lot, by replacing bolts, and nailing boards and planks over the most damaged sections of the ship. This work was intensified when the Vasa was dry-docked.
Picture 10. A temporary solution.
At the same time as work was done on the ship and her artefacts, the first museum was planned and built. The ship on her new floating platform covered by an aluminium building
was towed to the museum site in 1962, less
than a year after the salvage.
The old museum (Vasavarvet) was not perfect at all, but in many ways preservation came first, and the public second.
The building around the ship was there to enable the conservation process, so the spectators shared the same damp conditions as the ship. Exhibitions and other facilities were placed outside the ship’s building.
Picture 12. The final solution?
In 1990, the new Vasa Museum opened to the public.
In many ways it is a fantastic place. The architecture is exiting, you can see the ship from many different angles in the large ship’s hall, and there is a nice restaurant and a good museum shop.
You can even entertain your guests here at night, dining in style beside the ship or organise a conference or a business meeting.
Picture 13. What about the ship?
The new museum is a bit more complicated when it comes to preservation.
In fact, the large ship’s hall is one gigantic display area. The problem is that one usually does not let visitors into a museum display case.
So, it is difficult to maintain a perfect climate. The numbers of visitors and the volume of the place make it difficult to avoid different climate zones.
Also, it is difficult to work on the ship
during opening hours, without exposing the public to noise and risks.
Surveying the ship at the old museum was difficult. The Vasa was on a floating platform, she was being sprayed with polyethylene glycol and the restoration work was in progress. As the ship kept changing, it was hard to establish accurate plans or a base to work from.
Even in the new museum, with the restoration work more or less completed, a very large ship like the Vasa is difficult to monitor by hand or eye only. It is also hard to transmit observations from shipwrights or conservators into facts and figures, but in order to make decisions and for future planning these facts are needed.
Picture 15. The geodetic system of the
Vasa.
When we started looking for a system for monitoring the hull stability of the Vasa, an important consideration was finding a highly accurate system that would survive for a long time.
We also wanted something that could interest other museum staff, scientists, media people and the visitors as well.
After discussing various solutions, we decided to use a geodetic system, which would be designed by the Department of Geodesy at the Royal Technical College in Stockholm for the Vasa.
Picture 16. Pros and cons.
There were several reasons for choosing a geodetic system.
It should be highly accurate. We could except to detect movements in the hull that were below a millimetre.
We would be able to target a large number of points on the ship, creating a good overall picture.
We could extend the system to cover also the inside of the ship and also create further target points in sensitive areas.
On the other hand, the system would be expensive and we would need to rely on outside experts for design and maintenance, even if we planned to operate the system ourselves.
Picture 17. How the system works.
A geodetic system will measure distances and angles in three dimensions; X, Y and Z, by using a total station.
The results, in the form of co-ordinates, will define the exact positions of chosen target points on the ship. If you measure the same points again at another time (also called an epoch), any differences between the co-ordinates will be movements in the ship at those points.
By monitoring a large number of points and by doing a number of surveys (or epochs), you will end up with a clear picture of any deformations in the Vasa.
Picture 18. The Control Network.
Dr Milan Horemuz, a researcher at the Royal Technical College, started with designing the control network around the ship, finding stable reference points in the ship’s hall and suitable positions for the total station that would be used for measuring.
The control network is vital to establish the exact positions of the total station. The total station does not have to be placed in exactly the same place for every survey, but will find its positions with the help of the control network. (Free station).
Picture 19. Target points on the ship.
The points on the ship are made up of reflective tape, with a cross in the middle and look roughly like very small targets for shooting. In total, some 350 target points were fastened to the ship and cradle.
Picture 20. The total station.
The total station is programmed to find automatically a defined number of points on the ship from each position. The operator will only need to do the final adjustments to each point. Also, all target points are measured twice and from two positions at least, which makes the system even more accurate.
The measurements are saved in the total station’s computer, and are then processed by specially designed software in PC format.
Picture 21. Position G535.
This presentation makes it easier to understand. The total station is on the starboard side of the Vasa, and will establish its position using the prisms 108,125 and so on in the control network.
Picture 22. Target points on the ship.
When the position is established, the station will automatically target the programmed points on the ship, and the operator will do the fine adjustments.
Picture 23. Different angles.
The station is then moved to the next position, and some points already measured are targeted once again, but from different angles. Some new point are added and measured.
The total position is moved to the next position, some target points are measured again, and new ones added as well.
By moving around the ship and establishing the total station in 50 different positions, 4 people are able to measure all 350 points on the ship in a week’s time, creating an epoch, which in total consists of about 2000 measurements.
Two of us will then have to spend a few days computing the measurements and comparing the results with earlier epochs.
Picture 25. Results.
After measuring 4 epochs we have not detected any alarming movements or deformations in the hull.
We did not expect to do so, as the ship and the support systems for the ship; bolts, cradle, deck beam supports, stern and rig wires are maintained as well as they can be.
However, the day will come when these
systems will have to be improved or renewed. Then we will have a set of
measurements and a geodetic system that will ensure that the existing hull form
can be kept for the future.
Even if we see the geodetic system as a major component for the future preservation of the Vasa, it is by no means a finished product yet.
Recently, we have started looking at the possibility of laser scanning the ship, both outside and inside.
By laser scanning the ship, enough data will be gathered to enable us to built 3D-models, to get accurate CAD-drawings and to link climate control, geodetic surveying and other systems.
All these components will be used to produce a future, long-term preservation plan for the Vasa.
Leif Malmberg
Head of the Vasa Unit
The Vasa Museum
Stockholm
Sweden