Location -
Plymouth Sound, Devon, UK
Coordinates : 50 21.14N
004 09.80W (WGS84) Depth : 30m
Conditions : Sheltered, strong
tides, poor visibility
Type : Brigantine Built : 1782 Lost :
1786
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| History
In October 1973, whilst looking for the wreck of a
third-rate called the 'Harwich', a team of divers from Plymouth Sound branch of the
British Sub-Aqua Club found a bell on the seabed. They had discovered the wreck of
the brigantine the 'Frau Metta Catharina' from the Danish port of Flensburg, commissioned
in 1782.
The ship lay in 30m (100ft) in the mouth of the
river Tamar at the North end of Plymouth Sound. The seabed there is covered by a
thick layer of black mud, when this is disturbed the visibility drops to zero. The
team have been excavating the wreck with airlifts since the initial find and now a
significant portion of the stern is exposed in a large hole in the seabed. The site
is most famous for its cargo of Russian reindeer hides.
Download
the Site file to open in the free Site Reader program  |
Diver about to start work on the Catharina |
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| The image below
shows the position of the Catharina in the Barn Pool in Plymouth Sound as
shown in Site Recorder. The chart shows multibeam bathymetry
(MBES) overlaid with depth contours and the land surrounding the Sound. |
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Positioning Survey
As part of the ongoing survey work on the Catharina
site it was decided that the position and orientation of the site needed to be found.
The position and orientation of most marine archaeological sites is not usually
well known, this is largely due to the difficulty of establishing the relationship between
the site and any known reference points, especially difficult in deep water. In
addition, the positions of the control points on the site needed to be checked at the same
time.
A network of scaffold pole control points were
already installed on the site so all that was required was to devise a method to position
them. Sonardyne had already developed a technique for positioning seismic cables on the
seabed so this method was adapted for positioning the site. This method called
'array box-in' would calculate position errors as well as positions, so we could also show how
accurately the control points had been positioned.
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| Two site survey techniques were used in conjunction
for this exercise to improve the accuracy of the final result. The array
box-in method
has since been used for positioning sites such as the Mary Rose, the Resurgam submarine
and the Coronation but it was used first on this site.
Firstly, small acoustic transponder beacons were
attached to the control points on the site by divers. As the beacons were fitted to the
control points the divers could measure the distances between them acoustically, see the
network figure below. The network of control points does not form the ideal circular
shape as the wreck lies in a hole excavated into the seabed. The shape is a
compromise between the ideal required for survey and what is practical on this site.
The diver, Paul Dart, used a hand-held interrogator
unit called a Sonardyne Homer Pro which can be set to measure the distance to any one of
the beacons mounted on the control points. The Homer Pro works like an underwater
acoustic tape measure as it can measure the distance between itself and one of a selection
of beacons, see the picture below. The distance to the selected beacon in metres is
read by the diver from a numeric display on the rear end of the Homer. The diver
can then write the number on a slate or read the value to the dive control over voice
communications.
By swimming around the points once they were all in
position the diver could measure the distance to each of the other points on the site. The
depths of each of the control points was measured by the divers using a dive computer.
These measurements made between the points were used to help compute the positions of the
beacons on the seabed. |
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The Site Recorder screen image below
shows the network of control points and tape measurements overlaid on top
of the 1996 site plan. |
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Positioning in the Real World
For the next stage a transceiver unit was fitted to
the survey boat, this unit could be used to talk to the transponder beacons. The survey
boat was also fitted with a survey processing computer and a differential GPS (DGPS)
receiver used to compute the position of the boat.
The transceiver unit on the boat can measure the
distance from itself to the beacons by sending and receiving acoustic signals, pulses of
sound which travel through water. The beacons send back a reply signal pulse in response
to an interrogating signal pulse from the unit on the boat. The time between sending the
interrogation and receiving the replies from the beacons is measured by the interrogating
unit and then these times are sent to a processing computer.
The positioning survey was done on 13th August 1996
and took 12 minutes to complete. The survey boat was sailed around the outside of
the site and the interrogating unit was set to periodically measure the distance to the
beacons and send the results to the computer. The computer used these distances, the
previously measured distances between beacons along with positions from the DGPS receiver
to calculate where the beacons were. The computer could also calculate the accuracy of the
positions of the points. This technique is known as the 'top-down' method of
positioning beacons.
The distance measurements made by the Homer Pro on
their own could be used to position the network of control points relative to each other.
To work out where the site was in the world and its orientation we needed to use
the second method involving the boat. The second method works well without the extra
measurements from the Homer Pro but gives a better result if they are included.
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A Diver Using a Sonardyne Homer Pro
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Results
The survey data was processed using
Site Recorder, this program can use measurements from a Homer Pro as well as tape
measurements but it can also take in positions calculated using the top-down method.
All position, depth and distance measurements were processed together and used to
compute the positions for the control points.
Eleven control points were positioned in three
dimensions using 736 surface position fixes from the differential GPS, 42 baseline measurements from
the Homer Pro and 11 depth measurements.
The processing software showed that the error in the
Homer Pro measurements was 35mm, tape measurements less than 20m in length usually show a
standard error of around 20mm. Positions of the points were accurate to
approximately 0.5m absolute (95%) and 35mm relative to each other. The absolute
accuracy is governed by the accuracy of the 'top-down' positioning technique whereas the
relative accuracy is largely defined by the accuracy of the measurements made by the Homer
Pro. This is a typical result for a positioning exercise carried out with this equipment.
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Conclusions
Compared to a tape measure, the
Sonardyne Homer Pro appears
to be less accurate over distances of less than 20m. For distances of 20m to the
maximum range of approximately 500m the Homer Pro is as accurate or better
than a tape measure.
On this particular site it would be very difficult
to work with tape measures. The site is in 30m of low visibility water with currents
cutting across the site. In addition, the wreck is down in an excavation hole below
seabed level and is covered by a scaffold frame. There is a large chance that any
tape measures will be snagged on the scaffold frame in these conditions and it is not
feasible to use more than one tape at once. The Homer effectively acts as a tape
measure to each beacon on the seabed yet it can easily be used in such conditions.
- The time taken to complete this survey was considerably less than
that required for a tape survey. The distance measurements using the Homer Pro were
collected in just two 40 minute dives on the site.
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| The
image below shows the timbers around the main mast step along with
scaffold frame tubes (grey) and rolls of hides (hatched) |
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Bibliography
- The Wreck of the Metta Catharina,
Garbett G. & Skelton I. 1987 ISBN 0
9512916 0 2, [ABE]
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