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Section 3. Archiving Marine Survey Data#

3.3 Metadata and Documentation#

Comprehensive Documentation
http://guides.archaeologydataservice.ac.uk/g2gp/Geophysics_5
All metadata elements needed to be recorded especially 5.3.6 - maritime sonar surveys

App 2 Georeferencing Geophysical Data http://guides.archaeologydataservice.ac.uk/g2gp/Geophysics_App2 discussion of georef inc accuracy etc.

Specialised metadata

Metadata to be recorded alongside the data itself includes:

  • Equipment used (make and model)
  • Equipment settings
  • Assessment of accuracy
  • Methodology
  • Software used
  • Processing carried out

3.7 Positional and Navigational Data#

Datasets containing positional data can come from a number of sources on marine projects and are largely dependant on the data collection methodology. The VENUS project missions, for example, largely focussed on the collection of multiple data streams, including positional data, via unmanned Remotely Operated Vehicles (ROVs). Other projects may use a similar methodology to record the position of divers through a combination of acoustic tracking and GPS.


ROV Positional data

Within the VENUS projects, positional data was collected from two sources, sonar heads on the ROV itself to record its distance from the seabed and from an Ultra Sound Base Line (USBL) acoustic positioning system.

  • Sonar - Kongsberg
    • 2 heads - conical (2.7 deg) and fan (1.7 deg)
    • Only aim is to record distance of ROV from seabed
  • USBL (Ultra Sound Base Line) acoustic positioning
    • 0.5 Hz sample
    • Uses GPS or DGPS where available (DGPS 1m error, GPS 7m error)
    • Boat recordings
      • heading
      • pitch
      • roll
      • DGPS position
    • ROV recordings
      • Depth
      • X position
      • Y position
    • also records signal error/variability
    • Using 2 transceivers to track either
      • 2 ROVs
      • ROV and diver
      • Point on site and ROV - allows later correction of GPS positioning


Acoustic Tracking

Acoustic tracking can be used to keep a log of a diver's location throughout the dive. Sound signals are emitted by a beacon attached to the diver and picked up by a transceiver attached to the side of the boat. The relative position of the diver underwater can be calculated and these relative co-ordinates can be used to calculate an absolute location for the diver. Additional equipment may be needed to compensate for the motion of the vessel in the water. Acoustic Tracking was utilised for the Wrecks on the Seabed project .

Normal practice is to use a data logger for collection. Generally the data will be in the form of structured ASCII text. As such it will be easy to import into other packages such as a GIS or database. Wessex Archaeology supplied their Acoustic Tracking data as a Microsoft Access database


Reasons for archiving

For both Wessex Archaeology's Wrecks on the Seabed project and the VENUS project this data was seen as crucial to the project archive as it sets much of the other project data in context. It forms a vital reference point for the position of the ROV or diver individual photographs were taken, segments of digital video recorded or general observations made.


Problems and issues

Positional data may be possibly processed and not the raw data. Any processing carried out should be suitably documented.


Specialised metadata

Metadata to be recorded alongside the data itself includes: Equipment used (make and model) Equipment settings Assessment of accuracy Methodology Software used Processing carried out


Example data

Within the VENUS project positional data combined with photography to create Tiff world files for use in photogrammetry and rapid mosaicing. In each case the image was stored with a ascii .tfw file containing the locational information (figure 9).

               0.004755287425895
               0.000000000000000
               0.000000000000000
              -0.004755287425895
          703192.400211019100000
         4782488.815022848000000
Figure 11: Sample locational data within a .twf file.


3.4 Navigational Data#

In addition to positional data, navigational data relating to an ROV or vessel is often collected and again provides context for data recordings or observations.

Within the VENUS project, navigational data was acquired via an IMU (Inertial Measurement Unit). Such units record motion in a number or terms, as detailed below:

  • Pitch, roll, yaw etc.
    • Pitch
    • Roll
    • Yaw
    • Wx (Vx?)
    • Wy (Vy?)
    • Wz (Vz?)
    • Ax
    • Ay
    • Az
  • RPM (motor velocities)
  • Depth
  • Heading


Reasons for archiving

  • For the future interpretation of data e.g. seeing anomalies in the results not seen before
  • For monitoring condition and erosion of wreck sites
  • For targeting areas for future dives/fieldwork


Problems and issues

As with positional data, it is important to document any processing carried out on the original datasets.


Specialised metadata

  • Metadata to be recorded alongside the data itself includes:
  • Equipment used (make and model)
  • Equipment settings
  • Assessment of accuracy
  • Methodology (see section 5.3.7 Survey Methodology of the Geophysics guide)
  • Software used
  • Processing carried out

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