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Section 3. Archiving Marine Survey data#

3.3 Positional 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

Deposition and Archival File Formats

ASCII text (.txt,.dat, .xyz, etc)Published standard for ASCII Raw data, usually directly from a loggerE.G. Data logger outputs as structured ASCII text and incorporated into a database. There are well established archival procedures for databases in exporting tables as delimited ASCII text and documenting through an Entity Relationship Model (ERM) and a Data Dictionary.Suitable deposition and preservation format when stored with supporting documentation.
TFW: TIFF World file (.tfw)Proprietary ASCII format but associated image will be binaryA mechanism for geo-referencing images developed by ESRI (GIS software vendor). As such similar to GEOTIFF (see above) but in this case the metadata is held in a separate ASCII text file. TIFF World files will be small in themselves but may be associated with large images.That the metadata (spatial information is in ASCII could be seen as good for preservation.
XML: eXtensible Markup Language (.xml)A published open standard based on ASCII. Can be used for raw or processed data and within an increasing range of technologiesXML is a general-purpose markup language geared towards facilitating the sharing of data. An XML document is said to be 'well formed' when it conforms to XML's syntactical rules. It is described as valid when it conforms to semantic rules defined in a published schema. Many XML documents use a different file extension, for example .gml.Ideal for exchange and preservation if an established schema exists.

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).

Figure 11: Sample locational data within a .twf file.

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