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Techniques : Underwater Survey Techniques

There are a number of techniques that can be used underwater for recording sites. The techniques vary in accuracy, complexity and cost so not all techniques are applicable for all recording tasks :

Method

Assessment

Setting up survey control

Positioning artefacts

Recording detail

Recording structure

Recording Topography

Sketching

       

Radial

       

Offsets & ties

 

 

3D Trilateration

 

 

 

Drawing frames

   

 

Photogrammetry

     

 

Photomosaics

   

 

Acoustic positioning systems

 

 
Sketching

It is essential to study the site before starting any further survey work and generating a sketch is a good way to do this.

Radial Surveys

For a radial survey a diver records the position of features on a site using a distance and bearing back to a single control point. This technique is very simple, quick and can be done by a single diver. The technique is not accurate enough for pre-disturbance or excavation surveys but gives a good enough idea of the site for an assessment.

Offsets and Ties

Offsets and ties are used to position features relative to a baseline running through the site. An offset measurement positions a feature using a single measured distance at right angles to the baseline from a known point, a tie uses two or more measurements from known points on the baseline to position the feature.

3D Trilateration

 

Three-dimensional trilateration or Direct Survey Method (DSM) uses distance and depth measurements to position features on a site. The technique is similar to 2D trilateration with the exception that distances are measured directly to features and the processing deals with any difference in depth.

Drawing Frames

 

Drawing frames are used to record very small areas of the site in detail. A frame is placed over the area to be recorded and the diver draws visible features on a scale drawing or directly on to the drawing frame.

Close-Range Photogrammetry

 

Close-range three-dimensional photogrammetry is a very accurate way of recording complex structures. A number of photographs are taken of the structure from different viewpoints and processed in a computer program. The program then constructs a three-dimensional model of the structure to scale.

Photomosaics

If a series of overlapping photographs are taken of a site then they can be joined together to form a single large photograph. The photographs can be of the seabed taken vertically downwards or of the side of a structure. Since the tools to do this have become more widely available this technique is being used more often. Sufficiently good results can be obtained with a digital camera and the graphics processing software available found on most computers.

Like close-range photogrammetry and drawing frames, this technique is effective for small areas but must be used in conjunction with a control point network.

Acoustic Positioning Systems (APS)

Acoustic positioning systems are widely used for survey work in the offshore industry. These systems effectively replace the tape measures and dive computers used for DSM with distances measured with sound pulses. A diver fitted with suitable hardware can be positioned within an array of acoustic beacons, on the surface the archaeologists can then see where the diver is.

 

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