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Techniques : Radial Surveys

The simplest form of radial survey only requires a single diver with a tape measure and compass. One end of the tape is attached to a point in the middle of the site. The diver simply swims around the site recording the distance and bearing of each feature from the central point.

The positions of the points are assumed to be on the same level as the control point so this is provides positions in two-dimensions. It would be possible to correct for the depth of each point but this technique is not accurate enough to warrant it, depth information in the form of spot depths of features can be recorded at the same time.

Radial survey techniques are most easily used on flat sites with little kelp cover, places where the tape measure is not likely to be snagged. This technique is simple enough for deep sites and for sites with very poor visibility this technique can be easier to use than simple sketching.

The main practical limitation of this technique is the length of tape measure that can be used on the site. The main survey limitation is the ability to measure the bearing accurately, in this technique position accuracy decreases as distance from the control point increases.

Method
  1. Install a single fixed control point in the centre of the site as high as possible.
  2. Ensure the point is securely fixed, as it will be pulled on by the tape measure.
  3. Visit each feature and record the distance with the tape measure and bearing with the compass back to the control point.
  4. Record the depth of each feature and seabed type at the same time.
  • The distance from the control point should be limited to 30m, with this method position accuracy reduces with distance so keep the measurements short.
  • For larger areas additional control points can be used so long as you measure the distance and bearing between them.
  • In poor visibility it may be useful to have a second diver swim up and down the tape to ensure that it is not snagged.
  • When measuring the bearing avoid iron artefacts. To record the bearing for an iron object move further away from the control point before taking a measurement, while ensuring the tape stays over the object to be positioned.

Drawing up

Drawing up a plan view of the radial survey requires a scale rule, pair of compasses and a circular protractor.

If drawing on paper:
  1. Select a suitable scale for your drawing.
  2. Mark a point in the middle of your paper that represents the single control point and a line from this point representing magnetic north.
  3. For each feature, draw a line from the control point at the given bearing, longer than the line length required.
  4. Set the compasses to the correct distance using the scale rule and strike off the distance along your bearing line, this is the position for your feature.
  5. Add the names for the features, a scale and a north arrow to the drawing.

If drawing up on a computer:
  1. If a typical drawing program is being used then the scale does not need to be specified.
  2. Draw the control point at a convenient co-ordinate such as 500,500 to ensure all features have positive co-ordinates.
  3. Draw lines on your plan at the correct length and bearing, the ends of the lines will correspond to the positions of the features.
  4. Add the names for the features, a scale and a north arrow to the drawing.
 

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