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Distance Measurements

Points we are trying to locate on our site have positions given in three dimensions, X and Y horizontally and Z depth or height. Positioning under water is largely a matter of measuring distances between objects. The most typical tools used for this are the tape measure, the grid and the rigid rule.

Tape Measures

Many different varieties of tape measures are available, they come in different materials, lengths and designs. Because the tapes are to be used underwater some are more suitable than others.
  • A tape measure suitable for survey work underwater is no more than 30m long and has graduations at 1mm intervals.
  • Open frame tapes are better than the closed frame type as they can easily be rinsed off after use and can easily be dismantled to remove silt and grit.
  • Yellow tapes can be photographed and seen using divers helmet cameras but white tapes tend to flare in bright light and cannot easily be read.

 

The most common type of tape measure is made of fibreglass reinforced plastic and those with stainless steel or plastic fittings will last the longest. Fibreglass tapes are cheap and readily available from builders merchants but do stretch when being used. Steel cored plastic tape measures look very similar to fibreglass tapes but stretch very little, these tapes are slightly more expensive and can kink if not handled properly. Avoid steel tapes as they rust very quickly.

A typical steel tape is accurate to about 6mm at 30m in ideal conditions whereas a fibreglass tape is only accurate to 30mm at 30m. The accuracy does depend on the conditions underwater, any current will tend to make the measurement less precise. Measurements over 30m are not recommended as that length of tape is difficult to manage and the sag in the tape makes the measurements insufficiently accurate.

Other Tools

  • Rigid 1m long rules are useful for recording detail as are plastic folding rules.
  • A divers compass can be used for finding magnetic north so it can be marked on a sketch or drawing frame square.
  • A clinometer is very useful for measuring the tilt of ship's structure.
  • Large callipers are useful when recording thickness of objects such as timbers or the detail on cannons.
  • Drawing frames
  • Acoustic distance measuring instruments have been used in place of tape measures to measure distance underwater on some sites. The exposed frames of the Mary Rose were surveyed using this technique as early as 1975. More recent surveys have been undertaken using a Sonardyne Homer Pro transceiver on the Coronation and Catharina von Flensburg sites.
  • Laser distance measuring equipment has been tried underwater but it only works where visibility is good.
  • Acoustic positioning systems (APS) are used for surveying underwater
  • Techniques that have been used in the past include the plane table, the bearing circle, and an underwater theodolite.

Tape Measure Tension

Fibreglass tapes will stretch so the correct amount of pull has to be used to ensure the right distance is recorded. The weight of the tape itself tends to make it sag so measurements will tend to be slightly long, in pulling the tape straight the diver may end up pulling it too much and record a short measurement. Any water current will make the tape bow, this has the same effect as the sag.

It is possible to measure the correct tension but this is difficult to do underwater and is very time consuming. To get the feel of the correct tension to use, check against a spring balance on land. Getting the right tension can only come with practice, but try to be consistent with the amount of tension you use.

Only attempt to measure short distances as the effects of sag and bowing all increase with distance. Try to keep measurements less than 30m at all times and less than 15m where possible.

Recording

Measurements should be made to the nearest millimetre for most survey work, however for quick reconnaissance surveys measuring to less accuracy is acceptable.

The free end of the tape should be hooked on to a control point and unwound in the direction of the point to be measured. Pull the tape tight over the point to be measured and record the distance at the centre of that point.

The names of the relevant points should be clearly recorded on the form along with the measurement itself. Care is needed here as a mistake can be made when the measurement is written down and when it is read by whoever is processing the measurements.

The results should be recorded on a form clearly enough for someone who has never visited the site to be able to process the results. It is often necessary to refer back to the recording forms during processing to help decide whether a measurement should be rejected. A number of factors affect the quality of the measurements, working in low visibility, currents and deep water all tend to produce more frequent mistakes or blunders.
  • Write clearly in capitals
  • Use standard units and conventions
  • Include a sketch
  • Avoid smudging your results
  • Include estimates of visibility and current

Tapes are prone to being snagged on other objects between the two points being measured. Where both ends of the tape cannot be seen the tape should be checked for snags before the measurement is taken. Where practical and regulations allow, work in pairs where one diver makes measurements while another just ensures that the tapes are not snagged. Avoid pulling tapes to free them from snags as this can damage fibreglass tapes and put kinks in steel tapes.

Calibrating Tapes

Survey tape measures should be checked regularly. A standard tape measure should be used to check each tape being used for recording, preferably at 15m and 30m lengths under normal tension. Any difference larger than the required accuracy for the survey indicates a problem and that tape should not be used. The standard tape measure should be steel cored rather than fibreglass and ideally should not be used underwater so it does not get damaged.

 

Recommendations

  • Plan the measurements to be made before the dive
  • Make the measurements at slack water
  • Uniquely and clearly identify all points
  • Clearly mark the control points with bright markers and big labels
  • Use standard forms and teach the divers to use them correctly
  • Standardise on metres to three decimal places (000.000) or millimetres (000000)
  • Train the divers in the use of the techniques on land before they use them underwater
  • Demonstrate to the divers how and where to measure using the same type of control point
  • Where an offset is expected add a field on the recording form
  • Ensure the divers swim the length of the tape if both ends cannot be seen at the same time
  • Expect to make 15% mistakes, make repeat and check measurements.
  • Calibrate all tape measures
 

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