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Depth Measurements

The other component of our survey position is height or depth, in survey terms this is called vertical control and the principles described here are similar to those used for levelling on land. The most common tools for measuring depth or height are the digital depth gauge and the dive computer, these instruments measure the depth of water and display it on a screen most usually to a resolution of 0.1m.

Waves and tides affect any instrument that measures depth. Surveys underwater are frequently done relative to a point on the seabed, this cancels out the effects of tides. A point on the site is nominated to be the depth reference or temporary benchmark (TBM) for all depth measurements. The use of this benchmark is described in the section on fieldwork.

Another way of recording height is to measure vertical offsets with a rule or tape measure, this is most often used for recording ship's structure.  Since dive computers became available bubble tubes no longer need to be used to measure depth.

Types of Depth Gauge

The simplest way to make reasonably accurate depth measurements is to use a standard digital depth gauge or dive computer. Analogue gauges should not be used as it is usually not possible to read the depth with sufficient accuracy, say to better than 100 mm.

The depth measurement from a dive computer takes time to settle to the measured depth so allow time for the measurement to settle before recording it. Some dive computers are more accurate than others and will give different results at depth so try to use the same computer for all depth measurements.

The depth sensor reading is affected by large changes in temperature so measurements will change as the computer cools or warms up during a dive. If the temperature difference between on land and in the sea is very different keep the computer out of the sun or put it in a bucket of seawater for about 30 minutes before the dive.

Tides

Change in the height of tide will affect depth measurements made with a dive computer. There are a couple of things that can be done to minimise the effects:
  • Measuring depths at slack water usually gives the smallest change in tide and so affects the measurements the least.
  • Use one dive just to measure depths of points rather than doing it in a number of separate dives.
  • Make one control point a benchmark (TBM) or depth reference point. Measure the depth at the start and end of each dive, the difference in the tide height between dives can then be measured in the difference in the depth of this one point. The difference in depth of the TBM between the start and end of a dive shows how much the tide has changed during the dive.

The true depth of points on the site is not easy to establish accurately so for most surveys the depths or heights of points are given relative to each other. A rough estimate of the depth below chart datum can be found by measuring the depth of a point at a known time then taking off the published height of tide for that time.

Recommendations

  • Use a digital depth gauge or dive computer
  • Make depth measurements carefully, allow for settling
  • Allow the depth gauge to equalise in temperature with the water.
  • Use a TBM to correct for the change in tide height
 

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