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Charting the sea floor

by  Lionel Carter

Where the land meets the sea, it doesn’t end there. In the deep, dark world beneath the oceans, there are mountains, valleys, plateaus and volcanoes. The early explorers first charted this dramatic and inaccessible terrain by lowering ropes, weighted with lead, to measure water depth. Today’s map makers use more complex technology – multiple sound beams sweep across the seabed to give a precise record of its depth, shape, sediment character and much more.


Making and using charts

Charts are maps of the seabed and overlying waters. They were originally made for ships’ navigation and are commonly referred to as hydrographic charts. Hydrography is the science of measuring and describing the physical features of oceans, lakes and other waterways.

Historically, official hydrographic charts were produced by the Royal New Zealand Navy Hydrographic Office. In 1999 chart production was primarily the role of Land Information New Zealand, although the navy still carried out some marine surveys for charting.

What hydrographic charts show

Hydrographic charts show water depth. They can depict depth as a single point (like the height of peaks on a land map) or as contours, which are called isobaths. Coastlines are also displayed, along with coastal landmarks that may help navigation. Other information can include submerged and exposed rocks, navigational lights and buoys, submarine cable routes, seabed type and anchorages.

Modern marine charts may also show tidal and ocean currents, water properties such as temperature and salt content, the distribution of sediments and rocks on the seabed, submarine fault lines and landslides, and volcanoes. Almost anything that can be measured in or under the ocean can be charted.

Interpreting charts

Charts can be interpreted to identify how the seabed has formed. For example, off East Cape large crescent-shaped precipices at 100–200 metres are explained as the scars of giant underwater landslides. Underwater channels up to 2,000 kilometres long indicate the existence of sediment-laden currents. These currents flow along the channels and sediments are discharged at the end as vast submarine fans, similar to the alluvial fans which form the Canterbury Plains.

Advances in technology have improved the accuracy, detail and scope of charts. New mapping systems collect large amounts of digital data, which computers process into images that provide new perspectives of the sea floor.

Using charts

Charts are used widely as a navigational aid. All types of vessels, from recreational fishing boats to large container ships, rely on charts for safe passage especially in coastal waters and ports.

Charts are used to guide human activities that involve the seabed. They give essential information for defining sites for underwater cables and pipelines, and oil and gas platforms. The fishing industry uses them when trawling the ocean floor. They provide information for the construction of port facilities and the dumping of spoil dredged from harbours. Charts are also a tool for marine scientists, who interpret data about the seabed and overlying waters to gain an understanding of the marine environment.

Making charts

Because the safety of vessels, people and the environment are at stake, hydrographic charts must be accurate. They are made according to strict standards, which are outlined by the International Hydrographic Organization. For example, in shallow harbours, each depth measurement or sounding should be located horizontally to within 2 metres, and vertically to within 0.25 metres (depth is plotted on a latitude–longitude grid, the basic reference for navigation). Charts are continually being revised as new information becomes available.


First charts of New Zealand

New Zealand was discovered by Polynesian seafarers well before the arrival of Europeans. But they did not create written records, and this has meant that the first charts of New Zealand’s land and seabed were those drawn by European explorers.

Putting New Zealand on the world map

There is some debate about when New Zealand was first committed to a map. Parts possibly appeared on the world map of Jean Rotz, published in 1542 for King Henry VIII of England. A large southern land mass resembling Australia was shown with a distinct promontory, which some geographers have interpreted as New Zealand’s East Cape. However, evidence for this view is not strong.

The first time any part of New Zealand was outlined on paper was during Abel Janszoon Tasman’s voyage of 1642–43. Although he never stepped ashore, Tasman, aided by his pilot, the hydrographer Franz Jacobszoon Visscher, plotted New Zealand’s west coast from near Hokitika to Cape Maria van Diemen. That fragment of the coastline was to appear in atlases for the next 228 years.

Cook's remarkable chart

The now familiar long, narrow outline of New Zealand was initially drawn by Captain James Cook during his first voyage to the country, from 7 October 1769 to 1 April 1770. In just 175 days he sailed around the main islands, plotting the general pattern of the coast with amazing accuracy. There are errors, however. Banks Peninsula is shown as an island and Stewart Island is connected to the South Island.

Cook’s achievement was remarkable. His ship, the 30-metre Endeavour, was awkward to sail and he relied on a sextant and ship's clock for navigation. In places Cook was able to plot water depth, the location of rocks and the outline of sand banks. Some of his depth measurements, including those from Doubtful Sound, remained on hydrographic charts until well into the 20th century.

Chart to chart

The magnitude 8.1 earthquake of 1855 uplifted the entire Wellington peninsula and tilted it to the west. By comparing the 1849 chart, surveyed by the Acheron before the earthquake, with the 1903 chart by the Penguin, changes in water depth could be measured. Differences between the two charts show that the harbour entrance became shallower by up to 3.6 metres due to uplift and siltation. Without the 1855 chart there would have been no way of calculating these changes.

Early colonial mapping

Charts of New Zealand improved after surveys made by the French, British and other explorers who came after Cook. Much of their effort focused on bays, fiords and harbours – where ships took on supplies or were repaired.

Traders added to the hydrographic coverage of the country. The Snapper, a cargo vessel carrying flax, measured depth in Foveaux Strait. A need arose to chart harbours for the safe passage of immigrant ships. In 1826 Port Nicholson (Wellington Harbour) was surveyed twice. As coastal settlements sprang up, increased shipping saw greater demand for better charts. Between 1848 and 1851 the Acheron undertook the first systematic, detailed hydrographic survey of New Zealand.


Evolution of modern charting

Surveys and ships

New Zealand has the world’s fourth largest Exclusive Economic Zone (the water and sea floor over which the country has legal jurisdiction and resource rights). Covering more than 4 million square kilometres, it is the country’s most extensive environment, but also its least known. While coastal waters were systematically surveyed as part of early settlement and trade, the deep ocean was not charted until much later. One of the earliest ocean charts is dated 1895. It was based on data collected on the Challenger expedition of 1872–76, the first world-wide ocean survey. But depth soundings were scattered, and only a few large seabed features were identified.

Surveys expanded over the next 60 years as more naval and research ships visited the region. The period following 1949 saw the deployment of the hydrographic ship Lachlan, the creation of the New Zealand Oceanographic Institute, and frequent visits by foreign research vessels, especially the United States research ship Eltanin. As a result, by 1967 a broad outline of the ocean floor around New Zealand had been charted.

Even so, much detail remained unknown. From the 1970s, efforts by the Royal New Zealand Navy ship Monowai, the New Zealand Oceanographic Institute’s ships Tangaroa and Rapuhia, and the National Institute of Water and Atmospheric Research ship, also named Tangaroa, have revealed a host of features, which featured in the 1998 chart, Undersea New Zealand.

Undersea New Zealand

The award-winning chart Undersea New Zealand provided a new view of the seabed. To construct the chart, depth and navigational data were collected and inspected for errors. Computer software converted the data into a shaded relief model, revealing with great clarity the shape of the ocean floor. Colour indicated water depth, which ranges from the shallow continental shelf (0–150 metres) to ocean trenches where depths exceed 5,000 metres. The land topography was also presented to emphasise the continuity between land and sea.

Technology

Initially imprecise and slow, surveys have improved remarkably. James Cook gauged water depth by lowering a weighted, calibrated rope or lead line to the seabed – a method used since ancient Egyptian times. To fix position, Cook relied mainly on a sextant and ship's clock. Between fixes, the ship’s position was estimated from its speed and course; a process called dead reckoning.

When the Challenger arrived in Wellington in 1874, it opened a new era of depth-finding, this time by sounding machine. Although it used a weight similar to a lead line, the machine was quicker and more accurate as it mechanically measured the line as it wound onto a drum.

The invention of the first effective echo sounder around 1922 improved the accuracy and speed of gauging depth. In 1924 the development of radio-based positioning helped coastal navigation, but in the open ocean, navigators still relied on sextants.

This limitation was overcome in the 1960s with the launch of the first navigational satellites. By 1974, New Zealand naval and oceanographic ships operated satellite systems that typically gave positions to within 500 metres of a location. Positioning was better in coastal waters, where radar and microwave systems had accuracies to within 1 metre. The 1970s also saw the first commercial multibeam echo sounder, which heralded a new era of seabed mapping.


Multibeam echo sounding

How multibeam echo sounders work

The widespread use of multibeam echo sounders since the 1990s has revealed the seabed in unprecedented detail. Unlike normal echo sounders, which direct a single beam of sound to the seabed directly below a survey vessel, multibeam systems emit a fan of sound beams. By this method, a wide area of the seabed can be scanned in high detail.

Multibeam and climate change

As the climate gets warmer the ocean around New Zealand will change. Higher sea levels, increased winds, stronger storms and more intense rainfalls will leave their mark on the seabed and coast. A major storm off Wellington, for example, could shift sand and gravel, destroying old habitats and creating new ones. An event like Cyclone Bola could blanket the continental shelf with a thick layer of fluid mud. As multibeam systems can precisely map the seabed, it is possible to form baseline charts against which such future changes can be measured.

Fan width increases with water depth. At 2,000 metres the fan covers an 11-kilometre-wide strip of seabed. As the vessel sails along the survey line, several million depth soundings are collected for every kilometre. In a single day, over 3,500 square kilometres can be charted. When this is matched with the precision of modern global positioning systems, for the first time it is possible to rapidly survey large areas with exactitude.

Discoveries continue to be made. In October 2004, 12 underwater volcanoes were mapped for the first time during a multibeam survey by the Tangaroa.

Multibeam data

The vast amount of data captured by multibeam systems are recorded and processed on the survey ship. Special software allows preliminary charts to be plotted within hours. However, a range of corrections and checks are required before final charts are produced.

Multibeam data allows oceanographers to chart many different aspects of the ocean floor:

  • Depth can be shown as plots of selected soundings, as contours, or as colour-coded charts.
  • Seabed shape can be realistically depicted as a shaded relief model.
  • Slopes can be precisely measured and mapped to allow engineers, for example, to analyse seabed stability.
  • The type and distribution of sediment and rocks can be gauged by recording the strength of the returning echoes (called backscatter). Weak echoes are often associated with soft mud because it absorbs some of the sound's energy. Rocks absorb little energy and produce strong echoes.

Hononga, rauemi nō waho

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How to cite this page: Lionel Carter, 'Charting the sea floor', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/charting-the-sea-floor/print (accessed 19 January 2020)

He kōrero nā Lionel Carter, i tāngia i te 12 Jun 2006