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 They were originally made for ships’ navigation and are commonly referred to as hydrographic charts.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.
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.
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.
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.