Editing Nautical chart (section)

=== Depths and heights ===
[[File:British Admiraly chart colours.svg|thumb|upright=1.3|Use of colour in British [[Admiralty chart]]s]]

Depths which have been measured are indicated by the numbers shown on the chart. Depths on charts published in most parts of the world use metres. Older charts, as well as those published by the United States government, may use [[Foot (length)|feet]] or [[fathom]]s. Depth [[contour line]]s show the shape of underwater [[relief]]. Coloured areas of the sea emphasise shallow water and dangerous underwater obstructions. Depths are measured from the [[chart datum]], which is related to the local sea level. The chart datum varies according to the standard used by each national [[Hydrographic Office]]. In general, the trend is towards using [[lowest astronomical tide]] (LAT), the lowest tide predicted in the full tidal cycle, but in non-tidal areas and some tidal areas [[Mean Sea Level]] (MSL) is used.

Heights, e.g. a lighthouse, are generally given relative to [[mean high water spring]] (MHWS). Vertical clearances, e.g. below a bridge or cable, are given relative to [[highest astronomical tide]] (HAT). The chart will indicate what datum is in use.

The use of HAT for heights and LAT for depths, means that the mariner can quickly look at the chart to ensure that they have sufficient clearance to pass any obstruction, though they may have to calculate height of tide to ensure their safety.