Editing Nautical chart (section)

== Data sources ==

Nautical charts are based on [[hydrographic survey]]s and [[bathymetric survey]]s. As surveying is laborious and time-consuming, hydrographic data for many areas of sea may be dated and are sometimes unreliable. Depths are measured in a variety of ways. Historically the [[sounding line]] was used. In modern times, [[echo sounding]] is used for measuring the seabed in the open sea. When measuring the safe depth of water over an entire obstruction, such as a [[shipwreck]], the minimum depth is checked by sweeping the area with a length of horizontal [[wire]]. All depths on charts is measured with respect to a datum/reference level. This ensures that difficult to find projections, such as [[Mast (sailing)|masts]], do not present a danger to vessels navigating over the obstruction.