A hydrographic survey involves the gathering of data about a particular area of water. This will usually be carried out to confirm that it is safe for boats to navigate and to identify any potential dangers. There is a school of thought that the term can only apply to inland waters such as rivers and lakes, but the phrase is commonly used in reference to oceans.
Some of the measurements which will be taken in a hydrographic survey are tides, currents and waves. These interact to create the overall pattern of the water’s behavior. For example, waves are affected mainly by local wind patterns, though of course these can vary, while tides are regular and predictable. Currents are overall patterns of movement in the oceans caused partly by both winds and tides, but also by the differences in temperature and salt levels in different areas of the ocean.
A hydrographic survey will also look the land beneath the water such as the river or ocean bed. This will involve measuring its depth at different points. The survey will also take into account natural features such as reefs and rocks which lay below the water surface.
Because a hydrographic survey is primarily driven by safety, there are certain conventions which those carrying out the surveys will follow. For example, where the depth of a section of water is variable, the survey will usually record the lowest depth which is possible rather then base the figure on an average over time. This can cause debate since it can be argued that the charts created by the survey are not a true representation of the waterbed as a whole; the counter-argument is that it is better to err on the side of caution. In many cases, the people carrying out the survey will take multiple measurements so that they can produce both the safety-focused hydrographic results and a more accurate measure of the water bed known as a bathymetric chart.
The relevant authorities will often accept suggestions from the public for areas which need a hydrographic survey but are not currently on the authority’s schedule. One example of a suitable case for suggestion is when the pattern of use in an area has dramatically changed, for example if leisure use of a stretch of water has increased. Somebody could also suggest an area is surveyed if they believe an accident has been caused by errors and omissions in the most recent previous survey.