Inside the AHO
what is hydrography?
A Hydrographic Survey is the 'mapping' of an area of sea and surrounding coastal detail. The information from a survey enables a cartographer to portray a part of the Earth's surface on a flat piece of paper and indicate by soundings, height and depth contours and spot heights, the various irregularities both above and below the sea surface.
For a hydrographic survey, all the details of use to the mariner must be correctly described and positioned relative to each other and their positions located on the Earth's surface.
The Fundamental Parts of A Survey
In the simplest terms a survey can be broken down into five fundamental requirements:
Position: Position must be known in the survey area in order to locate the survey information. Position can be obtained by taking or extending control from a previous survey or be established using new positions obtained from navigation satellites.
Orientation: A direction or Azimuth must be known to orientate the survey. This can be established by accurately fixing at least two positions in the survey area or more traditionally by observing to the Sun or other heavenly body to obtain the true bearing of the second position from the first.
Scale: The distance between at least two positions must also be known to allow the survey to be represented at a particular scale. This can be achieved by direct measurement; or by calculation if at least two positions are known.
Shape: Having established position, azimuth and scale, additional positions will allow all the detail of the survey to be fixed.
Detail: From the established framework the survey can be conducted in earnest, gathering all the required information. The following lists some of the more important detail which is recorded:
- regular depths of the entire water area to seaward of the H.W. line
- the positions and least depths over all shoals, wrecks, reefs, banks and obstructions
- the position of the coastline (HW and LW mark)
- the nature of the bottom at regular intervals over the whole water area
- the positions of breakers, tide rips, fishing stakes, etcetera
- leading lines fixed and carefully sounded positions of topographical detail of use to the mariner
- tidal stream measurements
- tidal observations
- photographic views
- the positions of all floating navigation marks fixed on both flood and ebb tide
- Sailing Directions checked and amended details of wrecks found, swept, etcetera
- records of all sonar sweeping undertaken and investigation carried out
- light characteristics measured
Throughout the survey meticulous record keeping is required. Depth data is normally logged on computer. Additionally, there will be manuscript records of other observations and computations. These may include:
- geodetic observations
- coastline mapping
- bottom sampling
- buoy fixing
- calibrations and checks
- records of computer data file management
- echo sounder traces
- hard copy graphics
Results of the Survey
The final result is a completed survey which is rendered to the Hydrographic Office. It takes the form of a Report of Survey which details all the activities conducted during the survey together with all the final data and records.
A chart will be compiled or revised in the AHO using the information from the survey.