Survey Fundamentals and Terminology
Survey Fundamentals and Terminology
A candidate must command the vocabulary that underpins every hydrographic and maritime survey: datum (a reference surface, such as a geodetic datum or a tidal datum), benchmark (a fixed physical mark of known elevation), sounding (a measured depth), reduction of soundings (correcting depths to a common reference), position fixing (determining a vessel or feature location), tide and tidal stream, chart datum (typically Lowest Astronomical Tide, LAT), ellipsoid, geoid, and vertical datum. Understand the difference between depth (instantaneous water depth) and reduced depth (depth relative to chart datum). Know the terms feature (any object or phenomenon), obstruction, wreck, shoal, contour, and isobath.
Traps in questions often hinge on precision of language. For example, a question may ask for the reference surface to which soundings are reduced; the answer is chart datum, not mean sea level. Another trap is confusing accuracy (closeness to truth) with precision (repeatability). Also be alert to the distinction between horizontal and vertical datums, and between absolute and relative positioning. Questions may use 'sounding datum' or 'chart datum' interchangeably; in IFMP context, chart datum is the standard.
Rule of thumb: Always reduce soundings to chart datum, and always state the datum when reporting depths. A worked example: if a sounding is 12.3 m at a tide height of 2.1 m above chart datum, the reduced depth is 12.3 − 2.1 = 10.2 m. Remember that tide height is added to chart datum to get water level; to reduce a sounding, subtract the tide height from the measured depth.
Key terms to master: CD, LAT, MSL, ellipsoid, geoid, benchmark, sounding, reduction, position fixing, isobath, contour, obstruction, wreck, shoal.