Nick Hough, IMCA
The International Marine Contractors Association (IMCA) has published “Guidelines on the use of GNSS for tide calculations” (IMCA S 027). It provides an overview of how GNSS can assist in more accurate real-time direct measurement of tidal changes. IMCA S 027 is available for members to download free of charge, and costs £50 for non-members.
IMCA S 027 includes sections on
- tide theory
- geodetic reference systems and tidal datums
- tides from GNSS
- quality assurance and quality control
- glossary, references and a list of further reading
One reference is the recently revised IMCA S 015, “Guidelines for GNSS positioning in the Oil and Gas industry,” produced with the International Association of Oil and Gas Producers.
“As with horizontal positioning, vertical positioning is referenced to specified datums,” explained Nick Hough, IMCA’s technical adviser for Offshore Survey. “Unlike land surveying, where vertical measurements (elevations) are made from and to a known, fixed position, vertical measurements offshore (depths) are taken against a moving dynamic surface.”
“All absolute depths recorded from survey activities need to be adjusted for tide and reduced to a known constant vertical datum such as Mean Sea Level (MSL) or Lowest Astronomical Tide (LAT),” Hough said. “The effect of tides will result in depth differences at different times of the day, and at the same times on different days.”
“Advances in GNSS technology enable accurate and consistent calculation of height above a known datum, which means reliance on tide gauges or tide prediction tables is no longer necessary,” Hough said.
