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Sea level measurements

Tide gauges

The measurement of long-term changes in sea level from tide gauges involves the following:

At any one site:

  • Joining together data from a number of different instruments, which may include:
    • Highs and lows and their times measured by eye off a tide staff and written by hand in a book
    • A modern state-of-the-art acoustic tide gauge recording automatically every six minutes
    • Several generations of technology in between
  • Referencing them all to the same vertical datum, taking into account movements around the wharf area where they are installed and any movements of the structures that they were mounted on
  • Referencing the site's vertical datum to some standard vertical datum

Fortunately for us most of this work has been done by the nice people at the Permanent Service for Mean Sea Level (PSMSL). The University of Hawaii also maintains an archive of tide gauge data. For our work we normally use the monthly average data as we are mainly interested in the long-term behaviour, but higher frequency data is needed for some applications. Some high frequency data is available from the PSMSL and UHSLC, but much of it is in national archives, such as Australia's National Tidal Centre (NTC).

Tide gauge-derived SSH records are subject to contamination by vertical movement, which can take various forms:

  • It can be long-term and more-or-less constant, such as changes caused by Glacial Isostatic Adjustment (rebound from the last Ice Age)
  • Short term, e.g. the Boxing Day (2004) Tsunami, which was caused by an earthquake
  • Varying in time over periods of years or decades. This can be caused by sinking of piers because of unstable foundations or sinkage of land through (e.g.) groundwater pumping. A good example of the latter is the tide gauge record for Manila in the Philipines:

Tide gauge record at Manila

In recent years a lot of effort has been going into installing GPS receivers at or very close to high quality tide gauges to try to estimate this vertical movement directly and then correct for it. Some of the GPS records are now getting to the stage of providing useful estimates of this. See for example GLOSS and TIGA.

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Website owner: Neil White | Last modified 11/10/10


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