The first method for recording sea level observations was to graph the height of the sea as a function of time. A stylus plotted the curve on a roll of graph paper on a drum rotating at a certain speed. This tide curve is also known as a marigram. A tide curve does not represent the tide itself but rather the change in sea level, of which the tide is usually the major component.
How it works
If the movement of the drum completes one revolution every 24 hours, it is possible to get several days of measurements on the same piece of graph paper. The tide is mainly dominated by the movement of the moon, and because the lunar day is longer than the solar day by about 50 minutes, the curves are shifted relative to each other.
For a semi-diurnal tide, it is possible to leave the paper in place for two weeks. Beyond this period, the curves may overlap and cause errors when digitizing the graph. If the paper is left in place too long, the recording become unusable and the data are lost.
Sources of error
Several sources of error can affect this type of record:
- for measuring the height:
- incorrect vertical positioning of the paper on the drum;
- variation in the height scale of the paper due to moisture or temperature;
- for measuring time:
- incorrect horizontal positioning of the paper, often to compensate for play in the drum;
- the paper does not match the drum diameter resulting in a cyclic variation of the time scale;
- the clock runs fast or slow;
- fluctuations in drum rotation caused by defects in the drive gears.
Although these defects are significant, they can be corrected if regular checks are performed.
For certain time measurement errors, the movements of the Moon and the Sun, which are known with great precision, can provide an excellent clock to rectify such errors.
Height positioning errors are more difficult to correct and can sometimes go unnoticed. They are detectable only by comparison with measurements from neighbouring tide stations or if glaring discontinuities are observed in the tide curve.
To find out more:
- Simon B. (2007). La Marée - La marée océanique et côtière. Edition Institut océanographique, 434pp.
Last updated: 12/12/2012