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Kartbild på medelvattenyta

The traditional way of referring depth information in Swedish charts is by specifying a mean sea level for a given year (typically the chart production year). However, since the majority of the Swedish coast has a continuous land uplift, depth information in these areas gradually becomes obsolete. Uplift entails that both land and the seabed rises relative to the ocean surface as a result of having been depressed by the weight of the ice sheet during the last ice age. The uplift is greatest in northern Sweden, where it is about 1 cm/year.

As a consequence, both uplift and current sea level can be important for calculating the current depth. Each printed chart gives information about the rate of land uplift as well as the year the mean sea level relates to. This can be found to the right in the upper margin of the chart. Using this information, it is possible to recalculate the chart depths to the current year's mean sea level and subsequently adjust for the current sea level to retrieve the current depth.

Example: Chart 4151 indicates a depth of 5.6m for a shoal and the current water level is -10cm in relation to the current year's MSL. The chart indicates that its datum is the mean sea level in 1960 and that the rate of land uplift is 0.9 cm per year. This means that year 2014, the uplift correction will be (2014 - 1960) x 0.9 ≈ 49 cm. Therefore, depth values in the charts need to be reduced by about 0.5m due to land uplift. If you also take the current sea level into account, the current depth (in meters) for the shoal becomes 5.6 m - 0.49m -0.10m ≈ 5.0m. See the diagram below.

Skiss på medevattenyta
Click on the image to see it in a larger version.

See adjacent links for information and data about the current water level and the current year's average water level.

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