When there are power lines, bridges or other obstructions over navigable waters, signs are posted stating the height of the obstruction above the water. The stated height is referred to as “vertical clearance,” which is the clearance stated on the chart. Vertical clearance is calculated as follows:
Power lines
The distance from mean high water to the lowest point of the power line reduced by a margin of safety. (The margin of safety is 1.5 m – 2.75 m, depending on the voltage in the line.)
Bridges and other obstructions
The distance from mean high water to the lowest point of the obstruction, within navigable breadth, reduced by a margin of safety. The margin of safety is 0.5m-2m depending on expected swell in the fairway. Mean high water is the average value of the highest water level for the year calculated over many consecutive years.
Low bridges over protected fairways
Ordinarily, the lowest point of the bridge over the mean water level is stated without a margin of safety. Low bridges refers to bridges whose height people on small boats can easily judge for themselves, i.e., up to three metres high. The mean water level is the median value of the level of the water in multiyear observations. Note that the water level may be more than one metre above that level.