Let’s now find out the times and heights of high and low water on September 1st. Note that all times are in UTC (Coordinated Universal Time), formerly known as GMT (Greenwich Mean Time). Watch out – if you are on British Summer Time, you have to add one hour to UTC. On September 1st high water is at 09:56 and 21:30, and low water is at 02:50 and 14:30. Now, if you want to know the actual depth of water in Portsmouth at 14:30, you just have to add the charted depth of Portsmouth and the low water. Low water at 14:30 is 0.4 meters and the charted depth is 6 meters. We add 0.4 meters and 6.0 meters, so the actual depth in the harbor of Portsmouth is 6.4 meters. In case the height of low water is minus, you have to subtract instead of add.