1. Reference stations, found in Tide Table 1, have the times and heights of the high and low tides, listed by date.
2. Subordinate Stations, found in Tide Table 2, show the difference in times and heights for that station, based on the times and heights of the Reference Station.
3. To determine the tide level at times between high and low tide, you use the "Rule of Twelfths" Follow the AP form to apply this calculation. The form is found at:
The Reference Station is Boston. The Subordinate Station is Rainsford Isand. Boston is in Tide Table 1 - see page 68. Rainsford Isand is in Table 2, also on page 68.
First, look at the tides for Boston on 4/19/2004
Time Ht
0456 -0.1
1111 10.0
1710 0.5
2322 10.4
Next, look at the Subordinate Table for Rainsford Island
High tide is 0 hr and 00 min after Boston, (i.e., the same time). Its height is 0.95 times the height at Boston.
Low tide is 0 hr and 02 min after Boston, (i.e., 2 minutes after Boston low tide). Its height is also 0.95 times the height at Boston.
Finally, determine the height at Rainsford Island for the time in question, 1400 hrs.
1400 is nearly 3 hrs later than high tide. From the AP form, based on the rule of twelfths, the tide drops 50% of the range to the next low.
This results in the tide at 1400 being 5.0 above MLLW
