Finding Time for Given Height
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Because Blindly Assuming You'll Clear The Bridge Is A Bad Way To Sail

So lets say that you know you'll be coming into port on a given day, but need to know what time to arrive so that you'll have enough water beneath your keel to clear an obstruction (a sandbar for example). What do you do? Like solving for the height at any time, this problem is all about interpolation, and it's fairly simple.
Step 1: Find the high and low tide that bracket (are on either side of) the time that you want for your subordinate station. (If you haven't already corrected the height from the reference station to your subordinate station, do that first.) For example, if you need the height to be 3.2 ft, and the tide tables list a high of 3.1 at 0200, a low of -0.5 at 0815, a high of 5.1 at 1414, and a low of -0.6 at 2022, select the low at 0815 and the high at 1414 and continue on with the problem.
Step 2: Set up an interpolation table like the one at right to make your calculations easier. This table should consist of 2 columns and 3 rows, with enough space between each to allow for some writing.
Step 3: In the first column in the top row, place the time of the high or low tide occuring before the desired time. In the 2nd column, place the height of that tide. Repeat this step, placing the time and height of the high or low after the desired time in the bottom row. In the 2nd row of the 2nd column, place the desired height.
Step 4: As per the directions in the interpolation table (table 3) of the tide table, find the difference (range) of heights between the high and low, difference in time between the high and low (duration of rise or fall), and the difference in height between the nearest tide (either high or low) and the desired height (listed as height correction in table). These should each be noted along the margins of your interpolation table just so you can keep track of them.
Step 5: Using the interpolation table (table 3) found within the tide tables and the values you found in step 4 above, work backward through the table until you find your "Time from nearest high or low water"
 -Using your known Height Correction and your Range of Tide, find the column where they meet and follow it upward into the next table
 -Move upward through the column until you come across the row that describes your Duration of Rise or Fall
 -Where these two values meet, that is your Time from nearest high or low water
Step 6: Apply that Time from nearest high or low water correction to the time of the nearest tide (whichever you used to find the difference in height). If the desired tide is after the nearest, add the correction to the time. If the desired tide is before the nearest tide, subtract the correction.
Step 7: That's it man, you're done. It might also be useful to know the amount of time that the water will be at that level so that you don't enter the channel safely only to be stuck 10 minutes later.

Time of high or low prior to desired time

Height of high or low prior to desired time

Desired time

Height at desired time

Time of high or low after desired time

Height of high or low after desired time

Finding Highs and Lows

Finding Height At Any Time

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