1  Information from the
chart
Most often the chart
presents succinct tide tables for certain positions. These positions
are marked with the 'square'. The table below shows us an example for two
different positions. The first refers to Cowes (UK), the second to a
position south of Cowes.
Position 
Heights above LAT 
Mean HW 
Mean LW 
Spring 
Neap 
Spring 
Neap 
Cowes 
1.7 m 
1.5 m 
0.2 m 
0.4 m 

5.2 m 
4.3 m 
0.4 m 
1.2 m 
This data only provides us
with average high and low waters heights. Moreover, it is merely valid at
spring or neap tides. To use it we need to first find out how many hours
we are from high water. Secondly, we need to know if it is spring or neap
or sometime in between at that particular moment. We shall use this table
to solve two types of problems. Finding height of tide at a particular
location at a particular time:

To get over a shoal.

To pass under a bridge.
Almanacs and many other
nautical publications contain predictions of the times of high and low
tides at many major standard ports. Also listed are differences in times
of tides from these ports for additional secondary ports. To work with
this succinct data we need two extra tools:

To
interpolate between high and low water heights we use the
Rule of Twelve. We assume the tidal curve to
be a perfect sinusoid with a period of 12 hours. The height changes over
the full range in the six hours between HW and LW.

During first hour
after HW the water drops
1/12^{th}
of the full range.

During the second hour
an additional
2/12^{th}.

During the third hour
an additional
3/12^{th}.

During the fourth hour
an additional
3/12^{th}.

During the fifth hour
an additional
2/12^{th}.

During the sixth hour
an additional
1/12^{th}.
Hence, two hours after
the HW the water has fallen 3/12 of the full range.

To interpolate between spring and neap
tides we use the
Rule of Seven. Since the change from spring
range to neap range can be assumed linear (instead of sinusoid), each
day the range changes with 1/7th of difference between the spring and
neap ranges.
Hence, the daily change in range is (spring range  neap range)/7.
Shoal problem:
Our shoal near Cowes has a charted depth of 1 meter and we would like to
cross it at about 15:00 hours with our yacht (draft 1,5 m).
From any nautical almanac
we find that HW occurs at 03:18 15:53
and LW occurs at 09:45 22:03 at a standard port nearby. We also
find that at our location HW occurs one hour later and that spring tide is
due in two days. Hence, we have a HW around 17:00.

Via the rule of seven we
find out that today the range is:
spring range  2 x ( (spring range  neap range)/7 )
<=> 4,8  2 x ( ( 4,8  3,1)/7 ) <=> 4,8  2 x 0,25 = 4,3 m.

We also need today's
HW height:
which is Spring HW  2 days
x ( (5,2 4,3)/7 ) = 5,0 m.

Via the rule of twelve
we find out that at two hours before high water the height is:
5,0  3/12 x 4,3 = height at 15:00 hours = 3,9 m.
So, after three
interpolations we derive the water height at 1500 hours. Considering the
charted depth leads to an observed depth of 4,9 meters, enough for our
draft of 1,5 meters.
Bridge problem:
An overhanging rock, power lines or bridges have their clearances charted
with respect to another chart datum than LAT. Normally, 'high water' or 'MHW
spring' are used as reference planes.
An example:
Above our shoal hangs the 'Cowes bridge'. At 15:00 hours we would like to
pass this bridge, which has a charted height of 20 meters to HW. Our mast
is 23 meters high. In the example above we found that the water height was
1,1 meters below HW level at that time. Obviously, we will have to wait!
So, at what time will we be able to pass under this bridge?
The water height must be 3 meters lower than HW level (5,0 m). That is
almost 9/12 of the range (4,3 m) indicating four hours after HW.
Conclusion, we will have to wait at least six hours in total.
2  Information from tide tables
Instead of mere averages,
a tide table provides us each day with the times of high and low water for
a particular place. Basically, it is same table like the one we found in
the chart, but is extended for every day in a year. By using this method
we get more accurate water heights since it involves less interpolation.
The example shows us a part of a very detailed tide table, which even
includes heights for every hour.
3  Information from
tidal curves
In most tables the tides
can also be characterized by a tidal curve. This method substitutes the
rule of twelve providing more accurate heights. The left side contains the
water height information with the lowest heights to the left where also
the chart datum is indicated. The low water height will be marked at the
bottom and the high water height will be marked at the top.
The area under the curve will be marked with the time information.
To find the water height
at a specific time we need to know first how many hours before or after
the HW this is. Then
Often this is done when the curve is not sinusoid and the rule of twelve
is rendered useless.
Overview

Tide: The vertical rise and fall of the
surface of a body of water caused primarily by the differences in
gravitational attraction of the moon, and to a lesser extent the sun,
upon different parts of the earth when the positions of the moon and sun
change with respect to the earth.

Spring Tide: The tidal effect of the sun
and the moon acting in concert twice a month, when the sun, earth and
moon are all in a straight line (full moon or new moon). The range of
tide is larger than average.

Neap Tide: This opposite effect occurs
when the moon is at right angles to the earthsun line (first or last
quarter). The range of tide is smaller than average.

Range: The vertical difference between
the high and low tide water levels during one tidal cycle.

Tidal Day: 24 hours and 50 minutes. The
moon orbits the earth once earth month, and the earth rotates (in the
same direction as the moon's orbit) on its axis once every 24 hours.

Tidal Cycle: A successive high and low
tide.

Semidiurnal Tide: The most common tidal
pattern, featuring two highs and two lows each day, with minimal
variation in the height of successive high or low waters.

Diurnal Tide: Only a single high and a
single low during each tidal day; successive high and low waters do not
vary by a great deal. Gulf of Mexico, Java Sea and in the Tonkin Gulf.

Mixed Tide: Characterized by wide
variation in heights of successive high and low waters, and by longer
tide cycles than those of the semidiurnal cycle. U.S. Pacific coast and
many Pacific islands.

Chart Datum or
Tidal reference planes: These fictitious planes are used as the
sounding datum for the tidal heights.

Drying Height: Clearance in meters (or
feet in old charts) above the chart datum.

Charted Depth: Clearance in meters (or
feet in old charts) below the chart datum.

Observed Depth: Height of tide + charted
depth: the actual depth in meters.

Height of light: The height of light
above the bottom of its structure.

Elevation: The height of the light above
the chart datum.

Rule of Twelve: Assuming a tidal curve
to be a perfect sinusoid with a period of 12 hours. The height changes
over the full range in the six hours between HW and LW with the
following fractions during each respective hour: 1/12 2/12 3/12 3/12
2/12 1/12.

Rule of Seven: The change from spring
range to neap range can be assumed linear, each day the range changes
with 1/7th of difference between the spring and neap ranges. Hence, the
daily change in range = (spring range  neap range)/7.

