THE RAPID SURVEY OF AN ESSEX SALT-MARSH 297
than five hours, and 280 completed proformae were taken back
for analysis that same evening.
In a survey of this type, it is highly desirable that the data
should include a profile of the marsh, and the need for an accurate
levelling instrument has already been stressed. However, it is
also necessary to determine the position of the tidal levels and
thus to relate the distribution of the plants to the rise and fall
of the tide. Using the simple level available, determination of
the height of the marsh, relative to Chart Datum, by traversing
from the nearest bench-mark, would have introduced serious error
and would have been extremely time-consuming. It was there-
fore decided to refer the vertical height of the profile to the level
of high-water on that day, the time and predicted height of which
had been calculated from Admiralty Tide Tables.
There was insufficient wind to whip the waters of the estuary
into waves and thus it was a simple matter to mark the level
of the advancing tide by means of bamboo canes. These bore
a label indicating the time at which the canes were placed in
position and hence the time and place of high-water were accura-
tely known.
With the Artillery Director at the zero point, the relative
height of the corner of each 3-feet sample was recorded. The level
of the tide, as indicated by the highest cane, was also determined
and the vertical distance between high-water and the zero point
calculated. This value, when applied as a correction to the
original data, related the levelled profile to a tide of known height
and hence to Chart Datum.
After the levelling had been completed, the proformae were
carefully checked to ensure that none was missing and that no
gaps had been left in the sampling pattern. After clearing the
site of canes and strings, etc., the party returned to the laboratory
where the field records were assembled and partially collated.
II. In the laboratory
(a) "Frequency"
The proformae were separated into blocks of twenty and dis-
tributed amongst pairs of students together with printed sheets
ruled into twenty columns and listing the species as before. In
each column, which corresponded to a single proforma, the dis-
tance of the quadrat from the zero point and the number cf
squares occupied by each species were recorded. At this stage,
any obvious errors detected were discussed with the workers
whose names appeared on the appropriate proforma. This
simplification of the data also made it easy to see patterns and
trends.
(b) Density
Within a quadrat, the relationship between the number of
occupied squares (frequency) and the number of plants (density),
depends upon the contribution each plant makes to the total