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Figure 2.1
Ocean tide as a sinusoidal wave at a fixed location over an interval of time
(Forester, 1983).
The ocean tides are classified in four categories, according to
the uniformity of spacing and the number of HW (or LW)
during a complete lunar day. For the areas of interest, the ocean
tides belong to the semidiurnal (SD) type. The SD tide shows
two almost equal HWs and LWs about uniformly spaced over
the period of a complete lunar day.
ZT AIN fi
\
wee + / \
= fll
Ü |
2000 |
1000 4
ZIIIIIEEETETTTITTTITITIT
8888888888 gygyysysysssgysys
Figure 3.1
Tidal curve for August 2, 1992 for Saint John N.B.
3. PHOTOGRAMMETRIC MAPPING OF TIDAL AREAS
AT LOW TIDE
The tidal areas introduce special difficulties when
photogrammetric mapping methods are used. These problems
can be caused either by the tidal terrain or by the tide itself.
3.1 Problems caused by the ocean tide
The objective of the stereo aerial photography part of HY-GRO
project was to photogrammetrically map as large a tidal area as
possible. That means that the photogrammetric flight mission
must occur during or near the lowest low tide. The same is
valid also for the placement and measurement of the ground
control points. The HY-GRO project was carried out during the
period between June and September, when the weather is the
optimum for ground, aerial and acoustic surveys. For each of
these months the days that show the lowest low tide were
selected by looking at the tidal prediction tables for the areas of
interest. In each month a window of three successive days has
adequate low tides for the aerial mission and the establishment
of the ground control. Being a SD type, the tides in the areas of
interest show two low and two high waters during a 24 hour
interval. From the two low tides the most convenient one has to
be selected. For example for the flight mission, the tide that
occurs when the illumination conditions are better is preferred.
In any case, the low tide that occurs at night is not considered.
The tidal curve for the 2nd of August 1992 for Saint John area
is illustrated in Figure 3.1. It is clear that the available time for
performing the aerial and ground surveys is not only limited to
19
a window of three days monthly but for those specific days,
usually only one of the two daily low tides can be used and
only for a period of approximately two hours. That is very
critical because the aerial and ground surveys cannot be
performed in bad weather and on cloudy or foggy days. These
time restrictions add dramatically to the cost and effort of
carrying out the project.
3.2 Problems caused due to the special type of the tidal
terrain
a) Lack of well defined and stable features that can be used
as ground control points. Since there are no well defined and
stable features that can be used for ground control in the seabed
of the tidal area, permanent and stable artificial
photogrammetric targets have to be placed. This provides
great difficulty because the tidal forces pass twice a day over
the targets. Therefore the targets must be very stable to resist
the tidal water forces. A problem in designing artificial targets
is the contrast that has to be created with their background.
The variety and the dynamic nature of the radiometric
characteristics of the tidal terrain make it difficult and
uncertain to predict the contrast between target and background
in black and white photographs even when the targets reflect
adequately. But a highly reflecting target almost always creates
good contrast with the tidal terrain in colour photographs.
These problems were overcame in the current project by targets
(see Figure 3.2) that consist of a 1/2" (1.3 cm) plywood with
white glossy arborite (formica) glued to their face side, which is
attached to 1 m long, 3/4 " (1.9 cm) re-enforcement bar drilled
and tapped to accept a 1" (2.5 cm) long, 1/4" (0.6 cm) bolt. The
re-enforcement bar is anchored to the ground until the target
becomes unmovable. The designed targets have the following
characteristics :
* Stability. In order to test the stability of the targets, a number
of targeted control points were re-measured nearly 20 days after
their placement and initial measurement. It was concluded that
the targets showed virtually no movement from their original
positions. It is suggested that the time between the placement of
the targets and the acquisition of the aerial photographs must
not be greater than 20 days since the arborite starts to separate
from the plywood, and the plywood to tilt from its horizontal
position. The condition of the targets must be inspected at least
a day before the flight.
* Contrast. The white glossy arborite on the top surface of the
target reflects greatly and creates a very good contrast with the
tidal terrain when colour film is used. The identification of the
targets in the photographs was accomplished very easily and
without mistakes. These designed targets were not only used in
the seabed but also in places like the sides of roads, inside
bushes and on bare ground. It was found that if the flight was
carried out when the sun is directly above the area then all the
targets were very visible and distinct in the photographs.
Problems may arise in the case of very poor illumination and
low solar angle like the one that occurs on a late afternoon
flight. In such conditions targets maybe disappear mainly due
to the long shadows that are created by the low sun angle. In
order for the targets to retain their high reflectance, they have
to be cleaned shortly before the photographic flight mission.
* Shape and size. The shape of a target has to be symmetric
to its center to more accurately place the measuring mark of the
photogrammetric instrument and thus to increase the accuracy
of the photogrammetric observation of the target. Additionally a
distinctly symmetrical shape helps in the identification of the
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
