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Furthermore Ackermann and Schneider (1992) indicate that the
final accuracy of the DTM must take into account both the
standard deviation of the observed height differences and the
standard deviation of the check heights. If 6 is the standard
deviation of the check point heights obtained by tacheometric
field survey, op, is the standard deviation of the check point
heights obtained by DTM interpolation, and o, the standard
deviation of the "observed" height differences, then:
Ou” = ODTM + Och? 5 (5.1)
and the final accuracy of the DTM is 16 cm. This level of
accuracy is very satisfactorily if one takes into account the very
abrupt elevation variations on the surface of the rocks, and the
height accuracy of the ground control points (5 cm).
6. ESTABLISHMENT OF GROUND CONTROL ON THE
OCEAN TIDAL TERRAIN USING KINEMATIC GPS
The geodetic survey provides high accuracies in the
establishment of the ground control points but it is time
consuming. Since the duration of a low tide and thus the
available time for performing the geodetic survey on a tidal area
is less than two hours it is obvious that several days maybe
needed for the establishment of the ground control points. A
method that is not as accurate but much faster is the relative
kinematic positioning using GPS (Global Positioning System)
observations (carrier phase measurements of GPS signals), and
it was decided to be investigated and used for the establishment
of the ground control points.
Nineteen ground control points were established close to the
three rocks. The specially designed artificial target was used for
the targeting of most of them except for three ground control
points on the surface of the rocks that were targeted with white
painted crosses.
The ground control points were selected to be uniformly
distributed, close to the rocks, and at different elevations. Five
of the control points were established on the sea-shore, three on
the top of the rocks, and the rest on the flat tidal sea-bed.
The coordinates of a ground control point were known from
geodetic survey. This point was the common point of two
baselines along the shoreline outside the tidal terrain. The two
baselines were established with the conventional static GPS
technique just before the kinematic survey. Data were collected
for two hours to resolve the carrier phase ambiguities. The
common point of the two baselines served as a base (reference)
point and data was collected at that station throughout the
kinematic survey. The kinematic survey was initialized by
occupying the known baselines for two minutes. Then the
"rover" was moved to the next ground control point and one
minute observation was taken. At the end of the survey the
starting (initializing) point was revisited for data closure and
one minute of observations were taken again (closure of the
loop). Two kinematic sessions (two closed loops) were
completed by two independent groups. Each group consisted of
two persons: one carrying the antenna and the other the "rover"
receiver. The kinematic survey at the tidal area was completed
in less than two hours. The GPS receiver that was used was an
Ashtech XII.
The collected GPS data was processed using the NADTRAN
software. The base point and the initialized points were
obtained from the process of the static observations and then
23
they were held fixed during the processing of the kinematic
observations.
The GPS survey provided latitudes and longitudes (¢, A) and
geometric heights (h) with respect to the GRS80 which is the
ellipsoid that is used by the GPS community.
Since the DTM was required to be in UTM coordinates, the
ellipsoidal latitude and longitude (@, A) of the ground control
points were converted to UTM Easting and Northing (E, N) and
the geometric heights to orthometric heights (information about
the geoidal height of the particular ground control point was
acquired by using the Canadian Geoid Version 2.0(a) software
written by the Geodetic Research Services Ltd. )
The resultant orthometric heights were compared with the
orthometric heights obtained by precise leveling of the same
ground control points. The mean difference was 5 cm and the
maximum difference 7 cm.
It was shown that the relative kinematic GPS survey is fast (the
survey of the nineteen ground control points lasted less than
two hours) and gives orthometric heights with a 5 cm accuracy,
planimetric positioning with a 2 cm accuracy, and has the
additional advantage that no visibility is required between the
base station and the rover. Therefore it is an appropriate method
to use for establishing control points on the tidal terrain.
7. CONCLUSIONS
It is concluded that the use of any kind of survey is difficult to
apply for tidal terrain mapping. Photogrammetry, even though
it encounters some difficulties, seems to be the only effective
method for the mapping of the tidal terrain since it provides an
enormous amount of data with a minimum of required time
spent on the site. In featureless tidal areas of difficult stereo
vision, photogrammetry should be complemented by ground
field survey. This paper highlighted the advantages, discussed
the problems of using photogrammetry for the mapping of the
ocean tidal terrain and proposed some solutions that will be
hopefully used as guidelines for future and more extensive
applications of photogrammetry in tidal areas and may save
some time, effort and money.
REFERENCES
Ackermann, F. and W. Schneider, 1992. Experience with
automatic DEM generation. In: International Archieves of
Photogrammetry and Remote Sensing, Washington D.C, U.S.A,
Vol. XXIX, Part B4, pp. 986-989.
El-Hakim, S.F. and W. Faig, 1981. A combined adjustment of
geodetic and photogrammetric observations. PE&RS, 47(1), pp.
93-99.
Forester, W.D., 1983. Canadian Tidal Manual. Department of
Fisheries and Oceans, Government of Canada, Ottawa.
Moniwa, H., 1977. Analytical photogrammetric system with
self calibration and its applications. Ph.D. dissertation,
Department of Surveying Engineering, University of New
Brunswick, Fredericton, N.B., Canada.
Petrie G. and T.J.M. Kennie (Eds.), 1990. Terrain Modelling in
Surveying and Civil Engineering. McGraw-Hill Inc.
Warner, W.S. and W.W. Carson, 1992. Consequences of
enlarging small-format imagery with a color copier. PE&RS,
58(3) , pp. 353-355.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
