PHOTOGRAMMETRIC SEA WAVE MAPPING FOR HARBOR DESIGN

Prof. John N. Hatzopoulos, director of RSLUA Evaggelos Papapanagiotou, Ph.D candidate

UNIVERSITY OF THE AEGEAN DEPARTMENT OF ENVIRONMENTAL STUDIES

REMOTE SENSING LABORATORY (RSLUA) 17 Karadoni Street, Mytilene, 81100, GREECE

E-mail: ihatz@env.aegean.gr

ABSTRACT This work presents intermediate results of a research project, which deals with the determination of sea wave surface three-dimensional data for better harbor design. The problems rising from the use of a close range Photogrammetry system composed of two synchronized metric cameras are discussed and analyzed. Results involving stereo images from a test site are presented and analyzed. Further results from ongoing research involving image correlation are presented.

INTRODUCTION

Sea wave measurements are highly important for harbor design (Papadopoulos et al 1997) and the

task for such measurements usually involves wave recorders and buoys. Such devices usually are capable to measure and record accurately the height of the wave at a specific time and some times the direction of its course. Close range photogrammetry and Photogrammetric methods are capable to provide three dimensional data for the entire surface of the sea wave and for this reason it was chosen to provide such data in a project related to harbor design in Greece.

The experiments were originally tried at the appropriate harbor site in Northern Greece but due to unexpected difficulties (weather conditions, sea surface state etc.) it was not possible to obtain proper photography. By the mean time the evolution of Softcopy photogrammetry together with the improvement of digital cameras created new conditions and brought new ideas to the research team to approach the photogrammetric measurements. It was then decided to work on the project using conventional means of stereo photogrammetry and at the same time do research and investigate the use of digital images and automatic three dimensional surface data extraction using image correlation. The experimentation site was chosen to be the harbor of the City of Mytilene, which provides easy access to the research team to obtain proper photography and then experiments could continue to the harbors of Northern Greece. In this report the results of the experiment in the harbor of the city of Mytilene are presented.

DATA ACQUISITION

A set of photography was obtained on 18 of March 1997, in the harbor of Mytilene of the island of

Lesvos near the lighting house of the right wave breaker. There were used two identical metric cameras Rolleiflex 6000 with a Planar 2.8/80 mm metric lens (S.A. Veress and J.N. Hatzopoulos, 1979). Both cameras were synchronized and connected to an electronic device, which was adjusted to make simultaneous exposures every second. The cameras were loaded with color reversal film and were mounted on an aluminum bar of three-meter length and at about 10-meter distance over the sea surface. Two targeted points were placed 6.83 meters apart along the wave breaker to serve as control points and to establish a reference system for the measurements. The two cameras were located in a stereo position facing North providing stereo coverage to a sea water area approximately 30 m by 15 m. The firing of the cameras started at local time of 17:15 and 12 simultaneous exposures were obtained. During the exposures there was blowing a strong wind and the sea surface was rough enough to create adequate contrast and distinct recognizable patterns of the sea waves on both photographs. The general layout of the acquisition system is shown in Fig. 1.

FILM PROCESSING

The development of the film revealed that the first photograph had some problems and was dropped

from the set, for this reason the numbering of the photographs starts from the number 2.

In: Proceedings of ASPRS Annual Conference 2000, Washington DC, USA, CD-ROM, 22-26 May 2000, ISBN 1-57083-061-4

Figure 1. General layout of the photogrammetric acquisition system. A portion of a stereo pair of images is shown in Figure 2.

DIGITIZATION OF THE SEA SURFACE The digitization as stated earlier was performed using conventional means of stereo

photogrammetry and for that reason it took a quite long time and man power to perform. Supplementary analytical procedures were also used with in house developed tools (Hatzopoulos J. N., 1985). Stereo models were oriented in a PA-2000 Topcon analytical stereo plotter and for each stereopair of images all the characteristic points of the sea wave surface (peaks, valleys, wave patterns etc.) were digitized. In addition some extra points were taken from the wave breaker in land to establish a reference for viewing the dynamic sea wave surface. Such extra points were along the edges of the wave breaker, which had a zero elevation at the bottom and 2.05 m at the top (Fig. 1). The orientation process of the stereo images was performed with some difficulties due to the high tilt angle and the

relatively small base to height ratio, in addition the waves usually create many similar patterns which are some times hard to distinguish during orientation and compilation process.

Stereo pair

ID Total of Land

Points Total of Sea

Surface Points

2 61 1054 3 63 838 4 67 1046 5 57 982 6 87 312 7 65 878 8 65 963 9 62 876

10 58 1003 11 62 947 12 57 441

Table 1. Total number of digitized points per category and per stereopair

DATA PROCESSING The digital three dimensional coordinates of the points in Table 1, were saved by the CivilCad

software in an ASCII file. More processing was performed using in house developed software to prepare the data in order to be imported into the Arc/Info System through the Generate command. The Arc/Info system helped to create a Grid type of the sea wave surface with grid cell dimension of 20 cm. Further processing helped to transform the Arc/Info data into WinSurf 5.0 format for easier manipulation of the sea wave surface data and create desirable display forms represented by contour lines and perspective drawings. Figures 3, 4, and 5 show the contour lines of the same sea surface area, whose dynamic wave form changes are recorded every second by the 11 stereo pairs of photographs. Figure 6 illustrates in perspective view the sea surface as mapped from the photographs of stereo pair ID 4.

IMAGE CORRELATION

The problem with conventional photogrammetry is that it is too expensive in terms of man-hours

needed to map each stereo model. The data needed for harbor design must cover a quite large time period so that to provide good estimates of the sea surface topography. For this reason the research team started working on image correlation (Papapanagiotou E., 2000). The method developed is based on polynomials which practically model all the parameters of the photogrammetric system such as interior and exterior orientation but require a relative large number of control points. This image correlation method has been successfully tested to work with aerial photography and stereo SPOT/P images (Papapanagiotou E., 2000) and now it is tested with sea wave mapping. The preparation for testing on sea surface started with the construction of a control field frame as shown in Fig. 7. The retroreflective targets located in the control frame were measured with an accuracy of a fraction of a millimeter using the Australis Close Range software package. A platform was also manufactured to hold the control frame together with the two cameras in stereo position. At this time we are waiting to purchase two digital cameras synchronized to provide simultaneous images by a proper frame grabber. The exposure frequency is estimated at about three frames per second. Parallel to this it is also worked out a system to project an irregular grid pattern on the sea surface using proper beam of laser technology in order to make measurable images of any kind of wave form on the sea surface.

CONCLUSIONS The sea wave mapping process has many and difficult to solve problems as follows:

(a) Sea surface creates uniform patterns, which are hard to distinguish and perform stereo

matching, particularly with low and/or smooth waveform. This problem can be be tackled with irregular pattern projected using laser technology.

(b) The large amount of data required for harbor design increases drastically the costs when using conventional photogrammetry. Soft copy photogrammetry with image correlation and using digital cameras can solve this problem.

(c) Adverse weather conditions by the seaside (strong winds, rain, salty environment) create additional problems related to the system design which must take provision to protect the cameras and other sensitive devices.

The combination of photogrammetry with other type of wave recorders can provide a better solution and this is the direction for more research. In this project there were many attempts to combine conventional photogrammetry with wave recorders but did not succeed to obtain proper photography for stereo viewing due to the problems mentioned above. A sophisticated wave recorder such as the Medusa system∗ can be also equipped with a stereo photogrammetric system with digital cameras and image correlation software together with laser technology to obtain continuous flow of sea surface three dimensional data in near real time.

Figure 7. The frame with the control field

REFERENCES 1. Hatzopoulos J. N., 1985, An Analytical System for Close - Range Photogrammetry, PE&RS,

Vol. 51, No. 10, pp. 1583 - 1588. 2. Papadopoulos E., E. Papapanagiotou and J. Hatzopoulos, 1997, Sea Wave Surface mapping by

Close Range Photogrammetry, Proceedings of the ASPRS Annual Convention, Seattle WA. 3. Papapanagiotou E., 2000, Automated Correlation of Digital Stereo Pair of Images for Three

Dimensional Coordinate Extraction Using a Polynomial Geometric Model, Dissertation first draft University of the Aegean.

4. S.A. Veress and J.N. Hatzopoulos, 1979, «Photogrammetric Monitoring of Moro 1200-KV STR/Mech. Test Line", Final Technical Report for Bonneville Power Administration, under contract No. DE-AC79-79BP11090.

∗ Medusa is a floating system manufactured by the Greek Marine Technology Development Company S. A. (EANT), capable to perform precise measurements on the sea waves together with associated meteorological parameters. Medusa can be moved and anchored at a any desirable sea location, it generates energy through solar panels, and it has on board: sensors, computer, and data transmission system through radio frequencies.