SEA WAVE SURFACE MAPPING BY CLOSE RANGE

PHOTOGRAMMETRY

Efstratios Papadopoulos, Ph.D candidate Evaggelos Papapanagiotou, Ph.D candidate

Prof. John N. Hatzopoulos, director of RSLUA UNIVERSITY OF THE AEGEAN

DEPARTMENT OF ENVIRONMENTAL STUDIES REMOTE SENSING LABORATORY (RSLUA)

17 Karadoni Street, Mytilene, 81100, GREECE Tel.: +30 (251) 21-286, 42-849

FAX: +30 (251) 23-783, E-mail: ihatz@env.aegean.gr

ABSTRACT

This work is a part of a research project which deals with the determination

of parameters for better harbor design. The use of a close range Photogrammetry system composed of two synchronized metric cameras is described. Methods involving stereo, analytical, and digital Photogrammetry with image correlation are presented. Preliminary results from a test site are presented and analyzed.

INTRODUCTION

The necessity of studying the wave behavior in the coastal and sea

environment is important to the design, construction and management of harbors and other coastal works, as well as, to the transportation through the sea, the sea tourism, the navigation and the management of sea systems for the development of coastal areas.

There are noticeable difficulties even with the progress in fluid mechanics to simulate the sea surface environment due to the irregularities accounted on the wind generated sea waves. It is therefore difficult to establish simple quantitative relations between the reason of wave generation (wind) and the result (wave). The complexity and the irregularity of the sea wave form is mostly attributed to the turbulent field structure of the above the wave wind flow.

Generally it is assumed that at a given sea environment, there is an evolution of different sea states where each one of them is characterized from the following quantities: wave height, wave period, wave direction, where each sea state can be modeled as a stochastic process. The fundamental characteristic of such a stochastic process is the density - possibility function: f(Height, period, direction of sea waves per location and per time of the year) this fact makes it

In: Proceedings of the ACSM 57th Annual Convention and ASPRS 63rd Annual Convention, 7-10 April 1997, Seattle Washington, Vol. 3, pp. 485-489

necessary to have accurate data in order to approximate this function (Athanassoulis et al 1988).

The acquisition of accurate wave data at specific sea locations requires the employment of a network of wave recorders and buoys. The simplest wave recorders are limited in measuring just the wave height while the more sophisticated ones are allowed to measure the wave height and two wave slope parameters. The use of wave recorders creates obvious limitations and difficulties and requires significant costs. One way to acquire full range of wave surface information is to record and analyze different sea states by Photogrammetry. Photogrammetric methods (Gracian et al 1992) are perhaps the only ones capable to record and provide high precision maps of the detail elements of the sea surface elevation, similar photogrammetric methods on dynamic tests have been reported by (Veress S.A. & Hatzopoulos J.N 1979).

STEREOSCOPIC MAPPING OF THE SEA SURFACE

The objective of this research effort is to develop the methodology of

measuring the following wave characteristics: wave height, wave length, wave frequency, wave direction, wave slope parameters at spatial scale which will be defined from the particular requirements of the specific application. The method will be based on both stereo viewing and image stereo - correlation.

One of the basic questions raised on the developed methods is the definition of optimum resolution and the accuracy obtained which is investigated throughout this research. It must be noted that the sea surface is composed of features which on the one hand provide ideal stereovision, on the other hand create difficult problems. Specifically the continuity of the sea surface allows the application of all kinds of surface smoothing technics without related problems to discontinuity and at the same time the broken wave provides god quality of optical contrast. On the other hand the continuity of the wave surface creates difficult problems to the measurements of relatively small height wave forms (Jahne 1993).

It is evident that the problem of resolution of the stereoimages is directly related to the geometric features of the stereo image acquisition system, which in fact dictates a significant part of the research to be directed towards the analysis of such geometry in order to optimize the system configuration in every desirable scale. The resolution problem is investigated by using simulated data at the RSLUA.

The stereoimaging system is composed of two 70mm metric cameras which can slide on a vertical platform on up to 6.00 meters above the ground. The cameras are synchronized to provide practically simultaneous exposures.

The equipment used are as follows: 1. Two 70mm photographic metric cameras ROLLEIFLEX 6006 with

11x11 reseau plate, 80 mm planar lens, synchronized for simultaneous exposures with infrared remote control ROLLEI RC 031R, capable to exposure about 60 synchronized exposures in one minute.

2. Geodetic Total Station TOPCON GTS-3B 3. Analytical plotter TOPCON PA-2000 with CIVILCAD 5.0 software. 4. In house developed software for stereo image simulation. 5. In house developed software for using the PA-2000 as a comparator 6. In house developed software for analytical close-range Photogrammetry

with bundle adjustment and self calibration (Hatzopoulos 1985). 7. In house under development software in image correlation for digital

Photogrammetry.

EXPERIMENTAL PLANNING

The experiments to be made are located near to three under construction ports in Northern Greece at the cities: Kavala, Lagos and Alexandroupolis. The research project is financed by the Greek Secretariat of Research and Technology and the European Union. There are three more participating institutions: Democritous University of Thrace (coordinator), Aristotle University of Thessaloniki, Technical University of Athens, and the Marine Technology Development Company (EANT). The photogrammetric results are going to be used on calibrating wave recorders and on providing wave surface data which are not provided by other means.

The experiments are designed to cover various scales of sea surface starting from a 10 x 10 meter sea area to 150 x 150 meter sea area. Such scales are particularly interesting for optimum harbor design.

The first experiment was performed in the city of Mytilene to test the system and the results are shown in Fig. 1 and Fig. 2. The next experiment is planned to take place in the city of Kavala and the results will be compared to several simple wave recorders and to a highly sophisticated directional wave recorder which has been manufactured by EANT and is called MEDUSA.

Fig. 1. Perspective of sea wave surface mapping in the area of Mytilene.

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Fig. 2. Contours of sea wave surface mapping in the area of Mytilene.

REFERENCES 1. Athanassoulis G. A., Vranas P. B., Soukisian T. H., 1988, A new model

for long term stochastic analysis and prediction. Technical Report, National Technical University of Athens, Dept. of Naval Arc. and Marine Eng.

2. Gracian A. P.m Muller J. P., Challenor P. G., & Spokosz M. A., 1992, Automated 3-d Spatial Measurements of Short Gravity Capillary Waves, Proceedings of the ISPRS XVIIth Congress, Vol. XXIX, Washington DC.

3. Hatzopoulos J. N., 1985, An Analytical System for Close - Range Photogrammetry, PE&RS, Vol. 51, No. 10, pp. 1583 - 1588.

4. Jahne B., 1993, Spatio - Temporal Image Processing, Springer - Verlag. 5. 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.