Presizyonlu hidrografik ölçmelerde bat-çık etkisinin GPS yöntemi ile belirlenmesi

Abstract

21. yüzyıla girmemize bir kaç yıl kaldığı günümüzde, dünyanın nüfusu hızla artmaktadır. Bu artışa paralel olarak yalnızca karalardaki kaynaklardan yararlanma düşüncesi yeterli olmamaktadır. Günümüzde sadece deniz kaynaklan ile ekonomilerinde büyük gelişmeler kaydetmiş bir çok ülke vardır. Denizde bulunan kaynakların ne denli önemli rezervler olduğuna hiç şüphe yoktur. Örneğin okyanus sularının altındaki buz kristallerinin içinde hapsolmuş doğal gazın, dünyanın enerji ihtiyacını onlarca yıl karşılayacak kapasitede olduğu bilinmektedir. Bunların dışında taşımacılık alanında da denizlerin ekonomik oluşu, konforu ve hızı ile önemli bir alternatifdir. Hızla gelişmekte olan ülkemiz, büyük bir kısmı denizlerle çevrili olmasına rağmen denizlerden yeterince yararlanamamaktadır. Bu kaynaklardan verimli şekilde yararlanmanın ilk adımı söz konusu su ortamlarının güvenilir ve doğru hidrografik haritalarının yapılmasıdır. Bu çalışmada özellikle büyük ölçekli ve kritik bölgelerde yapılacak hidrografik haritaların konum ve derinlik bilgilerinin modern yöntemlerle yüksek doğruluklarla belirlenmesi amaçlanmıştır, özellikle dalgalı ortamlarda ölçme taşıtırım düşey yöndeki hareketi sonucu ortaya çıkan ve bat-çık (heave) etkisi olarak adlandırılan bozucu etkinin, ölçülen derinlik değerlerinden uzaklaştırılması için GPS ölçülerine dayalı bir yöntem önerilmişitr. Ayrıca klasik bir mareograf kurulmaksızın, GPS ve klasik ölçmeler birleştirilerek ölçme yapılan zaman aralığı için ortalama deniz seviyesinin belirlenmesi araştırılmıştır. Yapılan çalışmanın sonucunda, ölçme taşıtının bat-çık etkisi sonucu derinliklerde 20 cm'ye ulaşan büyüklüklerde hataların olduğu görülmüş ve belli bir sınır değerin üzerindeki bat-çık değerine sahip ölçüler düzeltilmiştir. Ayrıca klasik mareograflara alternatif olarak önerilen yöntem klasik mareograf ile karşılaşunldığmda, aralarında 4- S cm civarında bir farklılık olduğu görülmüştür. Dolayısıyla bu yöntem klasik mareografa ciddi bir alternatiftir. Bu yöntem ile belirlenen ortalama deniz seviyesinin kotunun doğruluğu ± (5-10) cm civarında bulunmuştur.

Today, while the population of the world is increasing, it is not sufficient to expect to use only terrestrial resources. Sea resources, along with the nutrition, oil and other gas reserves, are as rich as the terrestrial resources. For example, it is known that the natural gas captured in the ice bergs found in the ocean have a capacity to supply the world's energy needs for a couple of decades. Water resources of the world compared to terrestrial resources are offering economic solution for all types of transportation along with having the comfort and speed. The first step to be able to benefit from the indicated water resources, is to create the hydrographic maps in an accurate and safe manner. The maps that are mentioned above can be obtained by the hydrographic surveys including positioning by using the sounding method. Hydrographic surveys are similar to the classical surveying methods in many aspects. The most important difference between these two indicated surveying methods, are not being able to physically see the surveying sites unlike the classical surveying methods and the continuous motion of the water surface. The factors that effect the water surface and causes its change are as follows; a-) Meteorological Effects b-) Oceanographic Changes c-) Vertical Earth Crust Movement d-) Astronomic Tides The main reason of the changes on the water surface is meteorological, tides are not as significantly effective in Turkey. For example, the change of the tides in the Bosphorus ranges between 2.1 to 3.8 cm.. Positioning Surveys In a hydrographic surveying; positioning process is done for establishing control network, determining coastline and die position of the points on the horizontal plane which are sounded from the water surface. The most important factor that reduces the accuracy of the water surface measurements is the continuous movement of hydrographic surveying boat. Positioning methods, used for hydrographic surveying, can be grouped in to 6 categories; J-) Optical Methods, 2-) Electronical Methods, 3-) Acoustic Methods, 4-) Satellite Methods, S-) Inertial Positioning Methods, 6-) Very Long Base Line Interferometer (VLBI), Nowadays, optical methods is commonly used especially along the shoreline in practical surveying. However, improvements in GPS have shifted positioning surveying to satellite methods. As a result of the advantages of the GPS method such as being independent of weather conditions, having no requirement of the surveying points seeing each other, and being able to manage surveying during the night, it is being densely used for both terrestrial and hydrographic surveys. As a result of simultaneous surveys using at least two GPS receivers ± 2 cm vertical, and ±3 cm horizontal accuracy can be provided. It is now possible to obtain the position of the mobile objects with in accuracy below I dm using the advanced kinematic On-The-Fly (KOTF) method. Besides having such a high accuracy, the most important advantage that it provides is to define the initial integer ambiguity of the vehicle in motion. To define the initial integer ambiguity is becoming especially difficult in static survey in the water environment. Real-time positioning can be obtained during measurements by adding a modem to the system. It is possible to obtain real-time surveys that are 7- 10 km distant from the stationary (static) receiver using a classical modem. Recently, GLONASS, which was launched in 1993 by the Russian Federation, and GPS satellite systems, make it possible to provide combined signals from the simultaneous use of 24 GLONASS and 24 GPS satellites. Positioning is not only limited to the points that are obtained by depth measurements, but also can be used for shoreline measurements. As well as being able to benefit from the optical and GPS methods, it is possible to take advantages of using images for wide areas obtained from Remote Sensing Method. These images, for now, are limited to provide an accuracy of 1:25000 scale. The ease of wide area mapping for shoreline can be obviously seen by taking into consideration the images that covers 184 kmxl86 km area which are obtained from LANDS AT TM.. Depth Measuring (Sounding) Sounding is the method of measuring with any technique vertical length of a line that intersects perpendicularly beginning from the water surface level and ending at the seabed relief. Depth measuring methods can be classified into two groups as classical and modern. The classical method is sub-classified into three; latte sounding, robe sounding and mechanical. The precessions of these three methods respectively are ±(5-10) cm/6m, ±10cm/30mand ±0.01*H. xm On the other hand, the modern method is divided into 4 classes: 1-) Laser Sounding 2-) Remote Sensing Sounding 3-) Photogrammetric Sounding 4-) Acoustic Sounding The most common is the "Acoustic Sounding". The basic theory behind this method is to measure accurately the travel time of sound impulses through the water and by using the velocity equality to calculate the depth. 2 Where /; is travel time of transmitting and receiving of sound impulses, c; sound velocity in the water and H' ; is depth. The velocity (c) is a variable, which ranges between 1387 m/s and 1529 m/s with the concentration of salt, temperature, and depth of the water. The precession of depth using the "Acoustic Method" can be calculated by ±0.5 m ± %0.9* H'. The acoustic sounding is the most precise depth measuring method. Thus, there are different types of measurement errors as in all measurements. The causes of the errors arise from misconductive surveys, surveying equipment, and the water environment. The measured depth values are to be calibrated by minimizing or canceling the last two indicated error sources. Hd = H' + a + dHy +dHe + dHa + dHH +dHPR Where, Hj ; the calibrated depth value, H' ; measured depth value, a ; transducer depth, dHv, velocity correction, dHe ; slope correction, dHa ; calibration correction of equipment, dHfj; heave correction (the error caused by vertical movement of the surveying boat due to the waves), dHpR, pitch and roll correction, i-) Transducer depth correction (draft): The transducer is released into the water until a specified depth during measurements. The measured depth is the distance between the transducer and the seabed relief. To be able to calculate the whole depth, the distance between the water surface and the transducer should be added to the measured depth. xiv ii-) Velocity correction: The velocity of the sound in the water shows a variation with physical parameters of the surveying environment and the depth. If the theoretical and the experimental sound velocity values are different then a velocity correction should be added to the depth value. The mathematical equation is as follows; V -V dHv=H' °\ ° o Hi-) Slope correction: The sound impulses that are sent and bounces back from the seabed relief into the echo sounder follows the shortest path. The impulses that are sent in a conical shape have a negative effect on measurements especially when the seabed relief is rough. The slope correction can be defined as follows; dHe = H' Sw^- (7awy5e - 7awT*-) 2 4 Where ae ; beam width, fie ; slope of the seabed relief. iv-) Calibration of the equipment: v-) Heave effect: One of the important error sources of the hydrographic depth measurement is the error caused by vertical movement of the surveying boat, called heave effects, due to the waves. The heave effect causes a significant error in depth measurement especially in wavy water conditions. Heave effects can be canceled from measurements in two ways. These are; a-) Data Filtering b-) Measurement of the vertical movement of the hydrographic vessel The second alternative is more suitable. The error intensity can be obtained by using either heave compensator or conducting measurements from a stationary point on shore to the surveying boat. vi-) Pitch and Roll effects: Using a heave sensor can eliminate these errors. However, the error effects can be minimized by using the following ways. The first way is to place the depth measurement equipment as close as possible to the center of gravity of the vessel. The other is to provide the vertical position of the transducer of the echo sounder with an apparatus. In the application of this study, it has been especially emphasized to use a specified method to eliminate the heave effects without using a compensator. xv . Application There are two purposes of the study. These are; i-)Heave, pitch and roll effects in depth measurements in wavy environments are undesirable. Especially heave movement, having a large effect, can be eliminated by a compensator. The first aim of this study is to combine both classical and GPS measurements in order to cancel the heave effects from the measurement and as a result of this to apply a method and algorithm to increase the depth measurements precision. ii-)The second is, to define mean sea level by GPS measurement without using classical mareograph stations, which is necessary to construct in all hydrographic measurements. To conduct these stated two goals, measurements have been done at two different times at both the European side of the Bosphorus and the Golden Horn located at the south mouth of the Bosphorus. For this purpose, Wild-Leica System 300 GPS receivers and Atlas-Deso IS acoustic depth measurement systems have been employed. The position and the depth data from both receivers have been obtained simultaneously and input into laptop located at the measurement's vehicle. The PROFIMAP (Atlas Ltd.) software was used for this process. Mareograph Instantenaus Sea Level Hm hM Da Ha Ha d d' I ! i i.1. ^ *r Seabed Geold WGS-84 Figurel. Elimination of the Heave Effects Using GPS and Classical Survey Methods As a result of the depth measurement, shown in Figurel, elevation of point A can be calculated as; HA*=HM-d Where H^f, is the mean sea level elevation obtained from mareograph reading, d; measured depth. xvi However, the real elevation of the point A can be found by replacing d with d' in the following equation; Sd = d-d' = HA-HM=hA-hM The value of Sd found from the above equation can be neglected for depth measurements. In wavy environments, the error that has reach a few decimeters can be eliminated by extra measurements. hA and hM are used for this purpose in the above equation. Further explanation is given in the application section of this research. The second goal of this study is to prove that the//w value can be obtained by both mareograph reading and PDGPS measurements. According to the Figure 1, HM=hM-N Where N is geoid undulation. The Hm value for both applications has been calculated after obtaining the N value in the surveying area. The comparison of classical mareograph readings and Hm values are indicated in the table below. Tablel. The Comparison of Hu Values from Both PDGPS and Mareograph Readings As can be seen from the above table this method gives good results. It provides an average sea level determination with a precision of ± (5-10) cm.