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УДК 528.47:004.9
ОБОСНОВАНИЕ ВЫБОРА РАЗМЕРА ЯЧЕЙКИ ДЛЯ ПОСТРОЕНИЯ ЦИФРОВОЙ МОДЕЛИ РЕЛЬЕФА В ЗАВИСИМОСТИ ОТ ГЛУБИНЫ ОБЪЕКТА
Анжелика Таласовна Камза
Казахский национальный технический университет им. К. И. Сатпаева, 050013, Казахстан, г. Алматы, ул. Сатпаева, 22, аспирант, старший специалист по ГИС, тел. (707)451-48-20, e-mail: anzhelikakamza@gmail.com
Ирина Александровна Кузнецова
АО «Международная образовательная корпорация», 050043, Казахстан, г. Алматы, ул. Рыс-кулбекова, 28, доцент, тел. (777)257-55-95, e-mail: docent61@list.ru
Евгений Левин
Мичиганский технологический университет, Институт технологии, 1400 Townsend Drive, Хоутон, MI 49931, США, доктор наук, зав. кафедрой прикладной геодезии, сертифицированный фотограмметрист, школа технологий, e mail: eleven@mtu.edu
Батиметрические изыскания для построения цифровой модели рельефа (ЦМР) морского дна являются частью гидрографических изысканий, проводимых для дальнейшего освоения месторождений полезных ископаемых, расположенных на шельфе Каспийского моря. Результатом таких измерений является актуальная цифровая модель рельефа исследуемой территории, которая используется во многих прикладных науках о Земле. В статье освещаются инновационные методы создания регулярных сетей и цифровых моделей рельефа морского дна. В работе рассмотрены новейшие технологии и методы получения высокоточных батиметрических данных. В статье предлагается сравнительный анализ методов построения регулярных сетей для последующего использования данных с целью построения ЦМР с различными размерами ячеек. Даны рекомендации по выбору оптимального способа создания ЦМР в зависимости от дальнейшего целевого использования полученных данных. Описана разница построенных моделей в зависимости от размера ячейки эффективной 3D поверхности рельефа.
Ключевые слова: морская геодезия, цифровая модель рельефа, батиметрия, гидрология, ГИС, съемка морского дна, Каспийское море.
JUSTIFICATION OF THE CELL SIZE CHOICE FOR DIGITAL ELEVATION MODEL CONSTRUCTION DEPENDING ON THE DEPTH OF THE OBJECT
Anzhelika T. Kamza
Kazakh National Technical University named after K. I. Satpayev, 22, Satpayev St.,
Almaty, 050013, Kazakhstan, Ph. D. Student, Senior GIS Specialist, phone: (707)451-48-20, e-mail: anzhelikakamza@gmail.com
Irina A. Kuznetcova
JSC "International Educational Corporation", 28, Ryskulbekova St., Almaty, 050043, Kazakhstan, Associate Professor, phone: (777)257-55-95, e-mail: docent61@list.ru
Eugene Levin
Michigan Technological University, 1400 Townsend Drive Houghton, MI 49931, D. Sc., Head of Department of Applied Geodesy, Surveying Engineering Integrated Geospatial Technology
Graduate Program Director, Digital Mapping Enterprise Adviser, School of Technology, phone: (906)487-244, e-mail:eleven@mtu.edu
Bathymetric survey for construction of Digital Elevation Model (DEM) of the seabed are part of hydrographic surveying conducted for the further development of mineral deposits located on shelf of the Caspian Sea. The result of such measurements is the actual digital elevation model of the study area, which is used in many applied Earth sciences. This article highlights innovative methods of creating regular nets and digital models of the seabed relief. In the work the latest technologies and methods for deriving high-precision bathymetric data are considered. The article proposes a comparative analysis of methods of regular nets creation for the subsequent use of data in order to construct DEM with different cell sizes. Recommendations are given for choosing the best method for DEM creation depending on the further targeted use of derived data. The difference of the constructed models depending on the cell size of the effective 3D relief surface is described.
Key words: marine geodesy, digital elevation model, bathymetry, hydrology, GIS, seabed survey, Caspian Sea.
Introduction
Currently, there are raw hydrocarbons (HC) being produced at the field located in the North-Eastern part of the Caspian Sea, being one of the most promising sites for the development of offshore oil deposits in the Kazakh Sector of the Caspian Sea (KSCS). By Presidential Order of the Republic of Kazakhstan dated May 16, 2003, the State Program for the Development of the KSCS was approved, aiming at promotion of sustainable economic growth of the country and improving the quality of life of the population through the rational and safe utilization of the hydrocarbon resources of the KSCS, as well as the development of related industries [1]. Therefore, it is necessary to perform requires a number of seabed explorations to ensure environmentally safe offshore operations of the extraction and development of hydrocarbon deposits.
The use of Geographical Information Systems (GIS) allows solving various applied problems on the basis of DEM. Particular attention is paid to the choice of the method of DEM construction and estimating the accuracy of the model in morpho-metric terrain analysis, since many digital maps of various sizes are used in the analysis [2].
In this paper, we discuss DEM construction methods based on the results of a high-precision bathymetric survey. The purpose of this work is to perform a comparative analysis of the constructed DEMs and to identify the most optimal model for its further use in the design of engineering structures, for plotting various thematic maps needed during offshore operations.
Measurements of vertical depth are the decisive method for creating topography of the bottom of lake.
Depending on the height of the object, a 3D model can be created to estimate the habitat state, the study of precipitation, the measurement of the clarity of water and the search for historical shipwreck, etc. Previously, depth measurements were labor intensive and complex process. Old methods did not meet hydrographic standards
and accuracy due to poor accuracy since of hardware limitations. As the measurement techniques improve, vertical measurements can consistently meet quality control standards. Advances in sonar technology, such as interferometry sonar and multiphase sonar systems, have led to improved measurements of the depth range. Besides, satellite hydrographic methods can observe large geographic areas and collect consistent hydrographic data for the whole region [3].
Methods of research
Data on the bottom relief characteristics are required both to solve fundamental problems and for a wide range of applied research:
1) monitoring and determining possible changes in the seabed terrain and structure in the context of increasing anthropogenic pressure, including development and operation of hydrocarbon fields;
2) study of the terrain structure for the purpose of reconstructing sedimentation environments and terrain formation, determining its evolution and utilization the obtained data for the benefit of extracting industries, and in solving other practical problems, including defense tasks;
3) the study of geological and geomorphological processes associated with dangerous natural phenomena (seabed deformation, landslides, degradation of subsurface permafrost formations, etc.), resulting in a rapid change in the terrain, physical and granulometric properties of precipitation in order to minimize risks and prevent man-made disasters including in the areas of exploration, planned extraction or exploitation of raw materials;
4) the use of data on the seabed properties for design and construction works [4].
The bathymetric findings are a set of points with their location and depth. The
size and amount of data depend on the method of obtaining them. The experience of working on the continental shelf has shown that a single-beam echo sounder provides less data than a multi-beam sounder due to the width of the survey strip. After the survey, the information received is processed using special software (SW). The data obtained are cleaned; the depths are brought to the desired water level using daily depth gauge records. The survey findings are brought to one standard, depending on the nature of the problems being addressed [5].
Currently, a large number of software is being developed, which is used both for processing field measurements, and for further work with the bathymetric findings. One of the software toolsets used to work with geological and geophysical data is the Safe Software Feature Manipulation Engine (FME) developed in Toronto, Canada. The digital terrain models presented in this paper were constructed in the FME software using the Real 64 interpolation method. This model construction method is based on an algorithm that does not use approximation [6].
For the study, the coordinates and depths of a small section points (23 x 8 meters) were selected in the territory of the North-Eastern part of the Caspian Sea. As a result of bathymetric survey, a cloud of point data was obtained, which was used to construct the DEM.
Consequently, digital terrain models were obtained in GeoTIFF format. DEMs were visualized in the SonarWIZ 7 program in order to evaluate the results of model construction [7]. The digital terrain model presented in Figure 1 was built on a digital 0,5 x 0,5 meters grid.
Fig. 1. Digital seabed terrain model (cell size 0,5 x 0,5 meters)
Excessive amount of data on the grid of 0,5 x 0,5 meters makes it difficult to create a final DEM. For comparison, digital terrain models with a cell size of 1,5 and 2,5 x 2,5 meters and a magnification ratio of 5, shown in Figures 2, 3, were constructed.
Fig. 2. Digital seabed terrain model (cell size 1,5 x 1,5 meters)
3D model of the seabed with a cell size of 1,5 meters is a smoothed version of DTM 0,5 meters. On this model, small characteristics of the seabed are lost, the dimensions of which are less than 1 meter. This model is convenient during bathymet-ric surveys in shallow water areas, since the amount of information processed during the construction of the online model is not large, but at the same time it makes it possible to monitor the situation during surveys and to highlight interesting seabed surface areas for future processing and interpretation [8].
In the process of surveying large areas and several devices simultaneously, the online recording of the model will slow down due to the large amount of data. In connection with this, a model with a cell size of 2,5 meters is constructed, which allows preserving only the main characteristics of the bottom relief, allowing safe hy-drographic surveying [9].
Fig. 3. Digital seabed terrain model (cell size 2,5 x 2,5 meters)
Results and discussion
The obtained DEMs were analyzed by superimposing grids with different distances between nodes. An aligned DEM grid was obtained as a result. In order to reveal the difference in the accuracy of the construction of digital models, the surface of the difference between DTM was 0,5 and 2,5 meters. Figure 1 shows the distribution of values by the difference between the models.
The graph highlights the yellow bar, which means that the most difference of depths between two models is within 20 centimeters. This coincides with the requirements of the International Hydrographic Organization (IHO) for shallow-water territories [10].
■ -0,1- -0,02 □ -0,2 - 0,1 ■ 0.1-0,2 □ 0,2-0,4
Fig. 4. Range of depth differences between digital grids of 0,5 and 2,5 meters
When analyzing the aligned digital grid, it was discovered that at every tenth position the location of all points of the digital grid is the same; this allowed to perform depth comparison at these points. The grid of 0,5 x 0,5 meters grid was chosen as the base model for the DEM depths interpolation. Interpolation was carried out in the ArcGIS software using the Extract Value tool. For better visualization of the results of the experiment the data were presented in the diagram of depth changes along the lateral (see figure 5) [11].
Fig. 5. Diagram of Depth Determination Dependence on Digital Grid Cell Size
(0,5, 2,5 and 1,0 meters)
In diagram 2, it can be noted that DTM 2.5 varies markedly within the limits of 18-23 points, if in denser models in this section a positive anomaly is traced, and then DTM 2.5 m in this limit shows deeper values.
Comparison of the above diagrams made it possible to conclude that the model, constructed on a 2,5 x 2,5 meters digital grid significantly distorts the depth values data. At a time when DEM with a cell size of 0,5 x 0,5 meters and 1,5 x 1,5 meters largely repeat each other, the DEM, constructed in three-dimensional space, as well as diagrams comparing depths, showed that the DEM created on a grid of 0,5 x 0,5 meters describes the state of the seabed in more detail and more clearly, revealing characteristic terrain points that lose their relevance on the DEM with a large cell size [12].
Conclusion
These findings below are suitable for small and shallow areas. If we consider the bottom of the world ocean, the terrain DEM with a cell size of 0,5 x 0,5 meters will consist of a large amount of data that will not be convenient for the future use. Therefore, the cell size for DEM should be selected depending on several factors, such as the depth, scale and area, the target designation of the effective DEM, and other factors.
The general models of the bottom terrain should be the basis for determining the "key" shelf sections for further detailed study, including depth sounding. In the industrial development of shelf zones, they are needed at the stage of justification, planning and identification of geological hazards, exploration and construction, while at the operation stage they should serve as a basis for monitoring changes in the bottom surface, the ecological state of the study area, and geological and morphological study of the territory [13]. This approach is the most appropriate for optimizing financial costs, given the high and timely huge cost of full-scale field survey. It should also be taken into account that it is the general models that minimize possible and inevitable errors at all stages of exploration and operation of facilities. In this regard, before commencement of hydrographic surveys, it is necessary to determine the final DEM construction method and the size of the grid cell [14].
The choice of the final DEM's grid size directly depends on the depth of the survey area. In deep areas it is not so important to build the three-dimensional surface with the centimeter accuracy, since a possible danger of a large subsidence of the navigation vessel is excluded. While in shallow areas it is important to have a DEM, which most accurately describes the seabed, due to the fact that even a small object on the bottom surface might become an obstacle to navigation, also for the further engineering works on the site. Due to that, it is recommended to use the model of 0,5 m resolution for shallow, and the model with a cell size greater than 1,5 meters for deep areas with a subsequent increase, which is proportional to the seabed depth increase.
REFERENCES
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© А. Т. Камза, И. А. Кузнецова, Е. Левин, 2018
