Object-Oriented and lnt -2rated S -- - " - 1 Data Model for Managing Image, DEM, and

Vector Data Jlanya Gong and Deren LI

Abstract The new generation of geographic information systems (GIS) demand the integrated management of vector, image, and DEM data. This paper brings forward a vector, image, and DEM integrated spatial data model on the basis of the object- oriented idea. It also discusses the implementation method of this integrated spatial database management system. The spatial database management system has been used in the construction of the China Spatial Data Infrastructure as the base of a GIs software package, GeoStar. It reveals that the system has high efficiency and the feasibility to manage national, provincial, and city spatial data of multiple-scales and multiple-sources.

Introduction The integration of remote sensing and geographic information system (GIS) data is the long-term goal of both remote sensing and GI~. Remote sensing is the important data source for GIS and the means for data updating. Additionally, GIs provides the auxiliary information for remote sensing image processing and classification, which is used to automatically extract semantic information. In the history of the development of remote sens- ing, there are three different kinds of integration (Ehlers, 1989). 0;e is the separated but parallel integraiion (different user interface, different tools, and different database). The second is seamless (the same user interface, but different tools and dif- ferent database). The third is total integration (the same user interface, tools and database). However, all the integration is aimed at the single image or the integrated management of two data sets in one workspace.

With the rapid development of remote sensing and digital photogrammetry technologies, the amount of image data has dramatically increased, and has garnered more and more atten- tion (Fritsch, 1989). For example, one basic idea of a Digital Earth is to cover the surface with high-resolution remote sens- ing images and, by means of a digital elevation model (DEM) covering the entire world, to establish the virtual landscape of the Earth. On the other hand, the cybercity has gained a much higher profile. In the establishment of these models, the inte- grated management of image data, DEM, and vector data is in great demand, not just for multi-sources data management of a single image or a small area, but also for management covering a city, a province, a country, and even the whole world. Com- mercial GIS software packages can handle large spatial data- bases with vector data format. However, most of them are not efficient enough to manage image databases and DEM data- bases. Therefore, it is necessary to develop a spatial database

National Lab for Information Engineering in Surveying, Map- ping and Remote Sensing, Wuhan Technical University of Sur- veying and Mapping, Wuhan, Hubei, 430079, P. R. of China Email: jgong@rcgis.wtusm.edu.cn

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management system that can integrate various data types (Figuema, 1990).

With regard to the development of contemporary computer technology, the hardware fully meets the needs of multi-source integrated management. The external memory of computers is able to extend to tens of terabytes, and even thousands of tera- bytes. The current urgent problem is that commercial GIS soft- ware cannot meet the demand. Popular GIS software currently on the market has already developed the mature technology to manage vector databases. However, it has not yet been adapted to manage image databases, DEM databases, and the integrated three databases.

In order to manage the triple-integrated spatial databases, the Wuhan Technical University of Surveying and Mapping (WTUSM) has developed a new generation of GIS software, GeoStar, on the bases of object-oriented technology and an integrated spatial database management system. This paper first introduces the object-oriented integrated spatial data model. Then it presents the strategies for implementation of the proposed data model and the integrated spatial database man- agement system. This will be followed by an illustration of an exemplary project. The paper concludes with a summary of the major findings.

Object4rlented Integrated Spatial Data Model An object-oriented integrated spatial data model is the outcome of the combination of object-oriented and database technolo- gies. Object-oriented technology has become the mainstream of contemporary computer technology. This object-oriented tech- nology will become the vanguard of the new generation of soft- ware system structures among numerous fields. The object- oriented data model and object-oriented spatial data manage- ment system have always been the ultimate goal of the field of GIS (Gong and Li, 1992). Since the end of the 1980s, the scien- tific community has paid more attention to the application of object-oriented technology in the field of GIS, because the soft- ware technology is changing constantly. There are some object- oriented GIS software packages. Some spatial data transfer for- mats have also adopted the object-oriented logical model, e.g., the U.S. Spatial Data Transfer Standard (SDTS) (USGS, 1992) and the China GeoSpatial Data Transfer Format (CNSDTF) (Gong, 1998).

In the object-oriented data model, the core is object. Object is the abstract of entity of the objective world in matter space.

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The spatial object has two obvious features: (1) the geometric TwMlmensional OM& feature, which has size, shape, and position; and (2) the physi- Simple polygon: A polygon with one external border but with- cal feature, whether it is a road, river, or house. According to its out any internal island. physical feature, the spatial object will be coded generally. The Polygon with island: A polygon with external border and one country also has sorting and coding standards for the spatial or more internal islands. features. According to its geometric feature, the spatial object Compound polygon: A polygon composed of several simple is defined into the following types: polygons or polygons with islands.

Universal polygon: A polygon with internal island but without Pure Geometric Type. Its geometry is an independent point or external border. a contour line. There is no correlative relation among objects. Pixel: One two-dimensional image element. This is the smallest Geometric Topological Type. It has both the geometric position non-divisible image element. and topological relations, such as common arcs and nodes. Grid: The intersection grid point of two-dimensional lines. Pure Topological Type. It has only the topological correlative relation. It is often used during the procedure for defining spatial analysis. &fPgaUve Obi& Spatial Feature. It has corresponding feature coding and attribu- Digital Image: A two-dimensional matrix that can form one tion descriptions, such as oil wells, houses, and parks picture and arrange pixels in the space regularly. Non-Feature Type. It has no certain feature definition. It is used Raster: The regular or contiguous regular grids or pixels of a as a convenience for the expression of spatial data and organiza- certain surface. This is often a rectangle or a square. tion of the in-between object, as a pure node or the common Layer: Composed of one or more feature classes. This may be arc of a polygon. the aggregation of vector data, or image and raster.

The first three types are distinguished by the geometric con- In fact, we abstractly refer to zero-dimensional objects as cept, while the last two types are distinguished by the attribu- "point objects," one-dimensional objects as "linear objects," tive concept. The above definitions are helpful in describing and two-dimensional objects as "surface objects." In order to the following spatial objects. describe spatial objects and the state and quality of the sur-

roundings clearly, commonly recognized annotation is needed. ZerMlmenslonal Spatla1 Object Thus, it may also be referred to as an "annotated object."

Figure 1 shows the object-oriented integrated data model. Independent Point Feature. This belongs to the pure geometric I, this data model, there are four types of entity type, but it is a spatial feature corresponding to feature coding and attributive table. point object, linear object, surface object, and annotated object. Pure Node. This is not a feature but a geometric topologic~ These are considered as the super-classes of all spatial features. element. l-he node is used to describe its correlative relation Each super-class is divided into sub-classes according to its and geometric position within the arc. physical attributive features. One or more feature classes con-

e Node Feature. This either belongs to a geometric topological stitute a feature layer, which is logical. One feature class may type or is a spatial feature. For instance, the node among elec- travel across several feature layers, making data management tronic lines is often a power distribution station. much more convenient. For example, a running river may be in Annotation Reference Point. This is used as a reference to record the water system layer or the traffic layer. ~n this way, unlike the position of annotation. It can be stored in the data structure the coverage model, it is unnecessary to copy data from one of annotation. Polygon Label Point. This is the auxiliary information of the layer to another. Feature class is the core here. polygon and can be stored in the polygon data structure. A workspace is a working area including all feature classes

in the area or several feature layers. Several workspaces can The above first three types of spatial objects have a high degee overlap. On the transverse orientation, several workspaces may of comparability and overlapping concept relations. There- compose a project. A "project" means the region being fore, when designing the data structure, they are treated as one type of object, referred to as the node-point type. They are divided into different objects by means of the descriptive code.

One-Dlmensional object

e Topological Arc. This belongs to the geometric topological type. The arc does not have branches, but the starting node and the finishing node. It may be part of a linear feature or the border of a surface feature. It may even be either the border of a surface feature or one part of the whole of one or more linear features. Generally speaking, the arc itself has no feature meaning. How- ever, if an arc itself is a linear feature, it can directly endow the feature with coding and link the feature to the attributive table. The topological arc may have geometric types of links, circles, arcs, and smooth curves. Non-Topological Arc. This is a pure geometric feature. It may be called "spaghetti" in some systems. The contour line is a non-topological arc. It is usually unnecessary to take its starting node, ending node, left polygon, and right polygon into consid- eration. It is much simpler than the topological arc. According to its shape, the non-topological arc may be divided as smooth or not smooth. The topological arc and the non-topological arc may be merged into one type. They share one data structure and are distinguished by geometric descriptive code. Linear Feature. A linear feature is composed of one arc or several arcs. A linear feature is allowed to have branches and intercross, so that it can deal with problems such as fluvial drainage areas and traffic. A linear feature must have attributive coding and be linked to the attributive table.

620 May 2000 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING

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researched or the area relating to a GLS project. For example, it may be a city, a province, and even a country. A project is a spa- tial database.

In order to manage spatial objects of an integrated image, DEM, and vector, the image and grid DEM are defined to be two layers. The operation and management of the two layers is simi- lar to that of the feature layer. However, the storing method is different. The image layer and DEM layer can be placed in either a workspace or a project. When they are placed in the work- space, they may be the image or DEM of a single workspace. But when in the project, image and DEM layer databases are to be established at first hand, an integrated management system to manage vector, image, and DEM databases is accomplished.

Implementation of the Object-Oriented Integrated Spatial Database Management System In order to simplify the implementation, we put the vector, image and DEM data into three sub-databases. As shown in Fig- ure 2, the three databases can be established separately, i.e., by using three different approaches to manage the databases. After establishing various types of databases, a Dynamic Link- ing Library (DLL) is developed for the integrated management. Thus, it is able to use the image database and DEM database in the vector database management system. Similarly, it is able to use the vector database and image database in the DEM database management system.

The vector data can directly adopt object-oriented technol- ogy. Therefore, we have developed an object-oriented spatial database management engine in charge of operation and man- agement of vector data. As shown in Figure 3, spatial object management is mainly composed of an object storing manager and an object manager. The object-storing manager is mainly in charge of accessing various spatial objects and establishing a spatial index. In our system, a grid-based spatial index is used (Li, Gong and Li, 1998). The object-storing manager also accom- plishes the storage of permanent objects, creates a spatial oper- ation log, and accesses spatial objects when necessary. The object manager is mainly in charge of producing spatial

Application Program Interface L Object Manager LrJ

Figure 3. Spatial object management engine.

PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING

objects; distributing the unique ID between object and work- space; managing spatial objects; completing every fundamen- tal spatial query; maintaining the consistency of spatial objects; and accomplishing multi-users management and management of feature classes, feature layers, workspaces, and projects within the network.

The image database module divides an image block and establishes a pyramid. Due to the large data volume of one image, it is difficult to meet the demands of real-time manage- ment. Therefore, it is necessary to partition the image further. For instance, a 512 by 512 block may index to the pointer and access the data block directly, according to the spatial position by means of indexing the image block, when the image is pan- ning. Another probIem is that, when the scale is decreasing, we need to see a more abstract feature image. If we directly call up data from the original image and then perform extraction, the extraction will be slow. Thus, a multi-resolution pyramid is needed. Data on different layers of the pyramid are called according to different display scales.

The aim of establishing aDEM database is to organize all rel- evant data efficiently and establish a spatial index according to the geographical distribution. Therefore, data in the database can be rapidly accessed, so that seamless panning through the entire area can be accomplished. In particular, when adopting the pyramid data structure, data in different layers can be called up automatically, flexibly, and conveniently according to the size of the display area. For example, we can either view the whole area, or concentrate on a more detailed view of selected areas.

Through the structure of "Project-Workspace-Row," the elevation of a spatial position in the DEM database can be deter- mined uniquely. In order to enhance the browsing efficiency of the whole data, the DEM database has adopted the pyramid hierarchy and system of calling up data from different layers according to the size of the display area. The pyramid hierar- chy of the DEM database is shown in Figure 4.

Application Examples of Object-Oriented Integrated Spatial Database Management System With an object-oriented and integrated spatial database man- agement system as its core, WTUSM has developed the GIs soft- ware, GeoStar. This software is a package which includes the functions of GIS, remote sensing image processing, and digital photogrammetry. The system structure of the software is shown in Figure 5. The physical correlative relation of the software is shown in Figure 6.

Being the foundation of GeoStar, the object-oriented inte- grated spatial database management system is used to manage

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Figure 4. DEM database structure.

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Plate 1. (a) The general image map of Guangdong Province. (b) The image map at a scale of 1:250,000. (c) The image map at a scale of 1:50,000. (d) The image map at a scale of 1:10,000.

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Figure 5. The architecture of GeoStar.

622 May 2000

Figure 6. The physical correlative relation of the software.

PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING

multi-types of spatial data with scales of 1:1,000,000 and 1:250,000 for the whole of China. A provincial GIs pilot project including vector, image, and DEM data has been built in Guang- dong Province. The system involves TM remote sensing imag- ery, DEM data, and vector data with a scale of 1:250,000 covering the entire province; SPOT-TM image data with a scale of 1:50,000 covering the Zhujiang River Delta; and aerial digital orthopho- tos, DEM data, and vector data with a scale of 1:10,000 covering the Zhujiang River Delta. Together, these have formed a multi- scaled and multi-source spatial database. Since the develop- ment of the multi-scaled database, the system has been able to access data from different levels according to the users' preset scales. Plate l a shows a general picture of a province. When we are interested in a city, we may zoom out gradually and get images with scales of 1:250,000 (Plate lb), 1:50,000 (Plate lc), and 1:10,000 (Plate Id). This method is also used for vector data, DEM data, or a combination of all three types of data.

Conclusions With the development of remote sensing and digital photog- rammetry technologies, there will be more and more image data and DEM data. More attention is being given to the question of how to manage the two kinds of databases and integrate them with a vector database in a GIS. An object-oriented and integrated spatial data model has been proposed in this paper, and a spatial database management system based on the data model has been implemented in GeoStar, developed by the WTUsM in China. A pilot project based on the GeoStar platform has been completed. The project results confirm that the data model and the database management system are of high effi- ciency and great adaptability.

Acknowledgment This paper forms part of the project, "A Pilot Research on Pro- vincial Spatial Data Inhastructure" (under C95-02-02), funded by the National Bureau of Surveying and Mapping. It also forms part of the project, "An Object-Oriented Integration Data Model in GIS (49525101)," funded by the National Nature Science Foundation under "Outstanding Young Researchers." We also thank the colleagues for developing GeoStar.

References Ehlers, E., 1989. Integration of Remote Sensing with Geographic Infor-

mation: a necessary Evolution, Photogmmmetric Engineering 6 Remote Sensing, 55(11):1619-1627.

Fritsch, D., 1988. Object-Oriented Management of Raster Data in Geo- graphic Information System, Archives of the Congress of ISPRS, Kyoto, B4:354-362.

Figueroa, D., 1990. Integrated GIs Discussion and Examples, Proceed- ings of EGIS'S0 Amsterdam, 19-22 April, pp. 327-330.

Gong, Jianya, 1998. A proposal for China GeoSpatial Data Transfer Format, Proceedings of CAGIS'98 Conference, 16-20 July, Beijing, pp. 257-263.

Gong, Jianya, and Deren Li, 1992. Object-Oriented Model Based on the Unified Data Structure, Archives of the 1 @ ISPRS Congress, 11-24 July, Washington, D. C., pp. 772-779.

Li, Aiqin, Jianya Gong, and Deren Li, 1998. Organizing for Large Seam- less Geographical Databases, Journal of Wuhan Technical Univer- sity of Surveying and Mapping, 23(1):57-61.

USGS, 1992. Spatial Data Mnsfer Standard, U.S. Geological Survey, Reston, Virginia, 120 p.

(Note: The customary western practice of listing author's family name last, except in the list of references where only the first author's name is listed family name first, is followed herein.)

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