Geographic Information System (GIS) is a comprehensive application system developed in recent decades. It can combine all kinds of information with geographical location and related views, integrate geography, geometry, computer science, CAD technology, remote sensing, GPS technology, multimedia technology and virtual reality technology, and use computer graphics and database technology to collect, store, edit, display, convert, analyze and output geographical graphics and their attribute data. With the comprehensive development of digital city construction in China, GIS system has been widely used in urban management fields such as land, planning, housing management, communication and municipal engineering. The system structure has also entered a brand-new Internet era from stand-alone application and LAN application. As far as the network structure is concerned, the browser-server mode (B
-S) has gradually replaced the traditional client-server model (C-S), with the ubiquitous Internet as the carrier, which has broken through the limitations of time and space and attracted more and more attention.
In the daily work of China's land and resources management departments, a large amount of information comes from massive map data (clan map, land acquisition boundary map, land red line map, road red line map, benchmark land price map, land use planning map, etc.). ) and parcel registration information (user, area, service life, use, etc. ), and the storage carrier is a paper medium. In recent years, information construction has been further strengthened, and a large amount of data has been obtained through the establishment of cadastral management information system. How to make this information enjoy the widest and most convenient query under the premise of confidentiality and security will directly determine the work efficiency and quality of the land and resources management department. Based on the analysis of MAPGUIDE software structure and land management characteristics, a solution and data organization model of land information publishing system based on browser-server mode (B-S) are proposed.
MapGuide is an online map publishing software launched by Autodesk Company in the United States. It can receive various vector and raster data formats, and according to the specific requirements of graphic expression, browsing and information retrieval, make and publish maps on WAN and LAN through hierarchical management of spatial data and raster data and classified organization of attribute data. It can not only query map data and attribute information through the network, but also access related database resources. In addition, it can jump from the current map target to the related homepage, so that GIS users can enjoy all kinds of data resources anytime and anywhere with the help of the Internet, which breaks through the limitation that vector maps can not be browsed quickly on the Internet in the past and fully embodies the advantages of network maps.
2 The basic composition and working mode of MapGuide environment
It mainly consists of three parts, namely Autodesk MapGuide (plug-in or ActiveX), Autodesk MapGuide Author and Autodesk MapGuide Server. Other auxiliary data processing and conversion tools are SDF Loader, Raster Workshop and SDF Toolkit. With the help of these softwares, spatial data and attribute data can be stored separately. With the support of background relational database and ASP programming, online browsing of map graphics is very convenient, fast and safe, and synchronous browsing and interactive access of map spatial data and parcel attribute information can also be realized in one webpage.
Among them, the application tool Raster Workshop is mainly used to convert and process raster images, such as aerial photos, satellite photos or scanned maps. Images can be nested according to the specified size and vector graphics, and raster images can be seamlessly spliced. SDF Loader and SDF Toolkit are mainly used to convert different vector data sources, such as DWG format of AutoCAD (which is also the most widely used vector map spatial data format in land management departments at present) and SHP format of Arc/Info. Through this format conversion, it can be converted into a high compression ratio format SDF (spatial data file) file unique to MapGuide, and then the author compiles it into a map source file MWF (map window file) and publishes it on the network, and the MapGuide server publishes information facing the Internet. Then Autodesk MapGuide (plug-in or ActiveX) in the client browser interprets and restores the map information to complete the release of map graphics. The following focuses on three main parts:
(1) Autodesk MapGuide (plug-in or ActiveX) is a plug-in or control on the Internet browser used by end users, which is responsible for interpreting the transmitted vector map window file (MWF) in the WWW browser. With the help of this plug-in/control, users can access maps and GIS sites. Because the vector map is used, it can be operated in layers, which reduces the bottleneck problem in the data transmission of a large number of map data from the server to the client. With the increase of map scale, the details of map content can be continuously improved. In addition, end users can query and browse other related materials through the map hotspots in the webpage.
(2)MapGuide Author is a tool for making GIS information and publishing it online. According to the data provided by MapGuide server, it creates, modifies and publishes vector-based interactive maps on the website. The map browsed by end users is divided into four layers of feature types: point, line, area and annotation, and each layer of data is a collection of feature types.
(3)MapGuide Server is a server software that provides various GIS data to be published on the Internet, and it is a map data server software running on NT4.0 Server or Windows 2000 Server. By configuring the Web server, the MapGuide server can obtain the access request transmitted from the Web server and return the result to the Web server, and then the Web server sends the result to the user. Each MapGuide server supports concurrent link access to multiple map library files and multiple relational databases located on the network and local computers through ODBC, which truly realizes distributed access to data.
3 Classification of land information data
All kinds of paper thematic maps and topographic maps used by land management departments can only express graphic information of a certain scale, and their load is limited. Electronic map and network map can not only integrate map information of different historical periods, multi-scale and different levels of detail, but also display multimedia information such as related words and pictures, as well as linked external databases (storing all kinds of words and data information related to parcels). This requires us to do some research on them.
3. 1 graphic data
Graphic data can be expressed in various digital forms, and electronic maps used in daily life can be divided into vector form and grid form.
(1) vector data emphasizes the correlation between data, that is, topological relations, including various pairs of relationships between points, lines and surfaces. One feature of this format is to emphasize the relationship between graphic objects, such as the adjacent relationship between roads and plots. Secondly, the shape of graphic objects is recorded by mathematical equations, so the storage space required for the same amount of information is much smaller, and the requirements for network bandwidth and transmission speed are lower. Therefore, this is the preferred graphic file format for establishing land information publishing information system, and the SDF file format in MapGuide is vector format.
(2) In the grid form, the data is organized by horizontal scanning lines or stripes, so there is no direct connection between the geographical entity information string and the logical organization of the data. However, grid data looks more like a "real map" (paper map) because it uses techniques such as area filling and true color. In addition, raster images have the advantages of less digitization workload and low cost. In the process of converting paper maps into digital maps, some thematic maps that do not emphasize topological relations can be directly converted into bitmaps that can be received by computers through scanning, but the disadvantage is that graphics occupy a large space and cannot be connected to related attribute databases according to graphic objects. Generally, this kind of bitmap file (such as landform shading map or aerial image map) is used as the background layer of topographic map, and with vector map, the "real" landform can be displayed.
3.2 Attribute data
Attribute data mainly appears in the form of database, and graphic data and attribute data are generally stored in separate organizations to enhance the flexibility of data processing in the whole system. This is the organization principle of data in the previous map database system. In land information, this kind of data generally comes from various registers in cadastral management, mainly in the form of numbers or words. Fortunately, at present, land management departments at all levels have generally established cadastral information systems, and most of these information has entered the database. For example, the cadastral management information system of Leshan City adopts the cadastral management information system of Dixing Town, and the parcel attribute information is stored in DBF database. Therefore, when establishing a land information publishing system, it is only necessary to associate attribute information with graphic objects through ODBC.
MapGuide adopts the traditional organization form of main file+index file for spatial data, and CGI technology is used for data transmission, which explains the client's request, and the results are calculated by the server and sent to the browser through the Web server, thus realizing the visualization of online map. In addition, it can also be built with SQL to realize two-way information query from attribute to space and from space to attribute.
4 acquisition of land information data
4. 1 graphic information
According to the present situation and requirements of land management in downtown Leshan, we can collect1:500 cadastral map,1:2 thousand road red line map,1:5 thousand benchmark land price map,1:0/10,000 land use master plan map,1:. Among them, 1∶500 cadastral map is a vector map,1∶ 1,000 aerial photographs are raster maps, and the other three are paper media maps, which can be converted into digital maps by scanning vectorization.
4.2 Attribute information
The parcel information part mainly comes from the urban cadastral surveying and mapping results, including the application form, cadastral questionnaire and approval form in the process of registration and certification, which can be directly converted from the cadastral management information system according to the requirements of database structure design.
5 Organization of land information data
5. 1 spatial data organization
Based on the above data format characteristics, in the data organization stage of online land information release, different principles of data classification, grading and blocking should be adopted according to different data types.
5. 1. 1 Organization of vector graphics data
(1) classification principle. MapGuide environment classifies vector data according to feature types, and point, line, surface and annotation data can all appear in SDF file format. The graphic elements contained in the thematic map and topographic map to be published are classified and extracted according to different scales, which are used as different layers for graphic display. By controlling the display of different layers, different users can see the information of different scales that they care about at the client. Taking the scale of 1∶500 as an example, various layers such as administrative divisions, streets, parcel boundaries, buildings, waters, control points, parcel numbers, green spaces and corresponding labels can be extracted from the urban cadastral mapping results. Each layer becomes a file independently and becomes a layer of electronic map. If all the layers are superimposed together, a complete cadastral map can be obtained. Users can open or close the corresponding layers according to the information they care about, so that different layers can be combined to form different thematic maps, which not only simplifies the drawing elements, but also reduces the amount of data transmitted by graphics and improves the browsing speed.
(2) Classification principle. Under different scales, the amount of information expressed by maps is different. At the ratio of 1 ∶ 10000, streets and even buildings cannot be represented, but at the ratio of 1∶500, balconies and even telephone poles of houses can be clearly represented. It is unrealistic to express all kinds of information of different scales on the limited display screen at the same time, and it is impossible to highlight the theme. Therefore, although the same map object may come from various scales, it must be controlled at what scale, whether to display it or not, and what graphic symbols to use.
5. 1.2 grid data (mainly aerial photos, satellite photos, shaded relief maps) are organized in blocks.
Aiming at the characteristics of large amount of remote sensing data, such as aerial photos and satellite photos, and slow transmission speed on the Internet, the data processing technology of graphic block and display classification is adopted. The main basis is to divide the existing raster image into multiple graphic blocks according to the size of the display area, and then determine the display scale range according to the amount of information expressed by each graphic block. Because the raster data adopts the framing management mode of 1∶ 1 thousand after deformation correction, each image can be regarded as a graphic block. In this way, when transmitting data on the network, only the image file data within the visual range is transmitted, which greatly reduces the transmission volume and improves the browsing speed.
5. 1.3 Registration of graphics and images
(1) vector data registration. If vector maps with different scales are derived from paper maps, there will be some deformation and errors due to different selection and integration of paper maps in the map preparation stage, and the data in the same area should be consistent. In the specific implementation stage, based on 1: 500 cadastral map, small and medium-sized thematic maps such as 1: 5 thousand benchmark land price map, 1: 2 thousand road red line map, 1: 1 10 thousand land use planning map can be corrected to ensure normal connection.
(2) Vector raster data registration. As a geographical base map, raster data format should have geographical attributes, which is the basic condition for matching vectors with raster maps. For the format difference between graphics and image data, images can be registered to vector data, and vector graphics can also be projected to raster data coordinate system. It is a simple method to match aerial photographic images with cadastral map of 1∶500.
5.2 Connection of Attribute Data and Spatial Data
In addition to the spatial information such as the shape of the land parcel, a large part of the land information is the attribute information such as characters and numbers stored in the database. For each parcel, only the combination of attribute information and spatial information can become complete parcel information. How can I see these two different storage forms at the same time on the client? MapGuide provides such a solution: when SDF Loader converts vector graphics, each map object (such as parcel number, parcel name, etc. ) can be given a key value, which is similar to the primary key of the table in the database, and it can be automatically connected with the records with the same primary key in the specified table in the database. In this way, when you click a map object, you can easily access the related database attribute information. On the other hand, if you know the attribute information, you can also easily find and display related graphic objects.
5.3 Structure of Land Information Release Information System
Figure 1 Topological Structure Diagram of Land Release Information System
6 system function
6. 1 map display content selection
(1) You can overlay any map layer of interest.
(2) You can control the removal of irrelevant layers.
6.2 Map data range positioning
(1) is located according to the page number (1:500).
(2) Quickly locate according to parcel attributes.
(3) Fast positioning according to city coordinates.
6.3 spatial data and attribute data query
(1) According to the parcel space information, query the corresponding attribute information (user, service life, purpose, etc. ).
(2) Query the corresponding spatial information according to the parcel attribute information.
(3) Query the land grading, and visually display the benchmark land price of each parcel.
6.4 Measurement on Drawings
(1) Distance measurement between map objects.
(2) Measurement of parcel area.
6.5 Set different permissions for client access through user name and password management.
7 system application value
(1) Highly integrate maps and data of various formats of land management departments to achieve maximum enjoyment.
(2) Break through the limitation of previous basic map framing management, realize the horizontal splicing and vertical superposition of maps with different scales and different data sources, and improve the query speed.
(3) Breaking the time and space restrictions and realizing online inquiry of land information in different places.
(4) Realize the synchronous query of parcel spatial data and attribute data.
(5) Calculate the area and side length of any plot to provide scientific and detailed data for land investment and municipal construction.
(6) The system is safe and efficient. The client only has the right to browse and query, and the data cannot be modified. The server only dynamically transmits the layers in the display window to the browser, instead of transmitting all the map data to the client, thus avoiding the loss and leakage of map data.
(7) Construct the basic platform of spatial data of Leshan digital city. On the basis of this system, other public management departments can build their own GIS systems by adding corresponding professional layers, such as gas, public security, etc., to avoid repeated construction to the maximum extent.