The acquisition of spatial data is the foundation of the construction and operation of geographic information system. Different data sources and data acquisition methods have great influence on the generation of data models. How to use appropriate methods to obtain data according to different needs, how to ensure the accuracy of data, and finally make visualization closer to reality and improve the spatial query and analysis ability of the system.
Because of the diversity and complexity of the objective world, visualization involves many aspects of data integration, and more complex data models should be adopted. In order to effectively manage and analyze all kinds of data in 3D GIS, the data model of 3D GIS is required to have strong data expression ability. The three-dimensional GIS data model should not only meet the needs of three-dimensional spatial analysis, but also meet the needs of three-dimensional graphic space generation and management. How to choose a fast and effective modeling method to meet the needs of different applications?
How to make people look at the scene in all directions in the virtual three-dimensional environment through dynamic interaction, such as observing the scene from any angle, any distance and any fineness, selecting and switching various motion modes such as walking, driving and flying, and controlling the browsing route independently.
(2) Key technologies
1, spatial data acquisition method
Spatial data collection is the foundation of GIS construction and operation. Generalized GIS spatial data includes not only geographical and survey data, but also geological environment and engineering design data. In the process of understanding and transforming nature, human beings have discovered and invented a series of spatial positioning methods and tools, which enable human beings to understand the surface, internal and external space of the earth. With the development of modern surveying and mapping technology, geological exploration and geophysical technology, the acquisition technology of three-dimensional spatial data has been continuously developed and enriched, which has greatly improved the ability of human beings to understand nature.
1. 1 spatial data acquisition method
Spatial data can be directly measured in the field by total station or GPS, laser rangefinder, etc. Can also be obtained indirectly from aerial images or remote sensing images and existing maps. Among them, map digitization and photogrammetry are the two most effective ways to collect large-scale spatial data, and their applications are also the most common.
1. 1. 1 map digitization technology
In the modern sense, the previous large-scale and various-scale aerial surveys were all simulated paper maps, films or images. To enter GIS and realize computer management, it must be a digital electronic map. The process of inputting a large amount of information from the existing image load into the database is called digitization. Digitization in a broad sense refers to the process of transforming information into a form acceptable to computers, and digitalization in a narrow sense refers to the process of transforming maps/images into vector data structures that meet the requirements. At present, map/image digitization includes manual tracking digitization and scanning digitization. The former is the process of resampling from the existing paper map with the help of computer and plane digitizer to form digital coordinate point series data; The latter is the process of resampling from the existing paper map with the help of computer, flat plate or drum scanner to form coordinate point column data.
(1) digital hand tracking
The handheld tracking digitizer includes a digitizer for fixing the map and a cursor for sampling. The hand-held digitization process includes the following three steps: map preprocessing: before digitizing the map, it should be numbered according to the content and elements of the map. Numbering should be carried out in accordance with the unified requirements of numbering system, usually with small-scale framing or the latitude and longitude positions of different regions for unified numbering, so as to facilitate the splicing and processing of map sheets; It can also be numbered according to the management scope of the administrative region. When numbering regions, patches, nodes, chain segments and independent points should be numbered separately in advance, while the feature points and feature lines on the main chain segments can be numbered incrementally in turn during digitization. After the numbering is completed, necessary records should be made for the convenience of inquiry. The record contents include: map number, map coordinates and number content, etc. After the drawings are numbered, they can be placed on the digitizer.
Map digitization: Generally speaking, digitizers use point mode, line mode and data flow mode to collect data. In point mode, record each isolated point on the map by positioning the cursor at the location of the collection point and pressing the button; In online mode, the straight line segment is recorded by digitizing the two endpoints of the line segment, and the curve is recorded by digitizing a series of straight lines that make up the curve; In data flow mode, the curve automatically collects the coordinate values of each point on the curve at a fixed time interval or distance. The advantages of point mode and line mode are that the loss of feature points is reduced as much as possible and the resampling accuracy is high. The disadvantage is that the sampling efficiency is low, which is generally suitable for the digitization of cadastral maps and planning maps. The advantage of data stream mode is high resampling efficiency, but the disadvantage is that it is easy to lose feature points. It is generally suitable for the digitization of topographic maps and contour maps.
Graph-to-graph relation connection: Only the geometric coordinate data of point, line and area elements are obtained in map digitization, and the topological relationship between point, line and area elements can be generated only by inputting the attribute information of point, line and area elements. Topological relationship can be established by full polygon mode, manual mode or automatic mode.
(2) Scanning digitization
Scanning digitization is a method of vector conversion or screen tracking after scanning the whole picture with a scanner. This method usually requires pretreatment of the original materials. For example, the geographical elements of different colors in the map are separated and copied on transparent film before scanning. At present, there are automatic color separation scanners, and there are also documents that are automatically layered and built. The raster data structure of the map obtained by raster scanning method of optical scanner is stored in pixel mode and needs to be converted into vector data structure before use. Vector data structure has incomparable advantages in data redundancy, map scaling, roaming, storage space, editing, modification and map analysis, so it needs to be vectorized according to the map data storage format selected during system design. The methods of converting raster data into vector data are mainly divided into three categories, namely point grid vectorization, line grid vectorization and area grid vectorization.