The Exemplary Effect of GeoStar in the Spatial Data Infrastructure Construction in China
3. The production technologies and processes of DGDF
The DGDF mentioned above includes three kinds of data, viz, involving digital orthophotos, digital elevation models and vector data. In this section, the technologies and process to produce the various datasets will be considered in more detail according to different situations regarding equipment and quality standards of production.
The production of vector data involves mature technological procedures. However different methods exist according to different equipment held. These methods include digital stereo photogrammetry, analytical photogrammetry, vectorization of scanned maps and digitizing vector data on digitizer. In addition, if the digital orthophotos already exists, acquiring vector data from digital orthophotos is a good method.
According to different data resources and equipment, the digital orthophotos can be obtained by several methods, a main method is digital stereo photogrammetry ( Zhang and Zhang, 1997), whereby the digital orthophote and the DEM are obtained at the same time. If the DEM exists as a product of either analytical photogrammetry or by interpolating DEM from vectorizing contour maps, digital differential rectifying can be used to produce the digital orthophotos directly \. If the analogure orthophotos maps were made by using optical instruments line a rectifying orthophotos instrument , the digital orthophotos maps can be acquired after scanning the analogure orthophotos maps. With the development of remote sensing technology, the high-resolution image can be used as the data resource of digital orthophote. The above methods can usually meet the required standards in accuracy and reliability.
There also are several different technologies to produce DEMs including fully automatic digital photogrammetry, interactive digital photogrammetry, analytical photogrammetry and interpolating DEM from vector countours lines. But each method has a different accuracy and efficiency . tests have indicates the analytical photogrammetry and interpolating DEM by using scanned contour lines have high accuracy; the accuracy is better by using interactive digital photogrammetry than by fully automatic digital photogrammetry, because the interactive digital photogrammetry needs to capture the topographical characteristic lines and more artificial editing .
4. Establishment of geospatial database
The spatial data including digital orthophote, digital elevation models and several layers of vector data are collected by the above approaches. In order to manage and distribute the spatial data, it is necessary that an integrated spatial database with raster data and vector data has to be built. The spatial data has to be built. The spatial data can be stored in file, or in a commercial database management system. No matter which approach is used, the spatial database should be designed as logically seamless. The software for managing the spatial database should have the following functions:
-
The spatial data can be searched and queried in the whole spatial database
- The vector graph should overlay on the image transparency. The new image cane be directly used to update the old vector graph through capturing and editing.
- A multi-resolution image database should be built in pyramid form so that the system can read the image data from different levels according to different displays scales.
- The function should include displaying, querying and panning in an integrated interface with DEM and image.
- All spatial data can be transferred to other GIS or remote sensing processing system.
- Visualization or mapping should be made according to the sheet number or range user-given.
As a platform for collecting and managing spatial data in China, GeoStar has the function to the produce the digital elevation model, digital orthophotos map and vector data as described in section 3 of this paper, and to manage an integrated spatial database. The structure of GeoStar is shown in figure 1.
Figure 1. The Structure of Geostar
To establish the integrated spatial database, there are still many problems to be solved. Therefore, the National Bureau of surveying and Mapping in China has approved a pilot project for PSDI which is being carried out jointly by the Land Department of Guangdong Province and Wuhan Technical University of Surveying and Mapping.
