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Poster Session
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Flood risk forecsting and disaster countermeasures
information system in the Dongting Lake region
Experiment of the database updating method
Both DEM and existing landuse information in the database system vary with time. In order to ensure the currentness of the system, it is imperative to updating the data in a speedy and timely way.
- DEM Updating: The relief of the Region changes very quickly, being mainly expressed as the variatios in washing and silting of the river courses and lakes, the extensive silting up of mud in the rivers and lakes during the flood season every year, the washing of the river courses during the dry season; whereas the topography within the DSA exhibits no great change because of the protection offered by the dykes. Consequently, the DEM updating in the Region is in essence the partial updating. Three kinds of methods have been adopted with relation to the different situations: 1) To collect elevation data from the air photos, which means that, in accordance with the characteristics that in the flood seson Dongting Lake always cover a vast expanse, and during the dry season that becomes some flood channels, the air photos for the dry season are to be selected for treatment with air line survey method or regional mean interpolation for densifying the control points so as to collect the elevation points according to certain standard on C-12, plaincomp or Z2 Orthocomp; and the derived points are to be interpolated as elevation data for obtaining the DEM data concerning rivers and lakes. 2) To digitize the maps of topography ; with availability of relatively new maps of topography, it is possible to read grid points or digitize elevation points and contour lines so as to densify the sampling points, carry out conversion of data and elevation interpolation for obtaining partial DEM data. 3) To input the data derived from radar depth survey; the development of radar depth survey technique provides a speedy method for obtaining the underwater topographical data ; with the input of the depth survey data obtained during the flood season and the corresponding water stage data observed by the regional hydrologic stations, it is possible to carry out interpolation of elevations for obtaining the underwater DEM data. The adoption of the method makes it possible to complete the updating of the under water topographic data in the Region within half a year.
- Updating of the Existing Landuse Data: The remote sensing information with relation to the data updating requires registration of images with elevation data, classification of images, extraction of information and thereafter the synthesis of information, format conversion, entering into the system for comparison with the data in the system so as to replace the old data by the new ones. Image registration employs the mode of polynomial interpolation for the purpose of correction, in which it is necessary to determine the polynomial correction models through a certain amount of control points, using the least square method for deriving the polynomial coefficient. The pixel size is to be chosen in accordance with the characteristics of images and the resampling is to be conducted using the method of the most adjoining points. Geometrical corrections are to be made for both the correction of geometrical error of images and the positioning of the graphic coordinates into the geographical coordinates system. Image classification employs the most extensively used method of the natural supervision, classifying MSS, TM,SPOT images , air photos, sided-looking radar images respectively. The extraction of the special information uses the methods of threshold value, multiple layer analysis, structural analysis, image analysis, logic sliding or repressive sliding, multiple temporal analysis, etc. The database updating just means the regional synthesis of the classified images, the connection of the classifed images in terms of object types and then the conversion from grid to vector for obtaining the vector information with relation to the objects on the land, which are to be entered into the database for updating the related data. Nevertheless, as not all the data in a certain layer shall be updated, then artificial control is necessary. In connection with the different conditions, we have provided protection for certain object types such as the dykes, settlements, wood land and some other information as slow change elements, which are not to be updated, or chosen certain types such as the reeds outside the DSA, plots with water weed and land on lake beach for the purpose of database updating. Such method of data updating with relation to only certain object types and local areas can greatly reduce the amount of work in volved by data updating.
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