Research About The Spatial Data Sharing Based On Geographic Information System

With the development of science and fertility, information revolutionproduces all over the world, and the world is surrounded by the information allthe time. Information sharing has become an important mark of thedevelopment of modern information society. Spatial data interoperability is thecertain result of information sharing and will become an important part in theresearch field of the 21st century Geographic Information System (GIS).In the early map system, many DLG is based on the CAD mode, especiallyon the AutoCAD mode. Therefore it is significant to research on the datasharing between the CAD format and GIS format. And this is the stress of thisthesis. In this thesis, the main point is on the researching of the transformationfrom DLG to GIS, the intermediate file format that we use is VCT, which is thetransformation format of CNSDTF.1.Feature of the data structure1.1 Format of AutoCADAutoCAD figure consists of point, line, text, and fixed element, such asblock and shape(road lamp, booth, etc.), and it can describe the property. DXFformat can be shared between CAD and other software. In this study, thetransformation is realized by selecting the object selection and attribute. Andthe map elements must be divided strictly according to "Classification andcodes for the features of 1:500,1:1000,1:2000 topographic maps",and"Specifications for cartographic symbols of 1:500,1:1000 and 1:2000topographic maps".1.2 VCT formatVector of VCT format is consisted of geometric figure data and property.And geometric data can be divided to 3 kinds, point element, line element andpolygon element. The geometric data and property connects by the target mark(keywords),and every geometric object can be defined by element code. Thereare 6 parts in VCT format,Tab.1. the contents of VCT formatHead Data information of a file,such as mark, unit,coordinate system,projection,scale,time etc.Parameter ofelementParameter of element code, element category,geometric category, default color, property table's nameThe structure ofpropertyProperty structure: property and the number ofitem's name DescriptionGeometric figure Coordination and category of geometry, i.e. point,line, polygonRemark Coordinate, content, style, color, shape, size of a text,and so on.Property dataProperty data is consists of many property items,started by the item's name, and ended with "TableEnd".Each object property takes a single line.1.3 The corresponding relationship between AutoCAD and VCTWe can draw many kinds of geometric figure, while the VCT format,only three, point, line, polygon. The corresponding relationship betweenAutoCAD & VCT is :表 2 AutoCAD 与 VCT 格式的对应关系AutoCAD format VCT formatPoint, block, shape Point elementLine, circle, ellipse, are Polyline elementpolygon area elementtext textproperty Property information1.4 Transform between CAD & VCT1.4.1 Get the figure dataData must be got according to previous fingure. We can get the data of lineand point, but there is not area in CAD platform, the area is substituted bypolygon, so the area data can be got just like that of the polyline.. And the arealine must be closed , so we can get the area by judging it closed or not..1.4.2 Get the propertyThe property of point, such as that of elevation is text, and the valueshould be transformed as the property. We can get all of them by a software,and store them in Access database. And we store the handle of the object in thestorage of keyword "mslink". So the figure can connect with property by keywords, and add information to extend data. We can put it to VCT propertydata if there is extend information by checking data extend data.And the property of line should be got in different conditions. Such as thename of a road or a river, we treat it as point. The property of contour iselevation, the value of elevation is stored in elevation feather, and the valueshould be got when the transformation takes place. The area elementtransformation is just like that of line.2. Transformation between VCT & ARC/INFOAfter the data is transformed to VCT format, we can program inARC/INFO surrounding. There are some methods to transform VCT toARC/INFO. One is VCT is transformed to Coverage,Shapefile, and another isto E00. I choose the Shapefile format because of it open format.2.1 data transformation(1) setting of systemWe must set space parameter first. It is used to describe the coordinate,range, and precision, i.e.(2) transformation of pointPoint is the object which can not be separated, it may be abstract ormaterial, and we realize the transformation by the code belowTransformation of lineThe line is use to draw polygon such like river and road, and the kinds ofline are many, so we must transform them separately. The correspondingrelationship between them is :Tab.3 The corresponding relationship between ARC/INFO & VCTVCT format ARC/INFOLine,polyline PolyLinearc CircularArccircle CircularArcElliptic ElipticArcB BezierCurve BezierCurvewe can transform as below:(3) Transformation of area elementThe area symbol is used to the 2s spatial object or the extend ofphotographic environment. And the area object has one or some fences. And thearea object is often be filled with some figure. We used polygon to store thearea object, the transformation model is figure 2, and we realize thetransformation by run the code belowfigure 2 .area element model(4) The transformation of textThe text is transformed as point, and the content is stored as the propertyof a point. Right-click "Label Feature", and choose "Convert Labels toAnnotation", change the mark to text.2.2 Dealing with data after transformationSpatial data expresses the object with simple point, line, and area, cannotbe directly perceived through the senses. So we'd better symbolize the data.This can be completed by professional software, such as ArcGIS, then weprogram code with ArcGIS to edit the map.3. ConclusionThis thesis focuses on the character of CAD & GIS data format, studiedthe method of data sharing and the problems during the sharing transformation,and built 2 transformation models, realized the transformation with theintermediate file---VCT. This sharing makes information and data usedconstantly;costing reduced, and efficiency rose.