Indirect Rectification For Raster Image In AutoCAD

Translation,rotation and scaling of the object can be done at the same time with "align" command to achieve data registration in AutoCAD.If this object is a raster image,the rectification of the raster image can be achieved,and this rectification method is called the "align" method.But when the satellite image is rectificated with the "align" method,it is found that the error distribution shows polarization: the error in the line of two control points is minimum,the further from the line,the bigger the error.The following jobs are done to solve this problem.1.Analyze and point out the radical reason that the error distribution shows polarization with the "align" method: only two pairs of source and target points can be used,translation,rotation and scaling of the object are all done based on them.2.Analyze and summarize two preconditions that accurate rectification is achieved for raster image in AutoCAD: the coordinate datum and projection of the raster image must be the same with what is used in the project,and the sampling interval in x-direction and ydirection must be equal.3.An indirect rectification idea is proposed: first do rectification for the original raster image in professional rectification software,then export rectificated image and its coordinate file,finally realize accurate rectification for the exported image according to its coordinate file with the "align" method in AutoCAD.4.The JPEG satellite image is taken for an example to realize accurate rectification in AutoCAD according to the indirect rectification.5."Indirect rectification in AutoCAD",the common rectification method in AutoCAD for raster image is summarized and proposed.The results indicate that "indirect rectification in AutoCAD" is no longer limited to using only two pairs of source points and target points,so it avoids the polarization of the rectification error with the "align" method;in addition,it doesn't care whether the raster image satisfies those two preconditions or not,and realize accurate rectification in AutoCAD for raster image indirectly,and can be promoted in practical work.