Research On Datum Transformation And Urban Control Network's Relative Factors

At present, there are two issues in coordinate's establishment and application: first of all, geocentric coordinate system has become the current trend of development, and all of our national coordinate systems are Reference Centric Coordinate System. Reference Centric Coordinate System will be long-term coexist with Geocentric coordinate system. And then, the Datum Transformation are necessary. In order to resolve this problem, we must consider two Datums' transformation elements and ellipsoid elements.Secondly, in China's urban measurement standards, the choice of cities coordinate system should meet the projection and deformation length of less than 2.5 cm / km. As Chinese cities ' constantly expanding in size , there has a control network bandwidth shortage problem. And the coordinate system should maintain a certain continuity and stability. Therefore, how to meet the demand of projection deformation will be a very difficult issue while takeing into account the previous surveying results.This study follows:(1)By analysising the transformation between two different datums of molodensky model, we study the transformation parameters such as translation parameters,scale parameters or rotation parameters impact on the Gaussian plane coordinates; the ellipsoid parameters such as long radius and flattening impact on the Gaussian plane coordinate. Through above analysis we conclude : translation parameters have more notable effects; When the translation parameters changing 100m, the control points'y coordinates changes larger than x coordinates changes, y coordinates changes are increasing with the increase of latitude and longitude. Parameters of scale or rotation have less effect than the translation parameters, sometimes they can be neglected. Between the WGS-84 coordinates and the 1954 Beijing coordinates' transformation, the ellipsoid parameters have greatly influence. About the ellipsoid parameters, this paper has a conclusion that long radius and flattening's change existes proportional relationship with change of plane coordinates x, y. One hundred meters' change of long radius have more significant impact on plane coordinates than 0.5ppm's change on flattening. The two aspects'changing at the same time have also more significant impact than separate aspect changing.(2)relevant programmes of city control network bandwidth shortage problem, data processing and implementation:In order to solve city control network bandwidth shortage problems, this paper has five programmes :1, to alter the projection method from Gauss projection to Mecator projection2, to alter the projection method from Gauss projection to a dual-standard longitude projection3, to alter the central meridian, using Gaussian projection4, to alter centric longitude, using non-standard horizontal axis Mecator projection5, to alter the central meridian, using dual-standard longitude projection From the five programmes, we can see that the advantages of the programme 1,2 are to easily achieve. Their disadvantages are what the original urban control points have large change. Programme 3 still have a large distortion at the edge part of urban. Programme 4,5 have a small deformation in the original urban coordinates. So, we can draw a conclusion that programme 4 and programme 5 are practical solutions. Programme 5 is the best solution because of its maintaining conformal projection, meeting projection distortion's requirements and enabling the old and new urban control point coordinates the smallest change.This paper has a preliminary study on all of above questions.It is my faithly wish that this paper can provide a few of practical experiences or theoretical bases on practical applications and further studies.