Research On Improving The Method Of River-crossing Leveling

River-crossing leveling is widely used in the construction of extra large projects like bridges,dams,high-speed railways across great rivers and lakes.The purpose is to establish a high-precision vertical control network for construction and ensure the elevation engagement in project construction and building deformation monitoring.The current river-crossing leveling methods all require a spacious site,good intervisibility conditions,small height difference between banks and even atmospheric refraction coefficient,etc.In addition,GPS measurement technique is still immature and it is difficult to meet the requirement for high precision leveling.Therefore,in order to reduce the labor,material and production costs of enterprise,it is urgent to study the feasibility of methods with unequal fore and back sight distances and apply the improved method in future production and life,so as to improve the efficiency of river-crossing leveling and meet the requirements for project construction.In this paper,several river-crossing leveling theories and methods are studied and comparatively analyzed by taking into consideration the frequent conditions of unequal fore and back sight distances(i.e.it is unable to set up a station or pole with equal fore and back sight distances),with the dam deformation monitoring of Xiannvbao Hydropower Station as the research background.In this paper,directed towards the general requirements of equal fore and back sight distances in leveling,river-crossing leveling is carried out under the condition of unequal back and fore sight distances made by rational arrangement of survey station;the error influence of earth curvature and angle i of instrument on river-crossing leveling is eliminated by theoretical equation development.Common leveling method and total station trigonometric leveling method are used to verify the feasibility of improved method with unequal fore and back sight distances and whether its precision meets the requirements of Specifications for the First and Second Order Leveling.Based on the experiment data,the precision of leveling method with unequal fore and back sight distances and total station trigonometric leveling method are comparatively analyzed,and the result shows that the precision of the former is higher than that of the latter.At the same time after a large number of repeated experiments,the author suggests improving the method of general leveling measurement's(under unequal back and fore sight distances)back sight distance value range of 5 ~ 10 m is advisable,according to the actual situation appropriate adjustment of the back sight distances.Result of this research have practical significance for river-crossing leveling.It has certain guiding significance and practical values for reducing production costs,improving work efficiency and shortening construction period in special project construction.In addition,it could reduce environmental impact and measurement error,improve the precision of river-crossing leveling and provide strong data support for engineering design.The improved method can provide certain theoretical support for engineering practice of river-crossing leveling and also certain references for further research and improvement of the method.