The problem of validity of benchmark elevations for general engineering tasks in urbanised areas on the example of vertical benchmark network in the city of Wroclaw

To carry out many surveying operations, one requires, apart from location determined in horizontal plane also elevation of selected points in a project. In the case of work realised over a large area, particularly if the object to be measured is of elongated shape, these tasks are performed in relation to a greater number of national network benchmarks. At that point control of benchmarks locations in relation to one another and determination of the most reliable reference frame for surveying tasks are required. The need to carry out such control will be proved in this paper on the example of historical changes of benchmark ordinates of vertical network in the city of Wroclaw.