Deformation Analysis by Means of Extended Free Network Adjustment Constraints

Deformation parameters are estimated based on a time series of monitoring campaigns. For each monitoring campaign, the measurements’ contribution is in determining the relative positions of the network points and defining the datum of the network. When part of the datum definition contained in the measurements is not constant in time, the estimated deformation parameters could be erroneous. Measured distance, for example, can be assumed to have scale. But fluctuations in the instrument oscillator, when using the same instrument for all campaigns or when using a different instrument in each campaign, could affect the scale between monitoring epochs. For a single monitoring campaign, datum parameters contained in the distance measurements may be used for estimating the coordinates of the network points. However, in four-dimensional (4D) geodetic networks, a part of the datum definition determined by the geodetic measurements may not remain consistent. For example, the estimated deformation parameters related to scale could be wrong in distance-based networks. In this study, the measurements from each campaign are stripped from their datum content by extended free network adjustment constraints. The datumless measurements are used to define the datum by preliminary coordinates and linear constraints that remain constant for all monitoring campaigns, as well as to define the position of the network points. Subsequently, the variations in the network geometry can be modeled by means of a physical model. Following an introduction of the concept of extended free network adjustment constraints and a development of theoretical tools, the paper presents the deformation analysis of a small electronic distance measurement (EDM) network measured six times between 1989 and 2008 using the proposed method.