Abstract: GPS relative positioning is contaminated by several sources of errors. Some of them have systematic effects on the measured baselines. These effects,however,may change from one survey epoch to another due to changes in the error characteristics and/or the sky distribution of the observed satellites. To obtain accurate and reliable results, they must be properly modelled and eliminated in processing of the measurements of high precision. In the design of survey schemes a consideration of these facts should also be given. This peper discusses some of these aspects with two examples:a GPS network for monitoring ground subsidenceof an oil field, and SSC GPS control network.