Borehole data and hydraulic heads are some of the most important data in groundwater modelling, and they are often treated as if they were 100% correct. They may not be, and the problems arising from that are often neglected or not fully recognized. The result may be that comprehensive and time consuming modelling is built on erroneous data which in the end may lead to uncertain or wrong conclusions. Therefore, it is important to recognize the errors attributed to both the primary borehole data and to the hydraulic heads before using the data, both for simple assessments and for more complex modelling. Since a number of different participants are involved in the data sampling, it is important to have precise and generally accepted guidelines for the collection and storing of data in order to meet the future demands for reliable data for geological models, groundwater models, etc. The present work describes a methodology to evaluate the quality of borehole and sounding data. It focuses on the importance of checking reference levels and time series, and it offers a way to select the best data for water table maps and for future monitoring programmes. We believe that this methodology can also be applied to selecting the best data for groundwater modelling, etc.