Sometimes, when working in TSF safety analysis, the historic archives with the original ground topography or details about the TSF design are few or inexistent. The knowledge of the bedrock position, as the details about the embankment construction are essential to estimate, for instance, the tailings thickness and to check the type of embankment growth. Establishing a work base is essential for the analysis of the TSF current state stability, which is, in many cases, very difficult to define with few and low quality data. Often the definition of the original ground topography is obtained from aerophotogrammetric reconstruction, from satellite images or aerial photos, originally with low resolution. The uncertainty associated with these methodologies is often more than 10 m for the elevation. In this paper is presented a case study from a TSF where the initial data package had only the feasibility design and some very simplified reports that checked the stability of the dam before an upstream raising, i.e. it didn’t exist much information and the details about the site and about the structure were very low. To “add” difficulties to the process of defining the bedrock surface and to establish the TSF design, the embankments were built with local rocks (mainly schist and phyllite), and during the initial analysis of Google earth older images it was noticed that the original ground was excavated in different areas to increase storage area and obtain construction materials. This paper will be presenting the steps developed to establish the definition of the ground bedrock surface, the difficulties felt and are discussed the its impacts on TSF stability analysis.