In 2012, the hydrogeological model (HM) LAMO of Latvia has been developed by scientists of Riga Technical University (RTU). LAMO comprises an active groundwater zone that provides drinking water. 3D-body of LAMO is approximated by the 951×601×25 size finite difference grid. Its plane approximation step is 500 metres and HM accounts for 25 geological layers. The commercial software Groundwater Vistas (GV) is used to run LAMO. The GV system contains tools used worldwide for hydrogeological modeling. The most time consuming and troublesome part of developing a HM is to designate its geometry that is represented by thicknesses of geological layers composing the HM body. For LAMO, most of these layers are outcropping. They are not continuous and, for this reason, they are not present everywhere in the area of the HM. After emerging at the surface, such layers have zero thickness. Thicknesses of the layers are not used by GV as initial data. A modeller must prepare the set of z-maps that presents elevation surfaces of the layers. The GV system uses z−maps for obtaining thicknesses of the layers. To prepare z-maps for primary geological layers, three steps of data interpolation were performed: 1. selecting and checking borehole data that describe stratigraphy of the geological environment; the EXCEL system was applied, to carry out this step; 2. by using the graphical system SURFER, the preliminarily set of z-maps was prepared; 3. the final version of z-maps was obtained by the Geological Data Interpolation (GDI) system. It was rather difficult to create z-maps for the Quaternary layers, because borehole information describing their geometry was waste and contradictory. For this reason, mainly pointwise data and a few lines were applied as sources for obtaining z-maps of the Quaternary System.