The basin of "Ribeirão Cafezal" is the second main system of reception of water that supplies the municipality of Londrina, to the north of the State of Paraná - Brazil. This region presents extensive agricultural areas, with different exploration degrees. The slope information is fundamental to the elaboration of the several maps that approach the erosion hazards. So, an analysis of the precision was carried out in the mapping, for obtaining a slope map as close as possible to reality. Slope maps of a representative area of the basin were generated using two methods: the abacus (considering as the real slope terrain) and the GIS - Geographical Information System (with grid resolution 5x5, 15x15, 20x20, 25x25 and 50x50 meters). These maps were overlayed and the precision of the mapping was analyzed; the adjustment percentages and the omission and inclusion errors of the related maps were calculated, according to the equations of Mendonça (1980). From this analysis, the biggest adjustment percentage belongs to the cross-checked of the l5xl5 meters resolution (71,50%); so, this is the resolution chosen in the making of the slope map for the whole study area. This test was satisfactory to the analysis of the precision of the mapping when constructing the slope terrain map using the GIS. In case that a specific project needs higher precision in the determination of the slope classes, it is recommended to use the manual abacus.