A thematic map shows the spatial distribution of one or more specific data themes for standard geographic areas. The thematic maps are generated to represent the studied variables, so interpolators are used to determine their values in places not sampled. It is usually considered the kriging as the best interpolation method, but several trials found out good results for inverse-distance and inverse-square-distance methods. The evaluation of a map quality usually consists in comparing the estimated values for interpolation with the sampled ones. However, the visual aspect of a created map is also relevant and the smoothness of outline curves is an item to be taken into account, because it facilitates the visual interpretation and the site-specific of agricultural inputs in agriculture. So this study aimed at creating a software to evaluate the smoothness index of thematic maps and use it to compare smoothness of three thematic maps, created by different interpolators, drawn based on corn yield data in a 30 ha area. The selected interpolators were kriging, inverse of distance, and inverse-square-distance. The evaluation was accomplished through the smoothness index, proposed in this paper that calculates the frequency of classes’ change of a thematic map in horizontal, vertical and diagonal directions, through a software drawn for this purpose. The index was shown efficient to determine smoothness of thematic maps.