The TIN model of terrain, as a basis of the spatial analysis of the Botonega reservoir watershed in Croatia, resulted in a significant number of horizontal areas that were an inappropriate presentation of reality. The authors developed and applied the algorithm to overcome this anomaly in an acceptable way. The specific feature of the proposed algorithm is that it applies elementary analysis on the morphology of horizontal areas as the basis for interpolation of extra breaklines. The simple concept of the algorithm gives the possibility for its further improvement in the direction of developing more detailed analysis and correction of horizontal areas as well as in the direction of its use as an initial concept for solving other problems in terrain modeling.