From directional survey for calculating well courses only the errors due to the uncertainty of the survey tool have been analysed. The errors influencing the well path because of the uncertainty of the directional calculation model have not yet been analysed. Hence, at the department for drilling and production of the mining university Leoben, Austria, a method was developed to predict the model error as a function of number and position of survey stations by nearly known well course. If there the model error is compared globally with the error due to the uncertainty of the survey tool, it can be recognized that the tool error is powers of ten larger than the model error. Since the tool error is in contrast to the model error nearly independent of the number of survey stations, an optimal number and position of the survey stations can be estimated by a given and, in regard of the tool error, acceptable model error.