This paper has attempted to highlight the importance of having available a complete VES of the soil that allows choosing the most appropriate model to describe the electrical properties of the ground. Referring to the classic Wenner VES, small distances between electrodes are not usually considered which prevents an accurate knowledge of the layers near the soil surface. This results in some uncertainty about the choice of the most appropriate multilayer model for soil.The root of the problem is, how electrical resistance measurements are converted into values of the soil apparent resistivity, since conventional methods only apply efficient conversion from the values of the spacing between the Wenner electrodes above a threshold. In order to achieve a complete sounding of the soil, a characteristic function dependent on the type of electrode used in the survey has been proposed. In this paper the characteristic function associated with bare vertical rod electrodes is calculated. Although there is no restriction on the type and form of electrode provided, they can be treated as combination of thin wires. This function can be interpreted as a calibration factor which makes it possible to obtain the true apparent resistivity of the ground for any value of the separation between the electrodes. This makes it possible to choose the most suitable soil model minimizing the uncertainty in model selection