In this paper a new method for predicting wellbore position uncertainty whi
ch responds to the current needs of the industry is described. An error mod
el applicable to a basic directional measurement while drilling (MWD) servi
ce is presented and used for illustration. As far as possible within the li
mitations of space, the paper is a self-contained reference work, including
all the necessary information to develop and test a software implementatio
n of the method. The paper is the product of a collaboration between the ma
ny companies and individuals cited in the text.