the following covariance matrix cov n 100 0 0 0 0 0 0 169 0 0 0 0 0 0 25 0 0 0 0 0 0 100 0 0 0 0 0 0 169 0 0 0 0 0 0 25 the first and second overlapping surveys may be combined to generate the mpp as follows mpp h n t cov n u2212 1 h n u2212 1 h n t cov n u2212 1 v in this example the resultant position vector is equidistant between the two survey points as expected for this example the covariance matrix may then be solved as follows cov mpp h n t ue89e cov n 1 ue89e h n 1 50 0 0 0 84 5 0 0 0 12 5 the result of this process is then a resultant position 28 c mpp based on combining overlapping surveys at the same position r vii in the wellbore for simplicity this example incorporated positions with identical covariance matrices however it will be appreciated that different surveys may have different covariance matrices referring now to fig7 the wellbore 14 of fig1 is drilled further into formation 15 the wellbore 14 extends beyond original bottom 18 at position r.