18.3 The Variance Covariance Matrix

When accumulating the matrix, care has to be taken to ensure that errors that correlate from one survey station to the next are added to the delta values directly before summing but errors that can vary from one survey to the next are summed in their product form. This ensures that random and systematic effects are propagated correctly. Some errors also continue from one survey leg to another and some even from one well to another such as the error in magnetic north.

Once the matrix is complete a technique is required to derive the size and orientation of the three main axes.