Whenever existing positions are held fixed or weighted, the adjustment will become distorted if these positions are in error and the weights are not chosen carefully. The a posteriori variance of unit weight and the error ellipses will increase as the error of the existing station coordinates increases. In an effort to simplify the description of position accuracy, the FGCS proposal does not recommend use of the error ellipses directly but instead proposes use of a circular measure. As proposed, the circular measure does not reflect the correlation between the station coordinates, that is, the shape of the error ellipses. The FGCS proposal refers to local accuracy and network accuracy.
Local accuracy is derived from an adjustment with existing control being weighted.
Next, the relative error ellipses are computed for all directly connected, adjacent points. Each relative error ellipse is converted to a corresponding circular measure. The average of all circular measures constitutes local accuracy. Network accuracy is also derived from an adjustment with existing control being weighted. However, the station error ellipse, that is, not the relative error ellipse is converted to the circular measure.