Abstract: In this paper the probalility of the occurrence of Люгп's longitudinal and transverse errors for forward intersection is found to be 0.63.This nnmber is too small,it is recommended that an error-ellipse with a probability of 0.95 should be used in planning how to stake out the points of an engineering structure,It is recommended to draw such an ellipse graphically in estimating the staking-out error in a cerain direction,It is proved that the error-ellipse derived from the view point of probability and that derived from the variation of the mean square erreors of coordinates are one and the same,The case of multiple intersection can be transformed into an equivalent simple forward iatersection having the same error-ellipse whose center is the true position of the point sought,Thus,the probability corresponding to the error-ellipses for multiple intersections can be convenientlystudied.