A method is presented which was designed to: 1. Simplify computations in areas where velocity is known to vary, or may later be found to vary, laterally. 2. Considerably reduce the amount of recomputation necessitated by subsequent revisions in velocity. The conventional time-section and time-map satisfy these requirements but are indicated generally to be subject to serious errors, as illustrated. The proposed system is explained and evaluated. Advantages believed to be peculiar to it are tabulated, and a practical method of plotting the cross-sections is presented.