The INSERT option is used in two situations, illustrated in Figs.5.6 and 5.7, where adding a line connection to the grid implies creation of a new vertex. Fig. 5.6 shows the addition of a line segment from an existing interior node, C, to a point X on boundary segment EF. This requires creation of a new grid node at X, since every line of the grid must end at a node. This operation can be carried out by choosing option I and placing the cursor at or near the mid-point of line EF.

If the boundary points on either side of the new boundary vertex have the same computational code, the editor assigns that code to the new vertex also, otherwise the user is asked to enter an appropriate code. The option of setting the depth at the new vertex manually or automatically is then offered. Automatic depth setting means that the new vertex is assigned a depth equal to the average of the depths at the two neighbouring boundary points.

If the new vertex lies outside the former boundary, a yellow marker is placed at the new vertex as a reminder that the reference DEPTH GRID should be updated correspondingly, in case general re-evaluation of depths is carried out subsequently with program REDEP.

The other application of INSERT is to add certain extra connections within a quadrilateral consisting of two triangles sharing a common side. This is illustrated in Fig. 5.7, where there is an existing connection between two points , E and F, of the quadrilateral CFDE. Another direct diagonal connection CD cannot be allowed, as it would cross EF. Thus, if some connection in that direction is required, a new vertex X must be created, connected to C,F,D and E. This is done by choosing the I option and placing the cursor at or near the mid-point of EF.

If the user approves the new configuration displayed, a choice of manual or automatic calculation of depth is offered. If automatic evaluation is chosen, the depth at the new vertex is found by linear interpolation if the four surrounding vertices are all interior points of the grid. In the event that one or more of the surrounding vertices is on a boundary, the depth at the new vertex is set equal to the average of the depths at those surrounding points which have non-zero depths. In both of the uses of INSERT described above, the new vertex is placed at the position of the cursor, i.e. close to the mid-point of the existing connection. Its position can be adjusted subsequently using the MOVE option in {TOP.EDIT} if required.