Program GRIDIT is now used for triangulation of the set of depth nodes (in file dep.nod in the demonstration case). The triangulation algorithm used in GRIDIT was devised originally by Cline and Renka and modified by Bova and Carey to handle boundaries. It yields what is known as a Delaunay triangulation, one in which the triangles formed are as near equilateral as possible for the given positions of the nodes.
On choosing GRIDIT in the main menu, you will see first some information on the maximum size of node file which can be handled with your implementation, then be prompted for the name of the file containing the set of nodes for the depth grid, dep.nod in the demo case. The next prompt suggests spatial sorting of the interior nodes prior to triangulation. Sorting can speed up triangulation if a large number of nodes is involved and more importantly, may prevent GRIDIT from 'hanging up' when dealing with especially complicated grids. However, enter 'N', as sorting is unnecessary in the case of dep.nod . Next, enter your choice of name for the file to be output containing the depth grid. This file will be in NEIGH format, so use of extension .ngh is recommended for the file name.
Answer 'N' to the next prompt, 'Is triangle list available?'. Normally, of course, a valid triangle list is not available prior to triangulation. This prompt covers a secondary use of some code in the output section of GRIDIT when both a node list and corresponding triangle list are available from some external source and there is a requirement to convert this information into a grid in NEIGH format.
The next prompt asks if a triangle list should be output. Normally, no list is required at this stage, but answer 'Y' and choose a filename for the list; the extension .tri is recommended. GRIDIT will now carry out the triangulation which should take 2 to 3 seconds on a 486 with dep.nod as input. Messages will indicate when the triangle list and grid file are being output, after which the run is complete. The triangle and grid files output should prove to be identical to dep.tri and dep.ngh in trigrid/demodata (see Figure 2.3). The next step is to check the depth grid output by GRIDIT, using the EDITOR program.
Figure: The grid produced by the gridding program GRIDIT from the set of depth grid nodes shown in Figure 2.2