Introduction

This chapter is an introduction to the TRIGRID programs needed for production of a depth grid in the demonstration case distributed with the TRIGRID package. A description is given in sufficient detail to permit a novice user to repeat every step in producing the depth grid included with the demonstration files. If each file produced by the user is compared with the corresponding file supplied, any discrepancies found should probably be enough to indicate where errors were made at any stage. After working through the demonstration case, users should know enough about TRIGRID to begin work on their own applications. More sophisticated use of the programs will be covered later.

A detailed account of all the functions of each program in the package is given in subsequent chapters. Some features common to several of the interactive graphics programs are described in Chapter . Several of the programs are used again later in preparation of the model grid. Details of the file formats used are given in Part III.

Some users confuse the depth and model grids at first, particularly in view of the fact that both are irregular triangular grids, are in the same file formats, and can be operated on by the same programs. In summary:

1. The depth grid is developed first; its boundary nodes are obtained by sampling digitised coastlines; its internal nodes are soundings or contour samples; the spacing of the nodes should be such that the surface formed by the triangular facets of the depth grid constitute a good fit to the coastlines and bathymetry of the basin to be modelled;
2. The model grid is developed subsequently to the depth grid; it uses the same boundary nodes as the depth grid, but its interior nodes are entirely different from those of the depth grid, their spacing being governed by quite different criteria, as will be explained in the next chapter.

To avoid confusion between depth grid and model grid files, it is recommended that all files associated with a depth grid be given names beginning with D or DEP ; for instance, the depth node file in the demonstration case is named DEP.NOD and the depth grid file is named DEP.NGH ; for the same reason, all files connected with the model grid have been given names beginning MOD... . Use of the extensions .DIG , .NOD , .NGH or .NEI, .TRI helps to distinguish related files in the DIGIT, NODE, NEIGH(bour) and TRIAN(gle) formats used in TRIGRID.

In the demonstration case and throughout this manual, the first grid prepared (depth grid) contains the scalar variable which will govern the spacing of internal nodes generated subsequently for the model grid. This scalar field being fitted is assumed to be mean water depth. Even though "depth" and "depth grid" are used throughout this manual for the reference scalar and the corresponding grid, it should be borne in mind that "depth" can be whatever scalar is most appropriate to the physical problem being considered. For instance, a weighted combination of depth and depth gradient may be appropriate for some problem, while in another, temperature gradient could be the factor most important in design of the model grid, in which cases the reference grid file would contain values of the appropriate scalar instead of water depth.