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Obtaining boundary nodes for the model grid

As explained earlier, using identical boundaries for both the depth and model grids avoids difficulties which occur with methods of depth interpolation such as bicubic splines. Therefore, the first step in producing a suitable set of boundary and interior nodes for a model grid is to isolate the boundary nodes from the set of nodes already used to produce the depth grid.

First select the NODER from the main menu and after this program has been loaded, enter the name of the file containing the depth grid nodes, i.e. dep.nod . Most of the operations which follow only take effect within a designated 'working polygon'. In the demonstration case, the whole model domain will be treated at once, so the next step is create a polygon covering the whole grid. To do this, pick DEFINEGROUP in the top menu of the NODER, which brings up the sub-menu:

Select WHOLE, then SAVE; a working polygon, covering the whole domain will now be displayed. Now return to {TOP} and pick option EDITGROUP. This brings up the sub-menu :

Picking GENNODES brings up the following sub-menu, which contains all the options needed for generation of internal nodes for the model grid:

First, pick DELETE: all the nodes to be deleted, that is, all the interior nodes of the depth grid, are now shown in yellow. On confirmation that these nodes are to be deleted, the display will be redrawn, showing only the boundary nodes, which are all that need to be kept as a starting point for the model grid.

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