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Checking and smoothing the model grid with the editor

After triangulation, it is always advisable to check the grid output using the EDITOR option {.TEST}CHECKNODES, just as described in Section 2.6 . Any errors detected can be eliminated using the various editing options in {TOP.EDIT}.

Now window in on the very shallow area just south of the island, using {DISPLAY}IN. The regular pattern of small right- angled triangles here results from the clusters in this area each consisting of just one square from the Cartesian grid (see Figure 3.5). The shapes of these triangles and most other triangles in the model grid can be made more nearly equilateral by applying a smoothing algorithm built in to the EDITOR. First use {DISPLAY}OUT to get the whole model grid is on-screen, then pick option {.EDIT}RESHAPE. This adjusts the position of each interior node so as to bring the immediately surrounding triangles closer to equilateral shape (see Figure 3.6). Windowing in again on the area south of the island will show clearly how much triangle shape can be improved by smoothing (see Figure 3.7). Note that {.EDIT}RESHAPE moves only those interior nodes which are in the viewing window.

 [IMAGE ]
Figure: A part of Figure 3.4 shown in greater detail. The regular area just south of the island is very evident.

 [IMAGE ]
Figure: The effects of smoothing the grid shown in Figure 3.4.

 [IMAGE ]
Figure: A part of Figure 3.6 shown in greater detail. The right-angled triangles in the area just south of the island are now much closer to equilateral triangles.

To detect any remaining poorly shaped triangles, pick option {.TEST}TRIANGLES. In the right hand panel, pick SET CRITERION, then EQL. Answer Y to 'Set or reset color table?', and to 'Use prepared colour table?', then type in ecol.dat which is the name of a colour table appropriate for triangle shapes. The grid should redraw automatically, with a coloured marker in each triangle. The colours correspond to different ranges of the shape factor, a non- numerical criterion for triangle shape, independent of triangle area. The value of this criterion is unity for equilateral triangles and becomes progressively larger as the triangle shape becomes more obtuse. The range of shape factor represented by each colour can be displayed by picking option INFO, then {INFO}TEXT and entering the filename ecol.dat . Normally at this point in model grid development, very poorly shaped triangles would be corrected using appropriate options in {.EDIT}, but this step will be omitted here to save time.

The NEIGH file format (Part III) used in TRIGRID provides for an integer to be assigned to each node of the model grid to indicate what type of computation should be carried out at that node during subsequent modelling computations. The computational codes in use at present are as follows:

[IMAGE ]

There is a graphical test in the EDITOR for checking what computational codes are currently assigned. Pick option {.TEST}VERTICES . The left hand column in the right hand panel offers a selection of tests, C1, C2, ... Pick on 'OFF' beside C1 to toggle marking of nodes with code 1 to 'ON'. Then picking ACCEPT will result in the main window being redrawn, with a marker in the same colour as 'ON' indicating which nodes have a computational code of 1. Toggling on test C0 and C2 also will place markers at interior nodes and island boundary nodes. At this point, no other codes should be in use in the demonstration case. Return to the top menu and use SAVE and FINAL to save the model grid. The output file should be identical to mod1s.ngh in trigrid/demodata and is plotted in Figure 3.6.

The other vertex tests available under {.TEST}VERTICES can be explored at this stage, if you wish, except for EXT, which requires some instructions besides the prompts (see Chapter gif).


next up previous contents
Next: Placement of open boundaries Up: Preparation of model grid Previous: Triangulation of model grid

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