There are four ways to generate a grid.
These four options are explained in greater detail in the following sections.
When the user chooses to create a new model, the “Initial Grid Dialog Box” gives the user the opportunity to specify a uniform grid (see p. 11).
If precise control over the position of the grid is desired, the menu items Grid|Specify Grid Angle… and Grid|Specify Grid Lines… can be used to locate the grid at an exact position. Normally, it is best to specify the grid angle first. When the grid angle is changed, the grid is rotated relative to its own center. The positions of the grid lines are measured relative to the origin of the coordinate system so when the grid is rotated those positions change. See “Grid Angle Dialog Box” on p. 35 and “Grid Spacing Dialog Box” on p. 36 for more information.
If the user selects Grid|Add Vertical Grid Line or Grid|Add Horizontal
Grid Line
and then clicks on the top, front,
or side view of the model, a horizontal or vertical grid line will be
drawn at the mouse position. By doing this repeatedly, a complete
grid can be drawn. The grid can be rotated around its center by
selecting Grid|Drag to Rotate
and then
dragging with the mouse on the top view of the model. Other menu items
that are useful in drawing the grid include Grid|Delete Grid
Line
, Grid|Move Grid Line
,
Grid|Subdivide Grid Elements
,
Grid|Set Width
, and Grid|Smooth Grid...
See “Editing the Grid” on p 33 for more information on these menu items.
Objects can be used to set the size of elements in a grid. For objects drawn on the top view of the model, the object specifies the column and row width. Polygons drawn on the top view of the model in addition to specifying the grid element size also specify the grid location. The grid will be drawn to completely enclose the polygon or polygons. For objects drawn on the front or side view of the model, the object specifies the layer height and the vertical extent of the grid. To be used to specify the grid, the Use to set grid element size check box in the “Object Properties Dialog Box” (p. 48) must be checked. When it is checked, the user can enter the desired size in the Grid element size edit box.
In PHAST it can be useful to have an area with a refined grid where results need to be more accurate. To support this, GoPhast allows the user to specify overlapping objects that specify different element sizes. The final sizes of the elements will be determined by the smallest element size specified for a region. In some models (but not PHAST) it is important for the contrast in element size in adjacent elements not be too large. When the user creates a grid using objects, the Grid smoothing criterion in the “Generate Grid Dialog Box” (p. 35) allows the user to specify the maximum ratio between adjacent rows, columns, or layers that will be accepted. The default value is 1.2, which is well below the usual limit.
Figures 8 to 14 illustrate steps in creating grids using objects.
1. The user draws two objects: a polygon on the top view of the model and a line on the front view of the model (fig. 8) to define the grid location and the size of the elements. (To create objects, see “Creating, Selecting, and Editing Objects in GoPhast” on p. 17 and the Object menu item on p. 48.) In this example, the size of the elements are set in the Object Properties dialog box (p. 49) to 500 m in the horizontal direction (top view, fig. 8) and 10 m in the vertical direction (front view, fig. 8).
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Figure 8. The objects used to define the position of the grid. |
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2. The user selects Grid|Generate Grid... and the Generate Grid dialog box appears (fig. 9). When the user clicks on the OK button the grid is created (fig. 10).
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Figure 9. The Generate Grid dialog box |
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Figure 10. Grid and objects. |
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3. It may be desirable to have a finer grid in some regions than others. To do this, additional objects can be use to specify a zone where a fine grid will be used (fig. 11). In this case, the object specifies an element size of 200 m in the Grid element size edit box of the Object Properties dialog box.
4. After choosing Grid|Generate Grid... again, a new grid is created with a smaller element size in the area enclosed by the object (fig. 11).
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Figure 11. Grid with region with smaller elements specified by polygon object. |
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5. In some cases, it may be desirable to have a gradual transition in element size. A size transition can be achieved by checking the Smooth grid checkbox in the Generate Grid dialog box (fig.12). When it is checked, the widths of the columns, row, or layers will be adjusted so that the maximum ratio of the widths of adjacent cells is less than or equal to the grid smoothing criterion. An example of such a grid is shown in figure 13. See “Editing the Grid” on p. 33 for more information on grid smoothing.
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Figure 12. Generate Grid dialog box with grid smoothing activated. |
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Figure 13. Grid generated with grid smoothing. |
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