splitting and merging regions

by Nate Hekman on January 14, 2008

Oswaldo, a reader in Peru using the Student Edition of GeoStudio 2004, recently asked:

“How do you split and merge regions and elements?”

Splitting Regions 

Splitting a region is not too difficult once you know how, but it’s not immediately obvious.  Although the dialog boxes in v6 and v7 will be slightly different, the concepts are the same.  I’ll use v6 for today since that’s what Oswaldo is using.

Here’s a region I want to split:

One region

First use Draw Points to add some extra points where you want to split it.  I’ve added 5 and 6:

Added two points

The trick is you have to use KeyIn Regions to actually split it.  Choose KeyIn Regions from the menu and select the region to split.  You see the list of points that define the region.  Currently the list is 1, 5, 2, 3, 6, 4.  I’m going to select 2 and 3 and delete them from the list.

Delete points 2 and 3 from the region

You get this:

We’re down to one half-sized region.

Finally it’s a simple matter of using Draw Regions to draw a new region connecting the remaining points.  Or you can use KeyIn Regions to create the new region since you’re already there, and just type in the point numbers (5, 6, 3, 2).

Two regions

Merging Regions

Merging regions is of course just the opposite. 

You start by deleting one region.  If you use Modify Objects to delete it, the points will be deleted as well (unless they’re used by other regions already); so use KeyIn Regions instead and click the Delete button on the right to delete the region–the points will be left behind.

Then use KeyIn Regions to change the list of points of the remaining regions so it includes the points that got left.

Splitting and Merging Elements

In v6 you could split an element similarly, using Draw or KeyIn Nodes to create nodes, then KeyIn Elements to hook up an element to those nodes.  But it gets touchy and is not recommended.

If you do start messing with the mesh directly, keep these points in mind:

  • Elements must be 3- or 4-sided.
  • Any secondary nodes must be exactly halfway along the element edge.
  • In KeyIn Elements, secondary nodes are entered as nodes 5, 6, 7 and 8.
  • At one point there were problems with mixing clockwise and counterclockwise elements–I don’t remember if that’s still an issue in version 6.
  • If you edit an element that belongs to a region, and if that region is remeshed at any time, your changes will be lost.
  • Be careful about mesh compatibility–a node that is used by one element but not by the adjacent element creates an invisible no-flow “gap” in the mesh.

If you stick to dealing only with regions and let the regions generate the mesh, you don’t have to worry about any of that, it’s all taken care of for you.

In fact in v7 you no longer have any ability to directly edit the mesh.  You can control the mesh density, mesh pattern, and other mesh properties, but there’s no KeyIn/Draw Elements/Nodes any more.

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