Manage Your Model by Deleting Triangles Have you ever had extraneous triangles appear on the outer edge of your model? These triangles typically provide little information to the overall surface and may degrade the appearance of the terrain model display. One of the best ways to correct this problem is to take advantage of the Delete Triangle command. The Delete Triangle command allows you to manually delete one or several triangles at a time, but not the points that define the triangle(s). The areas covered by deleted triangles are not used in any subsequent processing. Note: Deleted triangles cannot be removed from memory. This is because every triangle in a surface stores information about adjoining triangles. This information allows the software to find its way from one point to another very quickly. As a result, all of the triangles, even if they are deleted, remain in memory. To use this command, first display triangles (use the View > Triangles command with the Write Lock on). Then, activate Delete Triangle (Palettes> Design Surface> Delete Triangles), and enter a data point inside the perimeter of one triangle to be deleted. A dynamic line appears attached to the cursor. Move the dynamic line over all of the triangles you want to delete -- all of the triangles that the dynamic line touches will highlight. Then, place another data point and the dynamic line disappears. You are prompted to confirm or reject -- <D> to delete the highlighted triangles, or <R> to avoid the deletion. If you only want to delete one triangle, just make sure that the dynamic line stays within the triangle. Note: This command does not modify the permanent graphics in the design file. Therefore, if there are triangles displayed in your design file, you delete some triangles, then update the screen, the triangles you just deleted will reappear. To be able to see the changes, delete the triangles, then redisplay them. The triangles you deleted will no longer be displayed. When surface point data is triangulated using the Utilities > Triangulate Surface command, all triangles within that surface are removed from memory and then reformed from scratch. Thus, if you delete a triangle using this command and then retriangulate the surface data, the triangle that you deleted will be regenerated. To correct this problem, we will create an exterior boundary. After deleting all extraneous triangles on the outer edge of your model, use the View> Perimeter command (Be sure the Write Lock is on). The View > Perimeter command displays the outermost edge of a triangulated digital terrain model. This command computes the perimeter of the active surface in one of two ways. If an exterior boundary exists for the surface, the command displays it as the perimeter because no valid triangles exist outside of this polygon. If an exterior boundary does not exist, the software works its way around the edge of the model, making sure not to use any deleted triangles. This path is displayed as the perimeter. Your surface should not have an exterior boundary yet. The second method will be used to show the perimeter. Once you have the perimeter displayed, use the Import Surface From Graphics command to create an exterior boundary (File> Import> Surface From Graphics). First, select the surface into which you want to import the exterior boundary. Then, toggle Point Type to Exterior. Set Curve Strings to Stroke and Mode to Single. Choose Apply, and you will be prompted to select the graphic element in the design file you wish to use as the boundary. The element will highlight, and you may either accept or reject the element. Choose Accept, and the number of points and lines that the software loads will display in the dialog box. You may now triangulate the data (Utilities> Triangulate Surface) and manipulate it (Save Surface, View Triangles, etc.). After you triangulate the surface, the triangles you initially deleted should be gone. Note: Use exterior points with care. You may only have one exterior boundary in each surface. If you load several different exterior boundaries in a single surface, the software will only recognize the last exterior boundary that you loaded. If any mistakes or problems are found with the Design Web pages, contact Jeremy Wooldridge.