Creating TINs

TINs, or Triangulated Irregular Networks, are three-dimensional planar items. A TIN is made up of many triangular surfaces which connect adjacent points containing height attributes. Each surface contains graduated height values, or a slope, thus contour and/or flow themes can be superimposed on TINs. Greater detail is obtained by using smaller triangles.

TINs can be created in one of the following ways:

Using the Fan/Strip command

This command creates a strip or fan of connected triangles. Double-clicking finishes one fan and allows you to start a new one.

1. Select Fan/Strip TIN [Create-3D]
   
   
2. Click the first position, or type in its coordinates.
   
   
3. Click two more points to complete the triangle.
   
   
4. Click further points. Each one defines another triangle.
   
   
5. Press Enter to finish the TIN (or double-click).
   
   
6. Add further TINs if required.
   
   
7. Press Escape to finish.

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From existing items

Existing items can be used to form the basis of a TIN. Triangular surfaces are created with their edges connecting the origin points of the selected items.

1. Select the items for triangulation.
   
   
2. Select Create TIN [Create-3D].

If you are joining items in 3D space, e.g. point items which have a Z value (height), the result is a 3D TIN.

3. The Triangulation Options dialog allows you to configure the TIN.

   

4. The TIN can now be generated as a single TIN or as several. You can specify the number of faces per TIN. There is also an option for creating single or several MultiPolygon TINs.
   
   The options selected in the Triangulation Options dialog will depend on the data, your machine's hardware and how you will use the resulting item(s).
   
   Note: The greyed out parameter "TopoPolygons" is not implemented in SIS.

The example below shows the TIN items further enhanced by the application of a Relief theme:

You can generate a Relief theme using Add Theme [Home-Map]).

The Theme Overlay dialog will be displayed.

Select the required overlay and click Next.

The Theme Types dialog will be displayed:

In the Theme Types dialog select Relief and click Next.

See the Relief theme topic for full details of creating a Relief theme.

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From an existing grid item

Grid items can be used to create a TIN. TINs have two advantages over Grids:

• they are faster at redrawing in 3D
  
  
• some 3D formats do not support grids, so by converting your data to a TIN before exporting you can import it into other 3D viewing packages

1. Select the grid item.
   
   
2. Select Convert to TIN [Edit-TINs].
   
   
3. You will be asked for a tolerance:
   
   The tolerance value can be entered in any linear unit and SIS will convert it to metres. The smaller the number you enter, the more triangles you will get.

Tolerance

When converting a grid to a TIN, SIS tries to use as few triangles as possible, because this saves memory and takes less time to draw the 3D image.

The result is large triangles covering lots of grid points. These large triangles are accurate at their corners, but may deviate in the interiors of the triangles. There are two ways of ensuring accuracy within the triangles:

• increase the tolerance
• once the TIN is drawn, use Subdivide [Edit-TINs] 

The first method will provide more accurate data initially; the second method will make any subsequent contours added to the display smoother.

Troubleshooting If you view your TIN in 3D you may find it does not look as you would expect. Lowering the exaggeration to less than 1 may correct this.

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