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TIN components are built from input vector data.
Building a TIN
You can create a TIN all at once from one or more kinds of input data, you can create it in stages, or you can add data to refine an existing TIN. TINs are made from mass points, breaklines, and polygons. Mass points are point height measurements; they become nodes in the network. Mass points are the primary input into a TIN; they determine the overall shape of the surface.
Breaklines and hulls
TINs allow you to model heterogeneous surfaces efficiently by including more mass points in areas where the surface is highly variable and fewer in places where the surface is less variable. The example below shows mass points categorized by the height attribute.
Breaklines, shown below, are lines with or without height measurements. They become sequences of one or more triangle edges. Breaklines typically represent either natural features, such as ridgelines or streams, or built features, such as roadways. There are two kinds of breaklines: hard and soft.
Hard breaklines represent a discontinuity in the slope of the surface. Streams and road cuts could be included in a TIN as hard breaklines. Hard breaklines capture abrupt changes in a surface and improve the display and analysis of TINs.
Soft breaklines allow you to add edges to a TIN to capture linear features that do not alter the local slope of a surface. Study area boundaries could be included in a TIN as soft breaklines to capture their position without affecting the shape of the surface.
Polygons represent surface features with area—such as lakes—or boundaries (also called hulls) of separately interpolated areas.
Hulls could define the shores of individual islands in an archipelago, or the boundary of a study area.
Polygon surface feature types
There are four polygon surface feature types:
- Clip polygons, shown below, define a boundary for . Input data that falls outside the clip polygon is excluded from the interpolation and analysis operations—for example, contouring or volume calculations.
- Erase polygons define a boundary for interpolation. Input data that falls within the erase polygon is excluded from the interpolation and analysis operations—for example, contouring or volume calculations.
- Replace polygons set the boundary and all interior heights to the same value. A replace polygon could be used to model a lake or an area on a slope excavated to a level surface.
- Fill polygons assign an integer attribute value to all triangles that fall within the fill polygon. The surface height is unaffected, and no clipping or erasing takes place.
Polygon features are integrated into the triangulation as closed sequences of three or more triangle edges. Including breaklines and polygons in a TIN gives you more control over the shape of the TIN surface. To get a sense of the difference that breaklines can make in a TIN, compare the surface created from mass points alone to the surface created from mass points and breaklines. The graphic on the left shows a TIN created from mass points; the graphic on the right shows a TIN of the same area created from mass points and breaklines.
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