Graphics Programs Reference
In-Depth Information
understanding Surfaces
As you might guess, the 3D game of connect-the-dots is a bit more sophisticated
than the 2D version. In addition, you need the result to be a 3D model rather
than just lines. To accomplish this, Civil 3D uses a computer algorithm that con-
nects the dots in the most efficient and accurate way possible. This algorithm is
known as a
Triangular Irregular Network (TIN)
.
The TIN algorithm works by connecting one point to at least two of its neigh-
bors using 3D lines. Because each point connects to two or more of its neighbors,
the resulting model looks something like a spider web made up of triangles (the
T
in
TIN
). Because the spacing between points is typically nonuniform, the tri-
angles come in many shapes and sizes, which is why it's called irregular (the
I
in
TIN
). The network (the
N
in
TIN
) part comes from all the points being connected
by lines, and the points and lines being related to one another. Figure 4.1 shows a
surface with its TIN lines visible.
FiGuRE 4.1
A surface model displayed as TIN
lines. Note the irregular triangular shapes that make up
the surface model.
There's even more to it than the triangles, though. In fact, the triangles are just
a handy visual representation of the algorithm at work, and, by themselves, they
aren't all that useful. What
is
useful is that the algorithm can calculate the eleva-
tion of any point within the area covered by the TIN model. So, even if you pick
a point in the open space inside a triangle, the elevation will be calculated. This
