
\begin{picture}(320,343)
\psfig{figure=clipping.eps,width=320pt,height=343pt}
\end{picture}

   Since the goal of this terrain rendering project is allows high school
   teachers and students to do fly-bys on the terrain data set,
   we can assume the world view volumn of the user is bounded by
   simple flight characteristic.  In other words, the user will most 
often be
   looking level and have a level flight path.  By projecting the view volumn
   onto a 2D plane, we can identify a number of grid point that may been seen
   by the viewer.
  
   As show in the following figures, a viewer
   turned 4 degree to the right.  The turning rate is 240 degree per minutes,
   40 revolutions per hour.

\begin{minipage}{4.46in}
\begin{picture}(321,344)
\psfig{figure=image1a.ps,width=321pt,height=344pt}
\end{picture}
\end{minipage}
\
\begin{minipage}{4.46in}
\begin{picture}(321,344)
\psfig{figure=image5a.ps,width=321pt,height=344pt}
\end{picture}
\end{minipage}

   The two pictures shows that two of the squares are deactivated on the 
right
   side and two new squares are activated on the left side.
   As a result, the locality of the viewer position can be exployed to
   elliminate the unseen polygons.  Furthermore, with a coarse grid, we can
   determine, which part of the data is needed in core.


