Training is an essential part in military 
  effectiveness~\cite{mil_train1,mil_train2}. Generally, the more realistic 
  training is to the soldier, the   better prepared that soldier will be when an 
  actual scenario arises~\cite{mil_train3,mil_train4}. Creating a   realistic 
  training environment requires significant preparation in planning, 
  coordination, time, money, etc. Resources such as equipment, fuel, vehicle 
  parts, and maintenance time are used extensively, making it difficult to 
  provide consistent, thorough 
  training for every one.

  The advent of computer technology has significantly impacted the way the
  military trains its personnel. Specifically, the field of computer graphics
  and visualization has become crucial for the creation of realistic training
  environments. With modern technology, the military can train its personnel in
  computer simulated situations, thus alleviating much of the need for travel
  and equipment transportation. Also, personnel can work their way through a
  simulation several times, honing their skills while using minimal resources.

  Visualization techniques help to establish realism, but real-world geographic
  data is also necessary to achieve a fully immersive and realistic simulation.
  The geographic data represents a true-to-life landscape in the simulated
  environment. The data can be visualized via computer graphics and virtual reality
  techniques. The end goal is to give personnel the feeling they are at the
  real-world location, even though they are actually in a simulated environment.

  In order to obtain accurate information about a location, specialists in
  geographic and geological disciplines must first visit and thoroughly analyze the
  location. This is viewed as a necessary expenditure of time and resources.
  However, situations occur where the resources and access necessary to collect
  ample data is simply not available. This lack of data can endanger missions,
  especially when specific training, such as helicopter flight, requires
  accurate knowledge and representation of an area.

  There is some data that can be gathered and then later analyzed off-site. One
  such piece of data is a Digital Elevation Model~(DEM)~\cite{dem_def}. The name
  has pragmatic significance in that it consists of spatially explicit digital values of
  height data. Other important data are Geographic
  Information Systems (GIS) vector layers~\cite{gis_layer_def}. GIS vector layers
  are maps (i.e. represent a location) that store two-dimensional
  shapes~(polygons) with attributes. Attributes can be assigned from a variety
  of sources including satellite imagery, site visits, etc. DEMs are also
  referred to as GIS raster layers, but from this point forward, any mention of
  a GIS layer refers to a GIS vector layer.
  
  DEMs and GIS layers have been combined before, but this paper proposes a 
  method for merging the two so as to be efficiently displayed in a three-dimensional
  virtual environment.
  % What is lacking now is a way to combine DEMs and GIS layers in an immersive, three-dimensional environment.
  Such an environment would give 
  specialists better interaction with the data being visualized. It is also 
  believed that an environment such as this would reduce the amount of time and 
  other resources necessary to perform a thorough analysis of geographic sites.

  The remainder of this paper is structured as follows. Section~\ref{sec:background}
  presents background information on geographic applications, computer graphics, and
  immersive virtual environments.
  Section~\ref{sec:prop_meth} presents our method for combining DEMs and GIS layers
  and displaying them in a manner conducive to fluid interactivity. We present the results
  of our methods in Section~\ref{sec:results} and state our conclusions in
  Section~\ref{sec:conclusions}. Section~\ref{sec:future_work} presents several
  possibilities for future work on this topic.