ition has 
aditional 
roach is 
zia Tech 
in flight 
ays been 
ning fast 
allowing 
interface 
ral 
er 
of 
ion 
IVE 
started to be used in practical GIS 
applications. Simulation techniques 
developed over a number of years by the 
flight simulation industry using specially 
designed hardware, are now becoming viable 
on general purpose hardware. 
The synergistic combination of these two 
worlds into a truly interactive’ virtual’ GIS is 
now becoming a reality. 
II. VISUALIZATION IN CURRENT GIS 
SYSTEMS 
Current generation GIS systems generally 
view, analyze, and perform functions on 
spatial data (vector/raster) is a top down two 
dimensional manner. Complex analyses may 
be performed on the spatial data variables 
using weighting indexes, proximity functions, 
and a variety of multi-layer models. Each of 
the source layers of information as well as 
the analysis result is usually shown as seen 
from above. In many cases visibility of one 
part of the spatial data set from another 
position within the data set becomes as issue 
that must be considered. Example questions 
might involve whether a strip mine area can 
be seen from a major scenic highway, or 
whether a company’s view from its building 
will be occluded by the proposed erection of 
another building between it and the ocean. 
Visibility is generally handled in one of two 
ways, terrain masking, and perspective 
viewing. For terrain masking, a number of 
line of sight calculations are made in all 
directions from the viewers position. A line 
of sight calculation normally involves the 
reading of the terrain and feature data along 
a particular direction from the viewer. If 
there are locations along this line of sight 
that are higher in elevation than other points 
along the line of sight but farther away, then 
some of the terrain will not be seen by the 
viewer. These seen and not seen areas in all 
directions create a terrain mask in which 
areas that can be seen from the viewer 
location are differentiated from areas that 
can not be seen. The areas that can not be 
seen are said to be masked by the terrain. 
115 
This is a two dimensional representation 
of a visibility index. 
Perspective viewing allows the calculation 
and display of exactly what will be seen 
from a particular viewpoint. The 
perspective presentation of spatial data 
mimics the way that the eye perceives 
what can be seen. A view can be 
calculated that presents a very natural and 
interpretable image to the GIS user. The 
image, depending on the quality of the 
input image and GIS data, may show 
exactly what the viewer will see from that 
location. GIS systems which include 
multispectral imagery along with 
topography and other GIS layers can be 
used to show for example, what can be 
seen from the top of Stone Mountain, 
near Atlanta if one looks to the north 
west. Images from satellite and aircraft 
can record information in the visible, 
(red, green," blue) ^parts of the 
electromagnetic spectrum. Thus, when 
these images are draped over the 
surrounding terrain, the view closely 
replicates the human's view from that 
location. Atmospheric haze can be added 
to the generated images, or one can look 
at the view as if no haze were present; for 
example, after a fall cold front passes 
through. 
Perspective views have been generated for 
a number of years to show the view from 
a given geographic position. In general, 
however, perspective views are only used 
as a presentation method for final results 
and are not used in the analytical process 
itself. One of the principal reasons for 
this is the time it takes to generate a 
realistic perspective image. We will show 
in this paper that if this rendering can be 
done in near real time with modern 
technology, the perspective view will 
become the most natural interface for 
many GIS applications.