users can access all available data sets, they are encouraged to 
supply UNEP with other data sets in exchange for utilizing 
the data already in stock. This approach to sharing data helps 
keep data cost down for everyone and speeds processing. 
Data acquisition can be a very time consuming and expensive 
task and often an analysis requires immediate action. 
Terrain Data 
Besides satellite data and aerial photos, 3D terrain 
visualization requires elevation data. Digital elevation models 
(DEMS) have been segregated into a category of their own in 
the past, but more and more users are realizing the benefits of 
incorporating terrain data into a GIS. The products you can 
create from DEMs add a new level of meaning to any 
analysis. 
Some of the layers you can create using elevation data include 
slope, slope aspect, shaded relief, contour and perspective 
views. These topographic data are essential for studies of 
trafficability, route design, non-point source pollution, 
intervisibility, siting of recreation areas and more. Terrain 
data can also be used in models and in classification routines. 
They can, for example, be a key to identifying wildlife 
habitats that are associated with specific elevations. Slope and 
aspect maps are often an important factor in assessing the 
suitability of a site for a proposed use. 
DEMSs can be purchased from the U.S. Geological Survey 
(Reston, VA) at 1:250,000 scale for most areas of the United 
States and at 1:24,000 scale for some areas. DEMS can also 
be created through traditional photogrammetric triangulation 
techniques or they can be created in the GIS environment 
with softcopy photogrammetry software. Softcopy 
photogrammetry eliminates the need for expensive and 
complicated instrumentation. This means that elevation data 
can be created for virtually any study area. 
The ERDAS Digital Ortho module (ERDAS, Inc., Atlanta, 
GA) is an example of a softcopy photogrammetry system. 
Based on a user-friendly menu and prompt structure, the 
software carries the user through the process of generating 
DEMs. These DEMs can be created from overlapping aerial 
photographs or from SPOT satellite stereopairs. Using 
information that comes with the aerial photographs or in the 
SPOT header file and user-selected ground control points, the 
coordinates of the DEM are calculated through 
photogrammetric triangulation. Then interactive matching and 
densification processes are used to calculate the output DEM 
surface at a user-specified resolution. 
By draping aerial photos, satellite images or a GIS analysis 
onto a DEM, users can create a three-dimensional perspective 
view. Based on the position from which the view is 
calculated, the graphic can look like the viewer is standing in 
or hovering above the landscape. Perspective views are 
extremely effective in showing before and after scenarios 
simulating the effect of proposed development or possible 
land use changes. When several 3D views are combined, they 
can simulate travel through a landscape. 
MEETING USER REQUIREMENTS 
As hardware has evolved to handle larger and more complex 
data sets, so have software packages. GIS has become one of 
the most rapidly growing segments in the computer industry. 
Once dominated by a few companies such as Environmental 
Systems Research Institute, Inc., (ESRI) and ERDAS, Inc., 
there are now a proliferation of software packages on the 
market. As GIS applications broaden, products must be 
designed to be flexible. Along with appropriate functionality, 
these packages must be easy to use. Today's GIS analyst 
may be an expert in real estate, insurance or civil engineering, 
but not necessarily an expert in GIS. 
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Users have particular problems that must be attacked from 
specific points of view. Their findings must be presented in a 
way that both analysts and policy makers can understand. 
The old cliche that a picture is worth a thousand words seems 
perfect for GIS. If a static picture is worth a thousand words, 
how many words is a 3D perspective view worth? What 
about a 3D view that can be changed as parameters change? 
Users must be able to query data in three dimensions by 
geographic region or by a particular theme. "Show me all 
possible sites where there are both Oak and Pine trees with a 
body of water not more than 300 meters away and with an 
elevation of at least 2000 feet that are visible from the 
proposed parkway." Visual impact is no longer a guessing 
game. It is now possible to see exactly what the new parkway 
will look like and what the passengers driving along the 
parkway will see. 
These same techniques were applied in Operation Desert 
Shield and Desert Storm. Dedicated military and civilian men 
and women used a GIS to produce some unique map 
products under very adverse conditions. The success of these 
endeavors has created a renewed interest in satellite imagery 
in other military applications such as Mission Planning and 
Special Operations/low intensity conflict. Satellite images and 
aerial photographs are invaluable in providing analysts with a 
current and accurate view of the terrain, creating base maps 
were none existed. These images can be input into a GIS to 
model possible effects of proposed actions before any steps 
are taken. 
The Graphical User Interface 
The emergence of the graphical user interface (GUI) has 
shortened the learning curve tremendously. Users do not 
have to learn a series of commands, or the order in which 
programs must be run. They can simply point and click. The 
first ERDAS software package was progressive in its use of a 
menu-driven set of image processing and GIS programs. 
Each program contained prompts with intelligent defaults to 
help guide both experienced and novice users. This 
environment was carried through the Version 7.5 software 
package and is still being used by thousands worldwide. 
However, the newest release, ERDAS IMAGINE, is based 
on the X Windows GUI. Now processes can be 
accomplished in any order and at any pace. 
Other companies are also embracing the GUI. With the 
popularity of GUI-based word processing and spreadsheet 
packages, users are demanding the same ease of use from 
GIS manufacturers. And since there are certain de facto 
standards in GUI development, users can easily move from 
one package to another. 
ftware Functionali 
Beyond the user interface, users are demanding more 
functionality. The applications of GIS, both locally and 
globally, are requiring that vendors cater to a much larger 
audience. A regional planner in California may use GIS to 
plan new subdivisions and a natural resource analyst in 
Norway may use GIS to study the effects of global warming 
on local vegetation, but both of these users require similar 
tools. The order in which they use the tools is how the two 
analysts differ. This is where the flexibility of a software 
package is tested. Analysts all over the world must be able to 
tailor the software to their particular niche. The sequence in 
which they each use these same tools must be saved so that 
others in the field can benefit from their research. In some 
cases, as with petroleum exploration companies, these steps 
are proprietary information. But, as with the data sharing 
concept used by GRID, software solutions can save valuable 
research time. ERDAS IMAGINE is designed so that scripts 
can be written for a particular application such as forestry, 
urban planning, mineral exploration or resource management. 
These scripts will be written by experts in the respective 
fields and will contain the common steps used to derive 
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