
\section{Geographical Information Systems} 


\subsection{ Introduction to GIS} 

The United States Geological Survey (USGS) defines a 
Geographical Information System (GIS) as
{\parskip=0pt
\begin{quotation}
\noindent
   ``a computer system capable of
   assembling, storing, manipulating, and displaying geographically
   referenced information, i.e. data identified according to their
   locations.''
\end{quotation}
}

Many types of geographical data can be used:
{\parskip=0pt
\begin{itemize}
\item Elevation 
\item Satellite image 
\item Digital line graph 
\item Land use / Land cover
\item Geographic place names
\end{itemize}
}
Integrating these different data sets is a non-trivial problem.






\subsection{ Elevation Data} 

Elevation data is available from USGS as a Digital Elevation Model (DEM).
This is obtained by digitizing contour lines on USGS topographical 
1:250,000 scale maps.
The data is set up as points on a grid covering a 1 degree quadrant 
with 90m resolution. 
30m resolution data is being developed, but currently is only available
for about half the US.

Data for New York state requires about 500 Mbytes, % ??
for the entire US it is about 10 Gbytes.
This will increase 9-fold for 30m resolution data.

The US Defense Mapping Agency has 90m resolution data for the entire world,
but it is classified. The only available world-wide data has 1km resolution,
which is the same as is available for Mars!





\subsection{ Example DEM Data} 

{
\begin{minipage}[h]{3.75in}
\begin{centering}
\psfig{figure=tumwater-2d.ps,width=3.75in}
2D image, with brightness increasing with height.
\end{centering}
\end{minipage} \ \
%
\begin{minipage}[h]{4.0in}
\centerline{\psfig{figure=tumwater.dem.ps,width=4.0in}}
\centerline{3D image with shadowing.}
\end{minipage}
}

\vspace{1cm}
Digital Elevation Model (DEM) data for part of the Cascade mountains
in Washington state.





\subsection{ Satellite Image Data} 

Landsat satellite image data for most of the earth is available at 
30m resolution, in several spectral bands.  
Satellite image data at better than 1 km resolution is also available 
for Mars and Venus from NASA JPL.

This data first has to be 
preprocessed to correct for distortion caused by varying elevations,
using the DEM data, so that it becomes like a 2D map. 
Then the values from the different spectral bands must be combined
to create a realistic true-color RGB image.
Both of these pre-processing steps are quite tricky.

Once the 2D images are created, they can be used directly for a 2D
visualization of the terrain, or 
``draped'' over the polygons created by the DEM data to make a 
realistic 3D image.






\subsection{ Example Satellite Image Data} 

\vspace{1em}

\begin{figure}[h]
% \begin{picture}(200,200)
%%BoundingBox: 211 313 401 478
\centerline{\psfig{figure=capecod.ps,width=4in}}
% \end{picture}
\end{figure}
\centerline{ Landsat image of Cape Cod}





\subsection{ Digital Line Graphs} 

Digital Line Graphs (DLG) are made up of vector data marking objects such 
as roads, waterways, city, county, state and country borders. 

\vspace{1em}

\begin{figure}[h]
\centerline{\psfig{figure=dlg.ps,width=4in}}
\end{figure}
\centerline{ Digital line graph of roadways}





\subsection{ Other Data} 

{\bf Land use / Land cover} data is available from USGS.
Areas are classified into 92 categories of land use (human activity such
as residential, agricultural, etc) and land cover (vegetation, water, etc).

{\bf Geographic place name} data is available from USGS, listing
all known places, features and areas in the US that are identified by
proper names. The database contains over 2 million place names
referenced by geographic position.

\medskip

Digital line graph, land use / land cover, and
and geographic place name data could be 
overlaid onto satellite and elevation data. 




