The next two pages familiarize you with how map data are stored for GIS. Storing map data in a computer is more challenging than storing simple text information that is non-spatial. The text you're reading is a good example of non-spatial information. Even the text created for Web pages is relatively simple since there is a universal text "language" called ASCII that any word-processor or Web browser can understand. Map data, however, must store complex information about location. GIS packages have developed two basic approaches to storing geographic data. A method of storing data in a computer is called a data structure. The two data structures used in GIS are commonly called vector and raster. This page examines vector data. The next page will add some raster data for comparison.
Imagine how you would draw a map of a road using a pencil and paper. You would probably draw a series of lines that makes up the road. The basic approach of vector GIS data is very much like this. Vector data use points, lines, and lines that close to form polygons in order to represent geographic objects such as light posts, streams and counties. Most GISs use only straight line segments, so what appears to be a curve in a GIS is actually made up of many small, straight lines that imitate the curve.
You have actually already seen vector data using ArcView before. But let's add some vector data and examine it more closely. You will work with some data that show aspects of the Sonoma State campus.
First, add a data layer that consists only of points.
Choose View-Add Theme 
and navigate to the data directory for this
exercise, which in the GIS Lab is H:\Class\280\Exer5 (if
you're outside the GIS Lab, change to wherever you installed the
exercise data). Among the listed themes should be Trees.
Click this theme to highlight it and click OK. After
the theme appears in the view's legend, click on its check-box to
draw it.
Next add a line theme. Choose View-Add Theme again and add the Roads theme.
Turn on the check-box in the view to draw it.
Finally, add a polygon theme with View-Add Theme and add Bldgs, and draw it in the
view. This theme outlines a few of the buildings around
campus.
You can change any of the colors for the themes you've just added using the Legend Editor, which you can bring up by double-clicking on any theme in the legend (see the page from Exercise 1 on Modifying Theme Display for details if you don't remember how).
Examine the differences between these three themes. You
can use the zoom tool 
to zoom in and examine the data more closely. As
you zoom in, the lines will remain narrow lines, even if the
roads they represent would have some true width of say 40
feet. Similarly, the points will remain small dots even if
you zoom in tightly. The polygons will get larger as you
zoom in, of course.
Points, lines and polygons are used in all GIS programs. But GIS programs differ in some of the particular ways they store those vector objects. You may find a friend who has GIS data to share, but she/he might use a different GIS program. Your GIS software may not understand the format of those other data. Often a translation program is available to convert data between particular GIS formats, but not always.
ArcView can directly read at least three formats of vector GIS data:
There are lots of details about file formats that need not concern us here, such as how to distinguish these data from each other in ArcView, or what kinds of changes you can make to each from within ArcView. If you need more information about file types used by ArcView, consult the Help pages in ArcView.
Bryan Baker, Sonoma State
University, bryan.baker@sonoma.edu
Updated 17 February 1999