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At this point you can no longer use the Getting to Know ArcView software; it does not include Spatial Analyst. To run Spatial Analyst (SA), launch ArcView and check off the "Spatial Analyst" option in the File|Extensions menu item (available from any Project or View window).
A digital elevation model (DEM) represents a continuous field of elevations over the earth's surface. Normally, DEMs are represented by regular grids of elevations, although in some cases they may be represented by triangular interpolated networks (TINs). From now on we will assume DEMs are represented by grids.
A DEM for Haiti and the Dominican Republic is available here in zip format.
Create a new View document. Use the Add
theme button 
to open the DEM as
a grid theme. The two most common mistakes made at this point
are: (a) not specifying a grid data source in the Add Theme
dialog--ArcView can add grids as images, but it's not the same thing--and
(b) failing to load the Spatial Analyst extension first, in which case the
Add Theme dialog will not offer grids as a possible theme data source.
ArcView's default legend for a DEM grid theme is a nine-class equal interval
classification. The colors are randomly chosen from ArcView's predefined
color schemes. The No Data class is always shown.
Make the No Data class transparent using the Legend Editor. This lets you focus on the actual data and permits any themes drawn beneath the grid to show through wherever the grid has no data.
Zoom to an interesting part of the grid.
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In the zoomed view at the right, individual cells are apparent. Each cell contains one number--an elevation--corresponding to the center of the cell.
Use the identify tool 
to explore the elevations. (Activate the theme if necessary.)
Note how ArcView provides complete information about grid data: X and Y
coordinates (for the cell centers) on the left, elevation value on the
right.
Explore the elevations in more detail by
creating contour lines with the contour tool 
.
Because it is a tool, you must apply it to a location on the
grid. (Nothing will happen if you apply it to a location with a No
Data value.) The ArcView status bar will temporarily report the
contour value. Change the contour's appearance by selecting it and
opening the symbol window (use the ctrl-P shortcut).
A contour is a line tracing out locations of constant elevation. There may be more than one contour for any elevation; the contour tool computes only the contour passing through the point of application.
Use the new histogram button to create a chart of elevation frequencies.
In the histogram, each bar corresponds to a classification in the legend. The values on the Y-axis are counts. Since they count cells, and each cell represents approximately the same area (because this grid covers a small area of the earth and uses a projection with relatively little distortion), the values on the Y-axis can also be interpreted as relative areas occupied by each elevation class.
To see the difference between grid themes and image themes, use the Add Theme dialog once more to add the same dataset to the view, but this time as an image data source.
Look at the differences between the two themes:
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The image theme has no legend in the table of contents. |
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The image is shown in grayscale by default. It appears more "smooth" than the grid. |
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When the image theme is active, many of ArcView's tools and buttons are disabled, including the identify and contour tools and the histogram button. |
These differences derive entirely from how ArcView presents the underlying source data. A grid contains data meant to be used for further analysis. An image contains data interpreted as colors on a map. It is intended for use only as a background or reference theme, not for further manipulation or analysis.
Obtain vector data for Haiti and the Dominican Republic. A good place to start is the ESRIDATA/World folder which contains country boundaries. Add these data as a theme to your Haiti view. Set the View's projection properties to match the grid's projection (see the metadata.) How well do the raster and vector data match? What are some reasonable possible explanations for any mismatch you see?
The Haiti image does have a Legend Editor, even though no legend appears in the Table of Contents. Activate the Legend Editor for the image view and experiment with its options.
Evidently, the Legend Editor creates 256 classifications (0, 1, ..., 255) and symbolizes each. However, you know that the Haiti DEM contains elevations from almost 0 to over 2,000 meters. How does the Legend Editor associate colors with elevations to produce an image in the view?
With the Haiti grid theme active, select the Surface|Create contours menu item. (If you have an earlier version of Spatial Analyst, this item will be in the Analysis menu.) Fill out the dialog--what it does should be evident. How are these contours different than the ones produced with the contour tool?
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