Some Concepts That Underpin GIS - Introducing Geographic Information Systems with ArcGIS - page 58

26. Dismiss the Identify Results window. If you right-click on the string Fire_Hydrants in the left
pane, you receive a menu of options. Of interest to us now is Properties. Click that. A Feature
Class Properties window appears. Under the General tab, you see that the feature type is Point
and that the data set is stored in high precision (of which more later). Click the Fields tab.
Here you see the names of all the attributes and the types of fields that are used to contain
the data. For example, click on the row that says COLOR to select it. Notice that COLOR is a
Text field of length 9, meaning that it uses 9 bytes of memory. (You will also sometimes find
“length” of a string referred to as “width.”) Now look at Y_COORD. It is listed as Float, meaning
it is a floating decimal point number. Since you previously saw that the field is stored in high
precision it will occupy 8 bytes of storage. (A Float number consists of two parts—a mantissa
and an exponent—which allows both high precision (lots of significant digits) and large range
(very large and very small numbers). (If this doesn't mean anything to you now, don't worry
about it.) Look at FLOW-RATE. It is also a numeric field, but it is an integer (a Long Integer
can store an integer somewhat larger than two billion) and may, therefore, be stored in a
simple binary format. It uses 4 bytes of storage for the flow rate of each fire hydrant. Dismiss
the window.
Looking at GeoGraphics
27. On the Geography toolbar, click the Zoom In icon (a magnifying glass with a “+” sign in it). Copy
the text in the ToolTip or Status Bar here: ____________________________________. Click on
the upper-left hydrant. Note that it moves the center of the window and the distance between
it and its neighbors has increased. You have zoomed in on the layer—not that you will see any
more detail in this particular image, but you get the idea.
28. Notice that although you have zoomed up on the image, the symbols representing the features
did not get any bigger. So, this zooming action is somewhat different from looking at a paper
map with a magnifying glass, which would increase both the distance between the points and
the symbol size.
29. Click again on the northwest hydrant and observe the results. Click between the hydrant and
its neighbor to the south. Click the Full Extent icon (on the Geography toolbar) to restore the
view of the entire layer and bring all hydrants back into view. With the Zoom In tool active, drag
a box around the middle three hydrants in the middle column and observe. The lessons: You
can zoom in by clicking on a point and also by dragging a box. The image is always re-centered,
either at the point clicked or the center of the box.
30. Click on the “hand” icon on the Geography toolbar—this is the Pan tool. Check out its
function with the ToolTip. Move the cursor over the middle point, and drag that point to the
left side of the pane. When you release the mouse button, you will see that the focus on the
image has been shifted (“panned”) to the right. The window center is now in between the
two columns of hydrants. Click the Full Extent icon. Now click the left-pointing arrow on the
Geography toolbar. Note the results. Continue to click it until it “grays out.” Now click the
right-pointing arrow several times. By using these arrows you can return to previous or later
levels of zoom or pan.
A “thumbnail” is a small sketch that can aid you in recognizing the contents of a data set. You make a
thumbnail sketch in the step that follows.