Some Concepts That Underpin GIS - Introducing Geographic Information Systems with ArcGIS - page 55

Deriving Information from the Table
Having attribute information in table form allows you to obtain statistics, to search for specific text strings,
and to sort the information.
11. You can find out the average of the values of a column along with the count of the number
of values, minimum, maximum, sum, and standard deviation. 19 To do so, you right-click the
column heading to bring up a menu of choices from which you choose Statistics. For example,
select the column FLOW_RATE by placing the mouse pointer over the heading of the column
(the pointer becomes a down-pointing arrow) and right-clicking. (You may have to use the scroll
bar at the bottom of the table to find the FLOW_RATE column.) Choose Statistics from the
drop-down menu. You see information about the values in the column from the Statistics
of Fire_Hydrants window. There are 21 values. The minimum flow rate is 250. What is the
maximum flow rate? ________. You also receive a frequency diagram that shows the number
of hydrants that have each given value (e.g. there are four hydrants with a low flow rate, nine
with a moderately high rate, and eight with a high rate). If all the hydrants were turned on
at once, how much water would flow? __________. The mean (i.e., average) and standard
deviation, although they probably don't mean much in this situation, are about 356 and 54,
respectively.
12. Once you have the Statistics window up, you can determine the statistics of other columns
by selecting the column name in the window's Field drop-down menu. What is the average
y-coordinate (to two decimal places)? ___________________. Dismiss the Statistics
window.
Sorting the Records
13. Suppose now that you wanted to have the list ordered so that all the westernmost hydrants
(those with the smallest x-coordinates) appeared at the top of the table. Click the X_COORD
column heading to make it active. The column becomes highlighted in blue/cyan/
turquoise—take your pick. Now right-click the column heading, then sort the values in the
column by picking Sort Ascending from the drop-down menu. Notice that the values of
the column are now in order from smallest to largest.
Note that when a value in a given record is moved, say during a sort, the entire record moves with it. That
is, all of the values in a given row stay together (including OBJECTID), regardless .
(If you are familiar with spreadsheets (e.g., Excel), you should note that this is different behavior from
results you may receive with that software.)
If you wanted the Y_COORD column values to be a secondary sort (sorted “within” the X_COORD values),
you could select both columns by holding down the CTRL key while clicking). (An example to clarify the
terminology: in a telephone directory the first names are sorted “within” the last names.) When two or
more columns are selected, the leftmost one will contain the primary sort, the next selected on the right
will contain the secondary sort, and so on.
19 You are probably familiar with these concepts. A discussion of basic statistical measures may be found in the
Overview of Chapter 6.