Analysis of GIS Data by Simple Examination - Introducing Geographic Information Systems with ArcGIS - page 384

7. Just to reinforce the idea that ArcMap does on-the-fly projections, add the US_20021105_MUAs
feature class. Make its color symbol distinctly different from MUAs_Albers. Notice that it lies
precisely on top of the Albers projection. Click it off and on. Flip back to ArcCatalog to see it
both in its decimal-degree form and in its Albers projected form. Switch back and forth between
the two. Close ArcCatalog and go back to ArcMap. Remove the US_20021105_MUAs feature
class.
Examine the attribute table of MUAs_Albers. As you determined previously, the units are
meters. So, the calculated areas are in square meters—not very useful numbers because the
areas are so large. Suppose we prefer square miles. A square mile contains about 2,589,988
square meters.
8. Under Table Options, click Add Field. Call it Area_SQMI. Make it a floating-point number.
Click OK. Right-click the column and activate the Field Calculator. Ignore the warning—if
you make a mistake, you can just do the calculation over. What expression should you
use in the Field Calculator? ____________________________________ Apply it. Run
Statistics. Is the largest area about 13,678 square miles? (If not, look at your expression
and correct it.)
9. How many square miles are there of Medically Underserved Areas? _________ If the area
of the coterminous (“lower 48”) United States is about 3 million square miles, what
percentage of the country fits the “medically underserved” designation, according to these
data? ________
Geographically Comparing Two Datasets
10. Create (by using the Project tool) HPSAPrimaryCare_Albers from US_200301_HPSAPrimaryCare.
Add it to ArcMap if that doesn't happen automatically.
Since the two data sets (MUAs_Albers and HPSA_PrimaryCare_Albers) tend to get at the same
concept—places where there isn't sufficient medical care—it might be interesting to see how they
compare.
11. Start by examining the layers visually. Make MUAs_Albers red and HPSA_PrimaryCare_Albers
green. Then observe what happens when you turn them off and on. Finally, leave them
both on.
One measure of how areas compare is whether or not there is overlap between them—that is, do they
intersect or not?
12. Under Selection, clear any selected features. Use Selection > Select By Location. Select
features from MUAs_Albers (make it the target layer) that intersect HPSA_PrimaryCare_Albers
(which will be the source layer). See Figure 6-18. Click OK.