Analysis of GIS Data by Simple Examination - Introducing Geographic Information Systems with ArcGIS - page 394

Assessing What We Have and What We Need to
Solve the Problem
What we are going to do is find the census Blocks polygons that lie within three miles of Galbraith School
Road. But since our objective is to find population we have to first join the census table to the Blocks
table, so that the number of people in each census block will be available to us. To join these two tables,
we need to find a key field in each table whose values serve as block identifiers. In the census table that
key field is GEO_ID_L4D. In the Blocks table the key field is BLOCK2000.
23. Look at the attribute table of Blocks. In the column BLOCK2000 we have the four-digit block
number. In the Layer Properties window check out Fields. Note that BLOCK2000 is a String field
of Length 4.
24. Look at the table Census4. In the GEO_ID_L4D column you see four-digit block numbers. In
the Layer Properties window check out Fields. Note that GEO_ID_L4D is a String field of Length
________. (We will hope that it will match up with BLOCK2000 string field of length 4 when
we join the tables.) Also note that we have P001001 which is the population of each block.
Dismiss both tables.
Next we need to join the Blocks table and the Census4 table so we can combine the geographic
information of the shapefile with the tabular information in the census file.
25. Right-click Blocks and choose Joins and Relates > Join. Under “What do you want to join to
this layer?” choose “Join attributes from a table.” For Step 1 choose BLOCK2000. For Step 2
choose Census4. What's Step 3? ________________. Click Validate Join for reassurance. Click
OK. (Read the Create Index box if it shows up. Since we are dealing with large tables, let's take
ArcMap's word for it that indexing the join field will improve performance. Click Yes.)
26. Right-click Blocks, open, and examine the table. The important thing to note is that now the
population of each census block (field: P001001) is part of the shapefile table. Dismiss the table.
27. Launch the Identify tool, click on the map, set the Identify from field to Blocks, and check out
the two blocks northeast of Galbraith School Road. What are their populations? _____________,
_____________. Dismiss the Identify Results Window.
28. Start the Select Features tool with its icon and, using Shift-Click, select the same two blocks. In
the Blocks table, choose Show selected records. Verify the populations.
29. Clear all selections. (If you don't, when you export the table, as you will you later, only the
selected records will be exported!) Dismiss the table.
You might recall that, even though you have the shapefile together with the table, the connection between
the original shapefile table and the census data is pretty tenuous. All you would have to do is leave ArcMap
and—poof—no join. So set this shapefile and its expanded table in concrete using the following step.
30. Right-click Blocks in the T/C and choose Data > Export Data. From the Export Data window
browse to
___IGIS-Arc_ YourInitials \Day_Care_Data