28 Jan 2000
Yesterday I had a request for help with locating information on population distribution for Rockbridge County --did I know of any maps, or any way to make a map, to assist in discussion of a location problem for a recreation center (site it so as to be maximally accessible to its intended patrons, etc.). I couldn't think of any existing map that met the criteria (topographic maps might approximate, but they're out of date and difficult to use as summaries), so I started thinking how such a map might be constructed with data available to us.

I knew that we had TIGER coverage for Virginia, tucked away in a folder on Miley (Va_tiger), and it turns out that one of the layers is outlines of the census blocks(22 for the County, plus Lexington City and Buena Vista City):

TIGER doesn't provide any direct information on population distribution, though I flirted with using the pattern of roads as a proxy for population distribution --obviously, where no roads, not many people, but that's not very useful:

I reasoned that we could probably get the numbers of households (or children under 15, or mobile homes, or whatever) for those census blocks, and those data would provide the basis for constructing maps that would give a coarse idea of distribution of population within the county.

How to find the requisite census data? We probably have the little numbers in the 1990 Census reports, but copying them out seemed too arduous if they could be found online. I tried www.census.gov and (after a bit of exploring) found venus.census.gov/cdrom/lookup and eventually to venus.census.gov/cdrom/lookup/949077899 which allowed me to choose a specific block group within Rockbridge County, and then specify tables to retrieve (total population, households, etc.). I could see the results as a web page, or opt to retrieve the data in tab-delimited form for import into a spreadsheet. So the data acquisition problem was solved fairly easily. Now how to make the maps, associating the various data with the correct polygons on the TIGER map in ArcView?

I knew that I would have to edit the attribute table of the TIGER layer that shows the block groups, adding values for the variables I had retrieved. Here's the step-by-step procedure I followed:
  1. Select the TIGER layer for the census block outlines
  2. Open the Theme Table to see the data:

  3. From the Table menu, choose Start editing:

  4. From the Edit menu, choose Add field:

  5. Assign an appropriate name in the Field Definition box ("Total pop" in this case):

  6. A new field is added, and you can type in values by clicking on the I-beam Edit button () and then clicking in the cell you wish to add a value to.
  7. Repeat the 'Add field' procedure for any other fields
  8. From the Table menu, choose Stop editing:

    and save the result with the same name as the TIGER layer and in the same folder (there must be a way around this, but at the moment I don't know what it is). You'll be in effect replacing the .dbf file of that layer with another .dbf, augmented with your added data.

  9. I found that I had to CLOSE the project, choosing to save it, and then REOPEN it to see my changes in the attribute table, and thus be able to access the added values in order to map them.

The procedure above led me to these maps of Rockbridge County:

Population density per square mile for the county census blocks from the 1990 Census:

And another showing households per square mile for the same census blocks:

I then addded data for Lexington and Buena Vista (the white spaces in the maps above) by

  1. adding Lexington and Buena Vista census block outlines

  2. choosing from the Extensions menu (1) Spatial Analyst and (2) Geoprocessing Wizard

  3. selecting Merge Themes Together from the Geoprocessing Wizard choices:

  4. combines the 3 to create a map including Lexington and Buena Vista populations:

    as a new theme ('merge.shp'), the attribute table of which I edited to add the data for the two cities (and stopped editing, saved, exited, reentered...)

Here are the two resulting maps:

Persons per square mile:

And Households per square mile: