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ESP 178 Applied Research Methods
Mapping Census Data with ArcGIS (based on an exercise created by Richard LeGates of San Francisco State University)
Overview
The purpose of this exercise is to familiarize you with the methods of downloading and processing Census data from the internat for use in GIS. In this exercise, you will download data on bicycling to work from the US Census Bureau website, download Census Tract TIGER data for Yolo County from ESRI, process the data for use in ArcGIS, and join the data to create a thematic map of bicycling to work in Yolo County.
Step 1: Downloading and Processing Census Data
Opening the website of the US Census Bureau
Census data can be found at many website, but the website of the US Census Bureau is the easiest and most reliable source for beginners.
Open an internet browser and go to www.census.gov A bar on the left side of the page will list navigation categories. Click on American Factfinder. This takes you to the American Factfinder web page, which has many links to charges and tables with aggregated data for the US. We want data that’s downloadable for GIS, so click on Data Sets then Decennial Census on the left bar. The Data Sets page lists the different summary files available for Census 2000 data. Note that the Summary Files 1 and 2 (SF1, SF2) contain 100% data, while SF3 and SF4 only contain sample data. We want to look at sample data, so click on Census 2000 Summary File 3 (SF3) – Sample Data then Detailed Tables.
Selecting Geography
You should now be viewing the Select Geography page, where there are several options under “Choose a selection method.” List, the choice currently displayed, will allow you to acquire data for only one specific Census Tract (or whatever aerial unit you choose) at a time. This would make it very tedious if you wanted to download Block-level data for all of California. Geo within Geo allows you to select any or all smaller units within a larger unit. For this exercise, we are interested in Census Tract within Yolo County.
Click on the Geo within Geo tab at the top of the page. The first two menus allow you to select the aerial units. In the drop-down menu under Show me all , click on Census Tracts.
For Within , County. The next menus allow you to select the geographic areas. For Select a state , select California. For Select a county , Yolo County. For Select one or more geographic areas …, select All Census Tracts. Click the Add button to move your selection down to Current geography selection. Click Next.
Selecting and Downloading Data Tables
You should now be viewing the Select Tables page, where you choose which data tables to download. The current selection method is Show all Tables, which allows you to browse through all the tables in the order of their name. If you know what data you’re looking for, but not the name, this method can be tedious.
Click on By Subject to view a more logical outline. Under Select a subject… , find Economic Characteristics , then select Commuting (Journal to Work), then click the Search button. Under Select one or more tables… , select P30 Mode of Transportation to Work for Workers 16+ years. Click Show Result.
You should now be viewing a page called Detailed Tables, which displays a table with a row for the data variable and a column for each Census Tract. This view does not make a lot of sense – notice that you need to browse through several pages to view all the data. Luckily, the data will be transposed when we download it.
On the blue bar below the page title, click on Print/Download and then Download. A new browser window will open with downloading options. Click the radio button for Microsoft Excel near the bottom. Click OK. [Note: If your computer beeps and nothing else happens, your computer may be blocking the download. Look for a message just below the internet address bar] Choose to save the file in the folder Z:\handy[your name]; to create this folder for the first time, click on the new folder icon, make sure the right path is indicated, and add the folder name. When downloading is completed, click on Close.
Processing Census Data
Now that we have downloaded the census data, we need to process it so that it can be used in ArcGIS.
Go to the location where you saved the Census output.zip. Double-click the file to open it with WinZip. Extract the contents to Z:\handy[your name].
On the Download Census 2000 TIGER/Line Shapefiles page, we now have the option to select by county or layer. Select Yolo in the Select by County drop-down and click the corresponding Submit Selection button. The next page presents many options for shapefiles to download. We just want one. Check the box for Census Tracts 2000. Scroll to the bottom of the page and click the Proceed to Download button. On the Your File is Ready page, click the Download File button. Save the file to Z:\handy[your name], then close the window. Go to Z:\handy[your name]. Double-click the zip file you downloaded, which should be named at_tigeresri…zip, to open it in WinZip. Embedded within this file is another zip file called trt0006001.zip. Double-click on it to view the contents in another WinZip window. Highlight all three files and click the Extract button. Enter Z:\handy[your name] in the Extract to field. Click Extract and then close the program.
Step 3: Joining Census and Geographic Data
Now that we have the census and the geographic data, the next task is to join them.
Open ArcMap (look for ArcGIS in the programs on your computer). Check the “a new, empty map” button.
Click the Add Data button. In the add data dialog box, browse to Z:\handy[your name].
Hit the CTRL key. While holding it, select biking.dbf and tgr06001trt00.shp. The two files should be added as two layers. (If you don’t see biking.dbf as a layer, click on the Source tab at the bottom of window at the far left). Click Add.
In the window at the far left, called the Table of Contents , right-click on biking and click Open. This should open the attribute table.
Now let’s look at the data in biking.dbf to see what we can base our join on. GEO_ID2 contains a series of 11 numbers. The first 5 of the 11 numbers are the same in each row, 06113. These numbers indicated the state and county. California is 06 and Yolo County is 113. The last 6 digits indicate the Census Tract and are the meaningful ones for our purpose.
Now we need to make sure there is a corresponding field in our Tract shapefile. Open the attribute table of tgr06001trt00.shp by right clicking on it and selecting Open Attribute Table.
Notice that SFID looks just like GEO_ID2 in the biking table. We will use these two fields to join the table to the tract layer.
To join census and geographic data, right-click on tgr06001trt00 in the Table of Contents window, click Joins and Relates -> Join… Choose the following options and then click OK: What do you want to join to this layer? Join attributes from a table
- Choose the field in this layer that the join will be based on: STFID
- Choose the table to join to this layer or load the table from disk: biking
- Choose the filed in the table to base the join on: GEO_ID
To make the join permanent, we need to export the data to a new shapefile.
Right-click on tgr06001trt00 in the Table of Contents window and click Data -> Export Data. Name the file tract_biking Click OK Click Yes to add the map to the current layout.
Step 4: Making a Thematic Map*
This section is the culmination of all the labor so far. We’ll use the data we’ve downloaded to create a thematic map of density of housing units in Alameda County.
Right-click on tract_biking layer and click Properties. In the Layer Properties window, click on the Symbology tab. Under Show on the left of the window, click on Quantities and then select Graduate Colors. Under Fields, click on BICYCLE in the Value drop-down menu. Click OK.
Notice that this map shows the number of workers bicycling in each area. This might be what you’re looking for, but more interesting is probably the percent of workers bicycling in each area. We need to “normalize” the data.
Right-click on tract_biking layer and click Properties. In the Layer Properties window, click on the Symbology tab. Click on WORKERS in the Normalization drop-down menu. Click OK.
Now notice that the map shows percent of workers bicycling in each area. But the map isn’t all that pretty or easy to read, and it’s probably more interesting to zoom in on Davis, rather than looking at the county as whole. Here are some things you can do to improve the map:
To zoom in on Davis: Click the little magnifying glass with the + sign on the Tools toolbar. Click on the map at a point somewhere near the middle of Davis. Repeat until Davis takes up most of the map.
- Technically, we should also check and define the map projection before creating the thematic map. This step is not essential for this particular exercise, but it is essential if you want to do anything that involves areas or distances (e.g. if you want to map people per square mile). This is not hard to do. Here’s what you do.
Right-click on the tract_biking layer and click Properties. Click on the Source tab.
You’ll notice that for coordinate system it says GCS_Assumed_Geographic_1, which is a geographic coordinate system, as opposed to a projected coordinate system, or projection. We want a projection in order to calculate distances and areas, so we’ll reproject the data using ArcToolbox.
Click OK to return to the main ArcMap window. If ArcToolbox is not currently displayed in ArcMap , click on the ArcToolbox button on the standard toolbar (a small red icon that looks something like a toolbox). In ArcToolbox, expand Data Management Tools by clicking on the + sign, then Projections and Transformations , and then Feature. Double-click on the Project tool. In the window for the Project tool, add tract_biking.shp as the Input Dataset and name the output Z:\handy[your name]\yolo_tract_biking.shp. Click on the button that looks like a little memo pad with a hand, to open the Spatial Reference Properties window. Click on the Select button. Double-click on the folder for Projected Coordinate Systems , then State Plane , and NAD1983 (Feet). Scroll through and find NAD 1983 StatePlane California III FIPS 0403 (Feet).prj , select it, and click Add. Click OK to exit the Spatial Reference Properties window and OK to run the Project tool. Close the tool window when it has finished running. Use yolo_tract_biking instead of tract_biking to create your thematic map.
