Community
and Regional Planning Program
The University of Texas
at Austin School of Architecture
Working with 2000 Census
Data in ArcGIS
ArcGIS Tips
I. Getting
Census Boundary GIS Data Sets (shapefiles)
II.
Getting Basic Census Data from the Geography Network
III.
Getting Census Data from the Census Bureau
IV.
Joining Census Bureau Data to the Tract Shapefile
V. Making
Your Data More User-Friendly
VI.
Resources
The objective of this tutorial is to explain how to get 2000 census data
into a GIS database. The tutorial uses the block group and tract levels of
census geography as examples. It can be applied to any part of the United
States, and users could select a different level of census geography.
Census GIS data and demographic/housing data are available from a number
of sources, the US Census Bureau being the primary source. State, regional,
and local agencies may also provide access to this data in various formats.
Some of these formats are impossible to bring into a GIS (e.g., a .pdf file),
others may need importing or some other manipulation, and some may already
be provided in a usable GIS format. In this tutorial, we use free downloadable
Census GIS data from a Geography Network web site run by ESRI. This data
is already in ArcGIS format. We then show how to access Census database tables
from the US Census Bureau web site and join these to the ESRI GIS data
layer with a small amount of data manipulation.
Before you begin this tutorial, please take a look at the US Census Bureau
web site (www.census.gov/). This should
be your first source for questions about the census, data, and data
documentation.
I. Getting
Census Boundary GIS Data Sets
To make a map of census data, you need a GIS data set showing census boundaries
plus a table or tables of the census data itself. Census geography will be
explained in class. In this tutorial, we are going to use Block Group boundaries.
Block Groups are collections of Census Blocks (blocks are the smallest unit
of geography used by the census).
The easiest place to obtain block group (or other census geography)
boundary data sets for ArcGIS is at the ESRI's Geography Network - Free
section
(http://www.geographynetwork.com/freeresources.html).
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From this site choose the Census TIGER/Line 2000 Data.
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Read about the data then choose Download Data
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Select a state.
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Under Select by Layer, choose Block Groups 2000
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Next check the county or counties you want (for Austin, choose Travis
County; for the MSA, choose Bastrop, Caldwell, Hays, Travis, and Williamson).
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Scroll to the bottom and press Proceed to Download
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Read the information and press Download File
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Save the file to your personal folder and give it a name you will understand
(but maintain the .zip extension)
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Unzip the data set after download
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The unzipped file contains another zipped file - unzip this one as well
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You can now bring this data layer into ArcMap.
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The data sets are in decimal degrees (GCS = Geographic Coordinate System).
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Open the table for the Block Group boundaries and look at what it contains.
It only has fields identifying the geography (e.g., tract, group). It has
no actual census demographic data. The STFID field will be the join
field we use next. It is a concatenation (adding together) of the
FIPS (Federal Information Processing Standard) codes for the state (e.g.,
Texas=48), county (e.g., Travis County=453), tract (e.g., 000101), and blockgroup
(e.g., 1).
II.
Getting Basic Census Data from the Geography Network
Now that you have a census GIS geography layer (e.g., block group boundaries),
you want to find and join actual census demographic and/or housing data to
it. In this section of the tutorial, we are going to download basic (SF1)
demographic data from ESRI's web site. It is already formatted to be easy
to join and easy to read.
Getting basic census data from the Geography Network
-
From ESRI's Geography Network - Free section
(http://www.geographynetwork.com/freeresources.html),
go back to the Census TIGER/Line 2000 Data link, and go to the download
data link again.
-
Go to the same state/county for which you have the block group boundaries
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For select by layer, choose Census Block Group Demographics (SF1).
This provides the basic data from the Summary File1 portion of the census
in an easily readable format.
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Download this data set and unzip it until you have the end product, a .dbf
file.
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Add this table to ArcMap and open the table to see what it contains. Nothing
will appear in your map because it is a table of information, not a geographic
layer. Note that it has data as well as geographic codes, including the
STFID field.
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With the Block Group boundaries and the block group demographics table in
ArcMap, you can now perform a join using the STFID field. A
join relates the attribute data in the table to the geographic data layer
by using a common attribute field. Right-click on the block group
geography data layer and choose Joins and Relates - Join...
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Create a join to the block group demographics table (.dbf) using the
STFID field as the common field (see
screen print example)
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With this join in place, you should be able to create a map of any of the
census demographic fields - try it and see (right-click on the block
group geographic data layer, choose Properties-Symbology and create
a quantities map using one of the fields. Note that most of the data
is raw numbers. If you want to see, for example, Hispanics as a proportion
of the total population, choose Hispanic as the value field, but for
Normalization, choose POP2000 (the total population of each
census block group). Normalization divides the mapped value by the
normalization value. Note that the Normalization option
called "percent of total" means percent of the chosen field value's total
sum, so if you chose this for the Hispanic population, the result would show
the proportion that each blockgroup held of the total Hispanic population
for Travis County, not the percent Hispanic of the blockgroup's total
population.
Creating a permanent shape file with the census data
A join between a geographic layer and a table is temporary - the join does
not create a permanent new file. If you save a map file with the join in
place, the next time you open that map file, ArcMap will look for your shape
file and the .dbf file and re-join them.
You can also create a permanent shape file with the census data attached.
To do this, make sure the data is joined as described above. Then
right-click on the block group geographic data layer and choose Data
- Export Data. In the dialog box that appears, leave the default settings
as they are but give the new shape file an appropriate name, like
2000block_groups_sf1.shp and make sure you use Browse to put it in
your personal folder. Next time you add this data layer to a map, the census
data will come with it.
III.
Getting Census Data from the Census Bureau
You can get more detailed census data from the Census Bureau and from
many state, regional, and local agencies (or you can buy it). In this example,
you are going to get tabular data from the Census Bureau itself. Note that
this data will not be as easy to use as the data that you previously
downloaded, but going directly to the source gives you access to more detailed
data for free.
In this section of the tutorial, you will be accessing census tract-level
data, not block group data as in the last section. However, you can use a
similar approach to download block group level data (just be aware that the
Geo_id in Part IV of the tutorial is different for block groups). Also, any
of your table choices (Detailed tables, Quick Tables, Geographic Comparison
Tables work basically the same way, except that the Geographic Comparison
Tables include an extra row near the top that will need to be deleted prior
to use in ArcMap - the instructions for that are given below).
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Go to the Census Bureau on the web at
(http://www.census.gov/ ). Then, click
on American FactFinder.
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Click on the link to Data Sets (on the left column, or under Detailed
Data Sets - Expert User)
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You will see several data sets (1990, 2000 and others). Click on the hyperlink
Summary File 3 - Sample Data.
2000 Summary File 3 contains data from the 2000 sample census form
and is useful for information about education, income, family
status, etc. For this tutorial, we suggest you use Summary File 3.
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Check the radio button for the Summary File 3 - Sample Data, and click
on "Geographic Comparison Tables"
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Once you are in the "geographic comparison tables page", for Select geographic
type, choose County. Wait for the screen to change.
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When you see Select State, choose Texas. Press Enter and wait
for the screen to change.
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For Select Geographic Area, choose Travis, and then for Select
Table Format, choose Census Tract, then press Show Table
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Pick a table from the list (only one) - e.g., you could start with Income
and Poverty in 1999:2000, and press Show Result
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The resulting table shows you the requested information summarized for all
of Travis County, then the data for each individual census tract within Travis
County. Look it over to understand what it is showing.
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To download the information for use in a spreadsheet or GIS, choose
Print/Download - Download (in the upper right
corn
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Choose to download a database compatible version in Excel,
and check mark the option to include descriptive data element names
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When prompted, choose Save. The downloading file will be named
output.zip by default. Give it a new name to match the data (e.g.,
income_poverty.zip), and save it to your personal folder.
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Open the windows file manager and unzip this file- it will open to a file
named something like gct_dec_2000_sf3_u_data0.xls
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When the Excel file has unzipped, double-click on it to open it in Excel.
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You now have the same data in an Excel spreadsheet:
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The first row contains the attribute field names in abbreviated format (e.g.,
GCTP14_CO1.C0)
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The second row contains the descriptive data element names you asked for
(e.g., Median income in 1999 (dollars); Households)
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Look carefully at the third row and note that the SUMLEVEL or Geographic
Summary Level says 050. This is the data for the county as a whole.
Everything below it (SUMLEVEL=140) is for each individual tract
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Next, to avoid problems later with field names, re-name the field headings
for the data columns to something that makes more sense. BUT - for
it to be compatible with with ArcMap, it must follow dBase table rules -
no field names longer than 10 characters, no spaces, no funny stuff. E.g.,
intead of GCTP14CO1.C8, you might name the field PctPovFam.
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Choose File - Save As, and name your new Excel (.xls) spreadsheet,
income_poverty.xls (or another appropriate name depending on your
data)
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You can use this Excel file later for reference regarding descriptive field
names and for comparing data against the county as a whole. The Excel format
is also easier for doing non-spatial analysis, and for graphing and charting.
Converting the Excel file to a database file that ArcGIS can read
The Excel file can still not be read directly by ArcGIS. It contains additional
information that will make it unreadable (databases should have one row with
the field names, followed directly by the data rows), and it needs to be
in a database format like dBase (DBF 4). So to use this information in ArcGIS
and to map it, you need to do additional processing. The row with the descriptive
names needs to be deleted as does the row containing the aggregated data
for Travis County as a whole.
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Click on row 2 (descriptive names) to highlight the entire row (click
on the "2" itself at the beginning of that row)
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Choose Edit-Delete
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There is now a new row 2 - the data row for the county as a whole - this
needs to be deleted as well. Click on the row 2 again and delete it.
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Now select all the cells in the spreadsheet, all the way from the upper left
(cell 1A) to lower right (e.g., cell O182 - this could differ depending
on the table you have downloaded)
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With the cells highlighted, choose File - Save As
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Save this modified version first as an Excel table - call it something like
modified_income_poverty.xls
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Next, choose Save As again to save it as a dbf file
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For Save as Type, first choose DBF 4 (dBase IV, *.dbf) - ArcGIS
can read this format directly)
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Name your new file appropriately (e.g., income_poverty.dbf)
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When you get a warning message, choose Yes.
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Choose File - Exit, and say No when prompted to save your file
again.
You should now be able to add this .dbf file to an ArcMap data frame.
IV.
Joining Census Bureau data to the Tract Shapefile
Now it is time to join the tract SF3 data to the tract shape file. (Those
of you using the GIS_Data CD, the tract shape file is called
census2000_tracts_sf1). If you don't have a tracts shape file, use
the same steps outlined in Part 1 of this tutorial to
download census tracts from ESRI's Geography Network - Free section
(http://www.geographynetwork.com/freeresources.html).
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Add the tracts shape file to an ArcMap session (don't try to use the block
group shape file as in the first section - we are now looking at a different
level of census geography).
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In ArcMap, if you haven't already, add the
income_poverty.dbf file (or whatever .dbf you created in the last
step) to your dataframe. It comes in as a table. Open the table and examine
its contents.You will see that it has a geo_id field, a sumlev
field (summary level, in this case 140 = tracts), and lots of
other columns with the actual data.
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You will see in the data table that there is no STFID to join to the
Tracts shapefile, only a GEO_ID. But in the table for the tracts
shapefile, there is no GEO_ID. Still you will see that the two ID
fields are very similar. The GEO_ID field simply has a beginning part
(14000US) and then has the same numbers as the STFID. We are going
to use this information to create a new field in the
Tracts shapefile table that can then be joined to the data table.
All we need is to add the "14000US" part to the STFID field.
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Open the table for the tracts shapefile.
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In the table, click on the Options tab and choose Add Field.
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Create a field called Geo_id, make it a text field, and set the length
to 25 (see screen print example), and press
OK
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The new field will appear at the very right end of your table. Check that
it is there.
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Right-click on the field name Geo_id to to get a short menu
and choose Calculate Values
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Respond Yes to the warning, that you want to continue.
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In the Field Calculator dialog box, note that the dialog box is indicating
Geo_id= and there is a box for you to type. Fill out the box
as in the diagram below - note that you have to type the first part, but
you can click on the "plus" sign and the STFID field shown in the
dialog box to add those to the equation. What this will do is put the "14000US"
in front of the STFID to create a Geo_id field that matches
the one from the Census Bureau. Make sure that you are using zeros
for the 14000US part, not the letter "o". Also, be sure to use quotation
marks around the "14000US". Press OK when finsihed. The new
Geo_id field should be filled with the concatenation of
"14000US" and the STFID.
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You can now join the tracts shapefile to the your
income_poverty.dbf file using the Geo_id - see a
screen print example of the join parameters.
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Check your work by creating a map (if you need to, consult the original Excel
file for reference as to what the field names mean). Make sure there are
no blank polygons that didn't join up properly. Open the tracts table and
check for any <null> values - this would indicate an improper join.
If you see this, examine the two geo_id fields carefully to see why they
wouldn't join.
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If the join worked properly, try creating some maps of the census data. Use
the Normalization option where appropriate.
You can repeat this for any table you download from American Factfinder.
Remember that in ArcMap, you can also export joined layers to create a permanent
shape file with all the information. I recommend this. For example, you could
export the joined table in this tutorial and call it
2000_tract_income_poverty. If you then download another table, I recommend
that you join it to the original tract shape file, and export that to a different
shape file (e.g., 2000_tract_financial_housing).
V. Optional:
Making Your Data More User-Friendly
The table you downloaded from the Census Bureau is very hard to understand.
There are many fields you don't want, and the field names are not comprehensible
without the documentation or the Excel file. There are a couple ways to mold
this data more to your needs, but unfortunately, neither of them is very
good, so this part of the tutorial is optional.
First, in Excel, you may already have renamed the fields to be somewhat more
explanatory, but in order for them to export correctly to dBase, the field
names have to be 10 characters or less and have no spaces or other unusual
punctuation (e.g., inc_percap is ok, inc/cap is not, nor is
inc_per_cap). This is the best option.
Another option is to set an understandable "alias" for fields of interest
in ArcMap and make irrelevant fields invisible. This will make your table
easier to read when it is opened. To be able to map using these aliases for
mapping, make sure that you create the aliases in the joined shapefile/data
table, not in the data table. To create field aliases and make fields invisible:
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Make sure the table has been added to ArcMap and joined to a shapefile table
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Double-click or Right-click on the shapefile name to access layer properties.
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Go to the Fields tab.
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Click on each field name and if you don't want to see it, uncheck visible.
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For the fields of interest, type an alias. This will show up in the table
when it is opened, instead of the real field name. (see
screen print example)
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As soon as you have done this, save the map file (.mxd) file! Your work is
not saved in the table itself, but only in this particular session. By saving
the mapfile, you can reload it again later and have all your aliases still
there.
You can also delete unwanted fields. Beware - you don't want to delete fields
unless you are absolutely sure you don't want them again. Right-click
on any column's field name and choose Delete Field. Alternatively,
you can use ArcCatalog, navigate to the .dbf file, right-click on the .dbf
file to get the file's properties, then delete fields there.
See additional Census Tips
for more guidance.
Resources:
