IPUMS - NHGIS Extraction and Analysis

Exercise 1

OBJECTIVE: Gain an understanding of how the NHGIS datasets are structured and how they can be leveraged to explore your research interests. This exercise will use an NHGIS dataset to explore slavery in the United States in 1830.

This vignette is adapted from the NHGIS Data Training Exercise available here: https://pop.umn.edu/sites/pop.umn.edu/files/nhgis_training_ex1_2017-01.pdf

Research Question

What was the state‐level distribution of slavery in 1830?


  • Create and download an NHGIS data extract
  • Unzip data file and open in R
  • Analyze the data using R
  • Validate data analysis work using answer key

Download Extract from IPUMS Website

  1. Log in to NHGIS
  • Go to https://www.nhgis.org and click on ‘Login’ in the top right.
  • If you have already registered on any Minnesota Population Center website…
    • If you remember your password, log in now. Otherwise, click the “Forgot your password?” link on the right and follow the instructions.
  • If you have not already registered…
    • Click on the “Create an account”” link on the right, fill in the required information, and submit your registration.
  • You will then enter the NHGIS Data Finder…
  1. Find Tables
  • Quick instructions
    • Apply any combination of the four filters below to find 1830 slavery related tables
      • Geographic Levels = ‘State’
      • Years = ‘1830’
      • Topics = ‘Slavery’
      • Datasets = ‘1830_cPop’
  • Guided instructions
    • Suppose you were interested not only in slavery, but in all that’s covered by the 1830 Census.
      • To view all available 1830 data, use only the Years Filter set to ‘1830’.

Q1) How many tables are available from the 1830 Census?

#     A: Fifteen (15)

Q2) Other than slave status, what are some other topics could we learn about for 1830?

#     A: Population that is urban, particular ages, deaf and dumb, blind, and foreign born
#        not naturalized.
    • Locate the Desired Table
      • Let’s focus in on the slavery topic. To narrow the results, apply the Topics Filter of ‘Slavery’. (You can find it at the bottom of the list of POPULATION topics.)
      • The Select Data grid now lists all the tables related to the topic of Slavery. If you don’t also have the Years Filter on, scroll down to find the 1830 tables, or utilize additional filters to further limit the available tables.
      • Locate this 1830 table and answer the questions that follow: “NT12. Race/Slave Status by Sex”
    • Learn About the Table in the Data Finder

Q3) Click the table name to see additional information. How many variables does this table contain?

#     A: Six (6)

Q4) For which geographic levels is the table available?

#     A: Nation, State, & County

Q5) Close the table pop‐up window and inspect the Select Data table… What is the universe for this table?

#     A: Persons

Q6) What differentiates this table from the other available slavery tables from 1830?

#     A: It includes the counts of "white" persons, in addition to "colored" persons

Q7) Name a percentage or ratio this table would allow us to calculate that the other tables would not, based on the counts available in each table?

#     A: Percentage of total population in slavery, or ratio of slave:free population
  1. Create a Data Extract Creating a data extract requires the user to select the table(s), specify a geographic level, and select the data layout structure…
  • Click the plus sign to the left of the table name to add table NT12 to your Data Cart.
  • Click on the “GIS Boundary Files” Tab
  • Click on the plus sign to the left of the State Geographic Level Table
  • Click the green Continue button in your Data Cart.
  • On the Data Options screen, select the geographic level of ‘State’.
  • Click the green Continue button in your Data Cart.
  • On the Review and Submit screen, select the “Comma delimited (best for GIS)” option (it doesn’t matter if you include the descriptive header rows or not), add an extract description if you wish, and click Submit.
  1. Download the Data Extract From the Extracts History page, you will be able to download your data extract once it has finished processing, typically within a few minutes. You may leave this page and return once you have received the email alerting you to your finished extract.

If you refresh your browser window (click on the loop icon at top, or press F5), you will see the extract status change from ‘queued’ to ‘in progress’ to ‘complete’, at which time you will be able to click the ‘tables’ link to download the data.

  • Return to the Extracts History page if not currently there.
  • Right‐click on the ‘tables’ link for the extract you created.
  • Choose ‘Save Target As…’ (or ‘Save link as…’).
  • Save the zip file into ‘Documents’.
  • Repeat the process for the GIS data (right-click, choose ‘Save Target As…’, …)
  • The R package can read the extracts as zip files, or if you wish to open in other programs, you can unzip them, by: Right‐clicking on the ‘nhgis0001_csv.zip’ file, and select Extract All… Then click the Extract button. (Repeat for the shape if you desire).

Getting the data into R

You will need to change the filepaths noted below to the place where you have saved the extracts.

Getting the data into R

You will need to change the filepaths noted below to the place where you have saved the extracts.


# Change these filepaths to the filepaths of your downloaded extract
nhgis_csv_file <- "nhgis0001_csv.zip"
nhgis_shp_file <- "nhgis0001_shape.zip"
nhgis_ddi <- read_ipums_codebook(nhgis_csv_file) # Contains metadata, nice to have as separate object
nhgis <- read_nhgis_sf(
  data_file = nhgis_csv_file,
  shape_file = nhgis_shp_file
#> Use of data from NHGIS is subject to conditions including that users should
#> cite the data appropriately. Use command `ipums_conditions()` for more details.
#> Reading data file...
#> Reading geography...
#> options:        ENCODING=latin1 
#> Reading layer `US_state_1830' from data source 
#>   `/private/var/folders/bp/l9qm36b10bq0jfwtkvty8q5h0000gq/T/RtmpGiAF3k/file7112353f06f2/US_state_1830.shp' 
#>   using driver `ESRI Shapefile'
#> Simple feature collection with 29 features and 7 fields
#> Geometry type: MULTIPOLYGON
#> Dimension:     XY
#> Bounding box:  xmin: -1389608 ymin: -1337508 xmax: 2258225 ymax: 1408188
#> Projected CRS: USA_Contiguous_Albers_Equal_Area_Conic

Note that read_nhgis_sf relies on package sf. You can also read NHGIS data into the format used by package sp with function read_nhgis_sp.


These exercises include example code written in the “tidyverse” style, meaning that they use the dplyr package. This package provides easy to use functions for data analysis, including mutate(), select(), arrange(), slice() and the pipe (%>%). There a numerous other ways you could solve these answers, including using the base R, the data.table package and others.

library(dplyr, warn.conflicts = FALSE)

Analyze the Data

Q8) How many states/territories are included in this table?

#> [1] 28

#     A:  Twenty‐Eight (28)

Q9) Why do you think other states are missing?

#>              Alabama   Arkansas Territory          Connecticut 
#>                    1                    1                    1 
#>             Delaware District Of Columbia    Florida Territory 
#>                    1                    1                    1 
#>              Georgia             Illinois              Indiana 
#>                    1                    1                    1 
#>             Kentucky            Louisiana                Maine 
#>                    1                    1                    1 
#>             Maryland        Massachusetts   Michigan Territory 
#>                    1                    1                    1 
#>          Mississippi             Missouri        New Hampshire 
#>                    1                    1                    1 
#>           New Jersey             New York       North Carolina 
#>                    1                    1                    1 
#>                 Ohio         Pennsylvania         Rhode Island 
#>                    1                    1                    1 
#>       South Carolina            Tennessee              Vermont 
#>                    1                    1                    1 
#>             Virginia 
#>                    1
#     A: In 1830, there were not any other states yet! Every decennial census is a 
#        historical snapshot, and NHGIS provides census counts just as they were 
#        originally reported without "filling in" any information for newer areas.

Q10) Create a new variable called total_pop, with the total population for each state, by summing the counts in columns ABO001 to ABO006. Which state had the largest population?

nhgis <- nhgis %>%
  mutate(total_pop = ABO001 + ABO002 + ABO003 + ABO004 + ABO005 + ABO006)

nhgis %>%
  as.data.frame() %>%
  select(STATE, total_pop) %>%
  arrange(desc(total_pop)) %>%
#>            STATE total_pop
#> 1       New York   1913006
#> 2   Pennsylvania   1348233
#> 3       Virginia   1211405
#> 4           Ohio    937903
#> 5 North Carolina    737987

#     A: New  York

Q11) Create a variable called slave_pop, with the total slave population by summing the variables ABO003 and ABO004. Which state had the largest slave population?

nhgis <- nhgis %>%
  mutate(slave_pop = ABO003 + ABO004)

nhgis %>%
  as.data.frame() %>%
  select(STATE, slave_pop) %>%
  arrange(desc(slave_pop)) %>%
#>            STATE slave_pop
#> 1       Virginia    469757
#> 2 South Carolina    315401
#> 3 North Carolina    245601
#> 4        Georgia    217531
#> 5       Kentucky    165213

#     A: Virginia 

Q12) Create a variable called pct_slave with the Slave Population divided by the Total Population. Which states had the highest and lowest Percent Slave Population?

nhgis <- nhgis %>%
  mutate(pct_slave = slave_pop / total_pop)

nhgis %>%
  as.data.frame() %>%
  select(STATE, pct_slave) %>%
  filter(pct_slave %in% c(min(pct_slave, na.rm = TRUE), max(pct_slave, na.rm = TRUE)))
#>            STATE pct_slave
#> 1        Vermont 0.0000000
#> 2 South Carolina 0.5426861

#     A: South Carolina (54.27%) and Vermont (0.00%)

Q13) Are there any surprises, or is it as you expected?

nhgis %>%
  as.data.frame() %>%
  filter(pct_slave > 0.5) %>%
  select(STATE, slave_pop, total_pop, pct_slave)
#>            STATE slave_pop total_pop pct_slave
#> 1 South Carolina    315401    581185 0.5426861
#> 2      Louisiana    109588    215529 0.5084606

nhgis %>%
  as.data.frame() %>%
  filter(STATE %in% c("New York", "New Jersey")) %>%
  select(STATE, slave_pop, total_pop, pct_slave) 
#>        STATE slave_pop total_pop    pct_slave
#> 1   New York        75   1913006 3.920531e-05
#> 2 New Jersey      2254    320823 7.025681e-03

#     A: Possibilities: Did you know some states had more slaves than free persons? Did
#        you know that some “free states” were home to substantial numbers of slaves?

Inspect the Codebook

Open the .txt codebook file that is in the same folder as the comma delimited file you have already analyzed. The codebook file is a valuable reference containing information about the table or tables you’ve downloaded.

Some of the information provided in the codebook can be read into R, using the function read_ipums_codebook().

Q14) What is the proper citation to provide when using NHGIS data in publications or researcher reports?

cat(ipums_file_info(nhgis_ddi, "conditions"))
#> All persons are granted a limited license to use this documentation and the
#> accompanying data, subject to the following conditions:
#> * Publications and research reports employing NHGIS data (either tabular or GIS)
#>   must cite it appropriately. The citation should include the following:
#>     Steven Manson, Jonathan Schroeder, David Van Riper, and Steven Ruggles. 
#>     IPUMS National Historical Geographic Information System: Version 12.0 [Database]. 
#>     Minneapolis: University of Minnesota. 2017. 
#>     http://doi.org/10.18128/D050.V12.0
#> * Publications and research reports employing school attendance areas data
#>   (either tabular or GIS) must cite it appropriately. The citation should
#>   include the following:
#>     The College of William and Mary and the Minnesota Population Center.
#>     School Attendance Boundary Information System (SABINS): Version 1.0.
#>     Minneapolis, MN: University of Minnesota 2011.
#> * For policy briefs or articles in the popular press, we recommend that you cite the use of NHGIS data as follows:
#>     IPUMS NHGIS, University of Minnesota, www.nhgis.org.
#> * If possible, citations involving school attendance areas should also include
#>   the URL for the SABINS site:
#>     http://www.sabinsdata.org/.
#> In addition, we request that users send us a copy of any publications, research
#> reports, or educational material making use of the data or documentation.
#> Printed matter should be sent to:
#>     NHGIS
#>     Minnesota Population Center
#>     University of Minnesota
#>     50 Willey Hall
#>     225 19th Ave S
#>     Minneapolis, MN 55455

#     A: Minnesota Population Center. National Historical Geographic Information
#        System: Version 11.0 [Database]. Minneapolis: University of Minnesota. 2016.
#        http://doi.org/10.18128/D050.V11.0.

Q15) What is the email address for NHGIS to share any research you have published? (You can also send questions you may have about the site. We’re happy to help!)

#     A: (You can also send questions you may have about the site. We're happy to help!)
#     nhgis@umn.edu

Bonus - Make Maps using R

One of the reasons we are excited about bringing IPUMS data to R is the GIS capabilities available for free in R.

Q16) Make a map of the percent of the population that are slaves.

ggplot(data = nhgis, aes(fill = pct_slave)) +
  geom_sf() + 
  scale_fill_continuous("", labels = scales::percent) + 
    title = "Percent of Population that was Enslaved by State",
    subtitle = "1830 Census",
    caption = paste0("Source: ", ipums_file_info(nhgis_ddi, "ipums_project"))