Performing Table Joins (QGIS3)

Not every dataset you want to use comes in a spatial format, and often the data would come as tabular data like CSV, TSV, or spreadsheet. You need to link it with the existing spatial data for use in your analysis. This operation is known as the Table Join and is done using the Join attributes by field value from the Processing algorithm Toolbox.

Overview of the task

We will use a shapefile of census tracts for California and a population data table from US Census Bureau to create a population density map for California.

Other skills you will learn

  • Loading a CSV files that do not contain any geometry in QGIS.

  • Eliminate additional headers in CSV.

  • Add an additional column to a layer.

  • Style an layer with transparent stroke.

Get the data

We want to prepare a population density map for the state of California, USA. We will first get a table with population counts for each census tract in the state.

Download Census Table

  1. Visit US Census Bureau and click VIEW TABLES.

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  1. Click on the Filters icon in top-left.

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  1. Let’s first filter to California. Click on Geography.

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  1. Click on Tract, which can provide the census tract information.

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  1. Choose California.

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  1. Check All Census Tracts within California. This allows us to download all information as a single file instead of downloading it separately for each tract. Now that we have set the geography, click on Topics.

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  1. Under Select Topics, choose Population and People.

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  1. Check the Population and People. Now we have selected all the necessary filters. Close the filter window.

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  1. In the Download tables pane scroll down to find S0101 | AGE AND SEX

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  1. This will open a detailed view of the data. We are concerned with only the population hence un-select the Margin of Error so we can get the necessary information only and save on file storage space.

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  1. Click on the Download button.

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  1. The selected data is available for multiple years and using different estimation techniques. The ACS 5-Year Estimate Subject Tables is recommended as multi-year estimates increases the reliability of the data. It also has no data gaps and contains information for all tracts. Select the year 2019. and click Download. This will download a zip file. Once it finishes, unzip it on your local disk.

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Download Census Tract Boundaries

To create a map from the census table, we also need the boundaries representing each census tract in the state of California.

  1. Visit TIGER Line Shapefiles and click Web Interface.

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  1. Under Select year choose 2019, in Select a layer type Census Tracts and click Submit.

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  1. Select California in Select a State and click Download. This will download a zip file containing the vector files.

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For convenience, you may directly download a copy of both the datasets from the links below:

tl_2019_06_tract.zip

ACST5Y2019_S0101.zip

Data Source [TIGER] [USCENSUS]

Procedure

  1. Locate the tl_2019_06_tract.zip file in the QGIS Browser and expand it. Select the tl_2019_06_tract.shp file and drag it to the canvas.

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  1. The Select Transformation dialog will prompt to convert from EPSG:4269 to EPSG:4326. This dialog presents several transformations to convert between the coordinates between these projections. Leave the selection to the default choice and click OK.

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  1. You will see the layer tl_2019_06_tract loaded in the Layers panel. This layer contains the boundaries of census tracts in California. Right-click on the tl_2019_06_tract layer and select Open Attribute Table.

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  1. Examine the attributes of the layer. To join a table with this layer, we need each feature’s unique and common attribute. In this case, there are 8057 individual tract records with the GEOID field. This column can link this layer with any other layer or table containing the same ID.

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  1. To load the tabular data, click on Open Data Source Manager.

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  1. In the Data Source Manager dialog, choose Delimited Text. Then in the right, click on the ... next to File name and browse to the unzipped folder with the California population CSV.

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  1. Now under Sample Data, we can inspect the data even before loading it as a layer. The representation shows that the data table contains 2 header rows.

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  1. To eliminate the additional header row, under Record and Fields Options set the Number of header line to discard to 1. Now the table will contain proper column headers. Click Add to add it as a layer and then click Close to close this dialog box.

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  1. The CSV will now be imported as a table to QGIS and appear as ACST5Y2019.S0101 in the Layers panel. Now right-click on the layer and select Open Attribute Table.

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  1. The ID column contains the unique id for each record, which can be used to join this table with the tl_2019_06_tract layer. If you compare the values of the ID with the GEOID column from the tl_2019_06_tract. you will notice that it is prefixed with 1400000US. To merge these two tables successfully, the values must match exactly. Let’s remove this prefix and add a new column with the last 11 characters which contain the value that is an exact match.

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  1. To create a new column with the last 11 digits, open the Processing Toolbox by going to Processing ‣ Toolbox, and search and locate the Vector table ‣ Field calculator algorithm.

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  1. In the Field calculator dialog, select ACST5Y2019.S0101 as the Input layer, enter geoid in Field name, and select string in Result Field type. Now search for substr in expressions. We can use this function to extract the required part from the id field.

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  1. Enter the below expression. We use the substr function and extract the value from position -11 (negative value is counted from the end). The final result can be viewed in the Preview section. Click Run.

substr("id", -11)
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  1. Now a new layer Calculated will be loaded in the canvas, lets inspect the attribute table. A new column geoid with the value that can be matched with the cencus tract will be present.

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  1. To create a table join, open the Processing Toolbox by going to Processing ‣ Toolbox, and search and locate the Vector general ‣ Join attributes by field value algorithm.

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  1. In the Join attributes by field value dialog, select tl_2019_06_tract as the Input layer and GEOID as the Table field. Select Calculated as the Input layer 2 and geoid as the Table field 2. Under Layer2 fields to copy, click on the ....

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  1. Check Geographic Area Name, Estimate!!Total!!Total population and geoid. Click OK.

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  1. Check the Discard records which could not be joined. This will eliminate any extra records in the population table. Click the button under joined layer to select the output file location and select Save to File....

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  1. Name the output geopackage as california_total_population.gpkg . Click Run.

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  1. Once the processing finishes, verify that the algorithm was successful if all 8057 feature(s) are joined. Click Close.

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  1. You will see a new layer california_total_population loaded in the Layers panel. At this point, the fields from the CSV file are joined with the census tracts layer. Now that we have the population data in the census tracts layer, we can style it to create a visualization of population density distribution. Click the Open the Layer Styling Panel button.

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  1. In the Layer Styling panel, select Graduated from the drop-down menu. As we are looking to create a population density map, we want to assign different color to each census tract feature based on the population density. We have the population in the Estimate!!Total!!Total population field, and the area field in ALAND. Click Expression button, to compute the percentage of total population in each cencus tract.

Note

When creating a thematic (choropleth) map such as this, it is important to normalize the values you are mapping. Mapping total counts per polygon is not correct. It is important to normalize the values dividing by the area. If you are displaying totals such as crime, you can normalize them by dividing by total population, thus mapping crime rate and not crime. Learn more

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  1. Enter the following expression to calculate the population density. The area of the feature is given in square kilometers. We then convert it to square meters by multiplying with 1000000 and calculating the population density with the formula Population/Area. Preview the result and click OK.

1000000 * ("Estimate!!Total!!Total population"/"ALAND")
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  1. In the Layer Styling Panel, click classify and enter the classes as 10.

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  1. Click on the color ramp to choose the color ramp RdYlGn.

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  1. The higher density concerns more so, let’s assign green to lower density and red to high-density areas. Click on the color ramp and choose Invert Color Ramp.

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  1. Now we have an excellent looking information visualization of population density in California. To make it better, let’s make the border of each census tract transparent. Click on the Symbol tab.

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  1. Click on Stroke color and click Transparent stroke.

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  1. The bins can be adjusted, click on the Values this will popup a dialog to enter the upper and lower bound value.

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  1. Once your satisfied close the Layer styling panel. We now have a nice looking information visualization of population density in California.

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