Linked Data replication of historical GIS: Roman Catholics per municipality in 1899

This datastory replicates a historical GIS map using an open dataset and SPARQL.

I. The paper

In 2015 Nynke van den Boomen & Peter Ekamper published a paper about infant mortality and causes of death in The Netherlands between 1875 en 1899. The paper makes use of innovative data and neat visualizations on Dutch maps to show the municipal distribution of infant mortality, and its potential causes. The data they used to plot these municipalities, and the share of Roman Catholic inhabitants, is largely open, namely the Historical Database of Dutch Municipalities (HDNG), and the NLGis shapefiles by Onno Boonstra. However, the GIS software used for the actual visualizations is not open, making it difficult to replicate their maps. Here we'll show how to use these data to plot historical results in Linked Data. This has the benefit of creating live, open, and interactive results. The maps created in this datastory can be shared with others by simpling copying the URL. What is more, they can be adapted live by changing the corresponding SPARQL query, for instance by selecting another year.

I. The datasets

In this attempt we will replicate Figure 2 from their paper, showing the percentage of Roman Catholics per municipality, 1899. First, we'll replicate the table underlying the map, using the HNDG, giving the share of Roman Catholics per municipality in 1899. For this, we can resort to the readily available 'go-to-HDNG-query'. We'll use the search function in the query to find the relevant variable in the HNDG, which is "RKA3". You'll notice that the "sex" field does not give a M/F code, meaning that the value of "RKA3" refers to both sexes. We also see that the value is available for 1899 - which is not a surprise, since Boomen & Ekamper used the same dataset. Now, we need to use the same steps to get population figures for 1899. These turn out to be variables "BEV1" (male) and "BEV2" (female). The query below uses these variables to get the share of Roman Catholics per Dutch municipality in 1899, for the fifteen largest cities.

Share of Roman Catholics per municipality in 1899 (table):

II. Replicating the map

We can now use this table to plot the results on a map. For this we also need the polygons of Dutch municipalities in 1899. Luckily, the NLgis shapefiles are already available as Linked Data, courtesy of Gemeentegeschiedenis. Because both Gemeentegeschiedenis/NLgis and the HNDG use the 'Amsterdamse code' to refer to municipalities over time, we can use this variable to retrieve the corresponding polygons. In order to replicate the map exactly as Boomen & Ekamper, the bin width is set using the categories from their Figure 2 ([0,20],[20,40], etc.), and the same color gradient is used (green).

Share of Roman Catholics per municipality in 1899 (map):

By hovering over the map, you will see that both the name of the municipality and the share of Roman Catholics will show. A nice interactive feature that Linked Data allows for. You can also zoom in and find your own municipality or region of interest.

Although the polygons are based on historical municipality borders (1899 or the closest one available before), the actual map is modern (OpenStreetMap). Using a function that is currently under development, we can project historical maps under these polygons. All that is needed for this is a WMS-service that provides georeferenced historical maps, such as Map Warper. Currently the number of maps to choose from is still limited, but the query below shows what is possible, using a map from 1832.

Share of Roman Catholics per municipality in 1899 (historical map LAYERS DO NOT WORK ATM):

III. Comparing the maps

The actual Figure 2 from Boomen & Ekamper can be seen below. Comparing it with our maps shows they are almost identical. What is more, using these queries as starting points, you're able to connect to all sorts of interesting data from the HDNG or other datasets, and visualize the results.

Source:

Nynke van den Boomen & Peter Ekamper (2015) Denied their ‘natural nourishment’: religion, causes of death and infant mortality in the Netherlands, 1875–1899, The History of the Family, 20:3, 391-419, DOI: 10.1080/1081602X.2015.1022199

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This datastory was created by the CLARIAH project (WP4)

For questions and suggestions please email Ruben Schalk

This version: July 2021