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choropleth-topojson.md

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Creating the choropleth TopoJson data

The Dutch municipal and safety region borders get updated quite frequently, so there is a chance that a new set of GEOJson data will have to be created eventually. This section describes how to generate this data with the correct projection applied.

Software requirements

  • QGIS (download)

    At this moment, QGIS is not yet notarized as required by macOS Catalina (10.15) security rules. On first launch, please right-click on the QGIS app icon, hold down the Option key, then choose Open.

  • Mapshaper (Online tool available at mapshaper.org)

Creating the GEOJson

Importing the source data

To create the data files we need, we will be using cbsgebiedsindelingen_2023.gpkg as the data source. This package can be downloaded from: www.pdok.nl/downloads/-/article/cbs-gebiedsindelingen. Download the XML file and find the link to the latest *.gpkg file in it. The URL looks like this: https://geodata.nationaalgeoregister.nl/cbsgebiedsindelingen/extract/cbsgebiedsindelingen_2023_v1.gpkg.

⚠️ ATTENTION: At the time of writing, the cbsgebiedsindelingen_2023.gpkg file is the latest version. It is of course very likely that by the time new data needs to be generated that a newer file is available. Pay attention to download the very latest version. If there may be a case that the latest CBS data is still not available and some manual work needs to be done, use the current TopoJson file and proceed to Generate TopoJson.

After downloading, import the package into QGIS (the easiest way of doing this is by simply dragging the package into the main QGIS window) and select the following layers to be added:

  • gemeente_gegeneraliseerd_2023
  • veiligheidsregio_gegeneraliseerd_2023
  • landsdeel_gegeneraliseerd_2023

Import with the CRS:EPSG:28992 - Amersfoort / RD New - Projected projection which should the default while importing.

Note: it is also possible to use and import *.cbf or *.shp files instead of the GeoPackage (*.gpkg) file. Those files are often inside the GeoPackage file. The steps below also applies to those other file extensions. Importing non-gpkg files could be helpfull when maps are generated manually instead of coming from CBS.

Cleaning up the imported data

To clean up the data we have to perform the following steps:

Create the municipalities data file (gemeente_gegeneraliseerd_2023):

  1. Right-click on the layer > Properties > Fields
  2. Rename (click the pencil to active edit mode):
    • statcode to code
  3. Apply > Ok
  4. Right-click on the layer > Export > Save Features As..
  5. Use the following settings:
    • Format: GeoJSON
    • File Name: Click on the three dots next to the file name on Step 5 of creating data files and choose a directory and save it as following: gemeente_gegeneraliseerd_2023.geojson
    • CRS: Project CRS: EPSG:28992 - Amersfoort / RD New
    • Open "Select fields to export..."
      • Deselect all and only select: code
  6. Export by clicking "Ok"

Create the safety regions data file (veiligheidsregio_gegeneraliseerd_2023):

  1. Right-click on the layer > Properties > fields
  2. Rename (click the pencil to active edit mode):
    • statcode to code
  3. Apply > Ok
  4. Right-click on the layer > Export > Save Features As..
  5. Use the following settings:
    • Format: GeoJSON
    • File Name: Click on the three dots next to the file name on Step 5 of creating data files and choose a directory and save it as following: veiligheidsregio_gegeneraliseerd_2023.geojson
    • CRS: Project CRS: EPSG:28992 - Amersfoort / RD New
    • Open "Select fields to export..."
      • Deselect all and only select: code
  6. Export by clicking "Ok"

Create the Netherlands data file (landsdeel_gegeneraliseerd_2023):

  1. Select the layer and go to Vector > Geoprocessing Tools > Dissolve > Run, this will merge the different areas
  2. Select the new layer and right-click on the layer > Export > Save Features As..
  3. Use the following settings:
    • Format: GeoJSON
    • File Name: Click on the three dots next to the file name on Step 5 of creating data files and choose a directory and save it as following: landsdeel_gegeneraliseerd_2023.geojson
    • CRS: Project CRS: EPSG:28992 - Amersfoort / RD New
    • Open "Select fields to export..."
      • Deselect all
  4. Export by clicking "Ok"

To make sure the coordinate system is correct we have to convert the exported files to lat and lon coordinates:

  1. Create a new project and add the three files:
    • landsdeel_gegeneraliseerd_2023.geojson
    • veiligheidsregio_gegeneraliseerd_2023.geojson
    • gemeente_gegeneraliseerd_2023.geojson
  2. For each layer:
    • Select the new layer and right-click on the layer > Export > Save Features As..
    • Format: GeoJSON
    • File Name: veiligheidsregio_gegeneraliseerd_2023_WGS84.geojson
    • CRS: (Default CRS:) EPSG:4326 - WGS 84
    • Export by clicking "Ok"
    • Make sure the output files contain longitude and latitude coordinates by opening the file and manually checking this by hovering over the map and looking for coordinate on the bottom toolbar, otherwise the next step will not work.

Generate TopoJson:

⚠️ ATTENTION: To make custom changes on the TopoJson using mapshaper, please take a look at this link for tips and tricks. It's possible that the map looks a little streched on MapShaper. This is most likely not an issue and it will display correctly once it's integrated in the Dashboard again.

  1. Upload the three different *_WGS84.geojson files to: mapshaper.org

  2. Rename the layers by clicking on the name in the dropdown:

    • Change: gemeente_gegeneraliseerd_2023_WGS84 to gm_features
    • Change: landsdeel_gegeneraliseerd_2023_WGS84 to nl_features
    • Change: veiligheidsregio_gegeneraliseerd_2023_WGS84 to vr_features

  3. Export to TopoJSON > nl-vr-gm-high-detail.topo.json;

  4. Upload the output nl-vr-gm-high-detail.topo.json to a new instance of mapshaper.org

  5. Do for each layer:

    • Select the layer and open the console; simplify using the following command: $ -simplify 27.5%

  6. When all the layers are simplified export to TopoJSON > nl-vr-gm.topo.json

  7. Add the new data file to the project at packages/app/src/pages/api/topo-json.

  8. Update the file topology.ts, if necessary. (located at packages/app/src/pages/api/choropleth)

Example of the topology.ts file

import { FeatureCollection, MultiPolygon, Polygon } from 'geojson';
import * as topojson from 'topojson-client';
import nlTopology from './nl-vr-gm.topo.json';

export type CodedGeoProperties = {
  code: string;
};

export type CodedGeoJSON = FeatureCollection<MultiPolygon | Polygon, CodedGeoProperties>;

export const nlGeo = topojson.feature(nlTopology, nlTopology.objects.nl_features) as CodedGeoJSON;

export const vrGeo = topojson.feature(nlTopology, nlTopology.objects.vr_features) as CodedGeoJSON;

export const gmGeo = topojson.feature(nlTopology, nlTopology.objects.gm_features) as CodedGeoJSON;

Note that the map data is stored as TopoJson. This is because of the size optimizations that TopoJson provides, at run time, after being downloaded by the choropleth component this TopoJson is converted to GeoJson.

Adjust municipal file

Please update the municipal configuration file accordingly. Make sure to remove the former municipalities so that the value of a gemcode only exists once in this file.

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