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README

Flexurba is an open-source R package to construct flexible urban delineations which can be tailored to specific applications or research questions. The package was originally developed to flexibly reconstruct the Degree of Urbanisation (DEGURBA) classification, but has since been expanded to support a broader range of delineation approaches.

The source code of the package is available on this repository and the documentation of all functions can be found on this website.

To acknowledge the use of the package and for an extensive description of its contribution, please refer to the following journal article:

Van Migerode, C., Poorthuis, A., & Derudder, B. (2024). Flexurba: An open-source R package to flexibly reconstruct the Degree of Urbanisation classification. Environment and Planning B: Urban Analytics and City Science, 51(7), 1706-1714.

The flexurba package can be installed as follows:

install.packages("flexurba")

Important notes for installation:

• The flexurba package uses C++ code for certain functions. Please make sure to have MAKE installed on your computer.

• While installing the package, R will give a prompt to install Rtools (if not already installed). Please click YES and make sure you have appropriate administrator rights.

The DEGURBA methodology classifies the cells of a 1 km² population grid into three different categories based on the following rules (detailed in the GHSL Data Package 2023):

• Urban Centres are clusters of cells (rooks continuity) with a minimum population density of 1500 inhabitants per km² of permanent land or a minimum ‘optimal’ built-up area threshold. In addition, the total population in these clusters should be at least 50 000. Gaps in the urban centres are filled and edges are smoothed.
• Urban Clusters are clusters of cells (queens continuity) with a minimum population density of 300 inhabitants per km² of permanent land and a minimum total population of 5000 inhabitants. Cells that belong to urban centres are removed from urban clusters.
• Rural grid cells do not belong to an urban centre or cluster.

We can reconstruct the standard grid cell classification for Belgium as follows.

library(flexurba)

# load the example data for Belgium
data_belgium <- DoU_load_grid_data_belgium()

# run the DEGURBA algorithm with the standard parameter settings
classification1 <- DoU_classify_grid(data = data_belgium)

# plot the resulting grid
DoU_plot_grid(classification1)

The function DoU_classify_grid() also allows to adapt the standard parameters in the DEGURBA algorithm. For example, the population thresholds for urban centres can be adapted by changing the following parameters:

• UC_density_threshold = 1250: the minimum density threshold for urban centres (UC) is changed to 1250 inhabitants per km² instead of the standard value of 1500 inhabitants per km².

• UC_size_threshold = 60000: the minimum size threshold for urban centres (UC) is increased from 50 000 inhabitants to 60 000 inhabitants.

# run the algorithm with custom parameter settings
classification2 <- DoU_classify_grid(
  data = data_belgium,
  # here, we can specify custom population thresholds
  parameters = list(
    UC_density_threshold = 1250,
    UC_size_threshold = 60000
  )
)

# plot the resulting grid
DoU_plot_grid(classification2)

For more information about the possible parameters settings, see the section ‘Custom specifications’ in the documentation of DoU_classify_grid().

Apart from DEGURBA, several other delineation approaches enforce thresholds on gridded datasets. The accompanying flexurbaData package provides pre-processed datasets that can serve as proxy to identify urban areas. We can construct urban boundaries based on these proxy datasets using the function apply_threshold(). The code examples below enforce a predefined threshold on (1) built-up area and (2) night-time light data.

# (1) predefined threshold of 15% built-up area

# load the example proxy data for Belgium
proxy_data_belgium <- load_proxies_belgium()

# apply the threshold
builtupclassification <- apply_threshold(proxy_data_belgium$built,
  type = "predefined",
  threshold_value = 0.15
)

# plot the resulting urban boundaries
terra::plot(builtupclassification$rboundaries)

# (2) predefined threshold of 15 nW/cm³/sr

# apply the threshold
lightclassification <- apply_threshold(proxy_data_belgium$light,
  type = "predefined",
  threshold_value = 15
)

# plot the resulting urban boundaries
terra::plot(lightclassification$rboundaries)

Besides a predefined threshold, the function apply_threshold() also implements other types of thresholding approaches. For more information on these, see vignette("vig8-apply-thresholds").

For more code examples, please consult the documentation pages of the individual functions. The following vignettes are also available with more information and workflows using flexurba:

• The vignette("flexurba") is a “Get Started” tutorial on using flexurba to reconstruct DEGURBA. It shows how to download the data from the GHSL website, construct a grid cell classification and a spatial units classification.
• The vignette("vig1-DoU-level2") showcases how the grid cell and spatial units classification can be constructed according to second hierarchical level of DEGURBA.
• The vignette("vig2-DoU-multiple-configurations") gives an overview on how to use the package to generate multiple alternative versions of DEGURBA by systematically varying parameters in the algorithm.
• The vignette("vig3-DoU-global-scale") explains how a global DEGURBA classification can be established in a memory-efficient manner.
• The GHSL released different versions of the Degree of Urbanisation in the past years. The vignette("vig4-DoU-comparison-releases") compares the method described in Data Packages 2022 with the method described in Data Package 2023.
• The vignette("vig5-DoU-computational-requirements") elaborates on the computational requirements of the package to reconstruct the DEGURBA classification, and compares the computational load with the existing GHSL tools.
• The vignette("vig6-DoU-comparison-GHSL-SMOD") compares the DEGURBA grid classification generated by flexurba with the official GHSL SMOD layer and explains few discrepancies between the two classifications.
• The vignette("vig7-DoU-population-grid") illustrates how the DEGURBA functionalities can be used with other population grids (e.g. WorldPop).
• The vignette("vig8-apply-thresholds") elaborates on the benefits and limitations of different thresholding approach implemented by the function apply_threshold().
• The vignette("vig9-different-proxies") illustrates how different proxy datasets can be used to identify urban areas using a combination of the data in the flexurbaData package and the functions in the flexurba package.

Disclaimer: The flexurba package includes a reconstruction of DEGURBA’s algorithm, and by no means contains an official implementation. For the official documents, readers can consult Dijkstra et al. (2021), Eurostat (2021) and the Global Human Settlement Layer website.

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