Voronoi Catchment Map for Central-Mexico Airports

This script builds an interactive Leaflet map that shows:

• Voronoi (Thiessen) polygons around a set of four airports
  (AIFA, AICM, AIP, QIA),
• every municipality intersecting those polygons,
• an estimate of the population served by each airport (total population inside its Voronoi cell).

The result is rendered in a browser tab (via RStudio Viewer or the default system browser) with layer controls for Voronoi cells, municipalities and airport markers.

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1. Prerequisites

R packages

install.packages(c(
  "sf",            # modern spatial classes
  "sp",            # legacy spatial classes used by deldir
  "deldir",        # Voronoi / Delaunay tessellation
  "leaflet",       # interactive maps
  "leaflet.providers",
  "dplyr",
  "htmltools"
))

Input files

Place these files in the working directory before running the script:

File	Description
Muni_2012gw.shp (+ .dbf, .prj, .shx)	Shapefile of Mexican municipalities (Geo-WGS 84).
NOM_MUN_POBTOT.csv	Two-column CSV: municipality name (NOM_MUN) and total population (POBTOT).

The script assumes that municipality names match exactly between the SHP and the CSV; adjust left_join() if your field names differ.

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2. How it works

1. Load spatial data

   • Read municipalities (sf), re-project to EPSG 4326.
   • Join population totals and keep the original area (area_orig).

2. Define airport seed points Hard-coded longitude/latitude coordinates.

3. Create Voronoi polygons

   • deldir() computes a tessellation within an expanded bounding box.
   • Polygons converted to sp::SpatialPolygons, then to sf.

4. Population assignment

   • st_intersection() clips municipalities by each Voronoi cell.
   • Population is apportioned by area fraction (pop_int = frac * POBTOT).
   • Summed per airport and merged back into the Voronoi layer.

5. Prepare municipality layer Only municipalities that intersect at least one Voronoi polygon are kept.

6. Leaflet map

   • Basemap: CartoDB.Voyager tiles.
   • Voronoi polygons: semi-transparent fill, airport-specific colors, popup with airport code and population.
   • Municipalities: thin outlines, tooltip shows name and population.
   • Markers: fixed labels for each airport (edit the hard-coded text to reflect updated population numbers).
   • Layer switcher included.

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3. Running the script

setwd("/path/to/your/folder")  # adjust to where the files are located
source("alfa.R")

The map appears automatically. Save it as HTML if needed:

m <- <leaflet_object_returned_by_script>
htmlwidgets::saveWidget(m, "airport_voronoi_map.html", selfcontained = TRUE)

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4. Customisation

• Add more airports Extend the airports data frame (same order: airport, lon, lat).
• Different basemap Replace providers$CartoDB.Voyager with any provider from leaflet.providers.
• Styling Modify the fillColor logic or add color scales with leaflet::colorNumeric() if you prefer a gradient by population.

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5. Troubleshooting

• Topology errors after st_intersection() Some municipal polygons may be invalid. Run muni <- st_make_valid(muni) before the intersection step.
• Population totals are zero Check field names in NOM_MUN_POBTOT.csv (NOM_MUN, POBTOT) and confirm they match the shapefile attribute table.
• Performance For very large shapefiles, consider dissolving municipalities into states first or simplifying geometries (st_simplify) before intersection.

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License

The script is released under the MIT License. Spatial datasets carry their own licenses (check the data source).