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NaturalEarth — France and China (2D)

This example demonstrates using GeoGmsh with the NaturalEarth.jl package, which provides built-in access to Natural Earth country boundaries — no files to download or manage.

Features highlighted:

  • Passing a GeoJSON.FeatureCollection directly (no read step needed)
  • Filtering by feature property (:NAME) combined with ring index
  • bbox_size to normalise coordinates into a dimensionless bounding box
  • MinEdgeLength simplification to remove short-edge redundancy

Mainland selection with ring == 1

_expand_rings numbers polygon components by area, largest first. ring == 1 therefore selects the mainland and excludes islands and overseas territories automatically.

FranceChina
France meshChina mesh
using GeoGmsh
using NaturalEarth

data_dir = joinpath(@__DIR__, "..", "data")
mkpath(data_dir)

Load all country boundaries at 1:10M scale. The result is a GeoJSON.FeatureCollection that GeoGmsh accepts directly.

countries = naturalearth("admin_0_countries", 10)

France

Metropolitan France only (ring == 1 excludes overseas territories such as French Guiana, Réunion and Martinique). Reprojected to Web Mercator (EPSG:3857) and simplified to ≥ 5 km edges.

println("=== France ===")

geoms_to_geo(
  countries, joinpath(data_dir, "france");
  select       = row -> get(row, :NAME, "") == "France" && row.ring == 1,
  target_crs   = "EPSG:3857",
  simplify_alg = MinEdgeLength(tol = 5_000.0),
  bbox_size    = 100.0,
  verbose      = true,
)

geoms_to_msh(
  countries, joinpath(data_dir, "france");
  select       = row -> get(row, :NAME, "") == "France" && row.ring == 1,
  target_crs   = "EPSG:3857",
  simplify_alg = MinEdgeLength(tol = 5_000.0),
  bbox_size    = 100.0,
  mesh_size    = 2.0,
  verbose      = true,
)

China

Mainland China, simplified to ≥ 20 km edges (larger tolerance suits the coarser 1:10M source data and the larger geographic extent).

println("\n=== China ===")

geoms_to_geo(
  countries, joinpath(data_dir, "china");
  select       = row -> get(row, :NAME, "") == "China" && row.ring == 1,
  target_crs   = "EPSG:3857",
  simplify_alg = MinEdgeLength(tol = 20_000.0),
  bbox_size    = 100.0,
  verbose      = true,
)

geoms_to_msh(
  countries, joinpath(data_dir, "china");
  select       = row -> get(row, :NAME, "") == "China" && row.ring == 1,
  target_crs   = "EPSG:3857",
  simplify_alg = MinEdgeLength(tol = 20_000.0),
  bbox_size    = 100.0,
  mesh_size    = 2.0,
  verbose      = true,
)

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