libgd-gis

add_geojson — GeoJSON Layer Rendering

The add_geojson method allows you to load and render GeoJSON files directly onto a map. Each GeoJSON file is parsed, classified, styled, and rendered as a map layer.

Note libgd-gis is evolving very fast, so some examples may temporarily stop working. Please report issues or ask for help: https://github.com/ggerman/libgd-gis/issues or ggerman@gmail.com

---

Basic usage

require "gd/gis"
require "gd"

TOKYO = [139.68, 35.63, 139.82, 35.75]

map = GD::GIS::Map.new(
  bbox: TOKYO,
  zoom: 13,
  basemap: :esri_satellite
)

map.style = GD::GIS::Style.load("solarized")

map.add_geojson("railways.geojson")
map.add_geojson("parks.geojson")
map.add_geojson("wards.geojson")

map.render
map.save("tokyo.png")

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What add_geojson does

When calling add_geojson, libgd-gis:

1. Loads and parses the GeoJSON file
2. Iterates over all features
3. Classifies geometries (lines, polygons, points)
4. Applies the active YAML style
5. Renders the layer respecting projection and zoom

Each call adds a new layer on top of the previous ones.

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Supported geometries

• Point
• MultiPoint
• LineString
• MultiLineString
• Polygon
• MultiPolygon

Rendering behavior depends on the active style.

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Layer ordering

GeoJSON layers are rendered in the order they are added:

map.add_geojson("base.geojson")
map.add_geojson("overlay.geojson")

Later layers are drawn on top of earlier ones.

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Styling and YAML integration

add_geojson relies entirely on the active GD::GIS::Style.

• Geometry types are mapped to style sections
• Feature properties are used for classification
• Visual output is fully style-driven

See examples/styles/ for reference styles.

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Adding post-render overlays

After rendering, you can open the generated image and add custom overlays using ruby-libgd directly.

Example: adding a label overlay

img = GD::Image.open("tokyo.png")

font = "../fonts/DejaVuSans-Bold.ttf"

img.filled_rectangle(24, 24, 264, 88, [0, 0, 0])

img.text(
  "TOKYO",
  x: 48,
  y: 68,
  size: 32,
  color: [255, 255, 255],
  font: font
)

img.save("tokyo.png")

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Summary

• Load and render GeoJSON files as layers
• Multiple GeoJSON files supported
• Fully style-driven rendering
• Layer order matters
• Compatible with post-render overlays

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