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maplibre-contour

maplibre-contour is a plugin to render contour lines in MapLibre GL JS from raster-dem sources that powers the terrain mode for onthegomap.com.

Live example | Code

To use it, import the maplibre-contour package with a script tag:

<script src="https://unpkg.com/maplibre-contour@0.1.0/dist/index.min.js"></script>

Or as an ES6 module: npm add maplibre-contour

import mlcontour from "maplibre-contour";

Then to use, first create a DemSource and register it with maplibre:

var demSource = new mlcontour.DemSource({
  url: "https://url/of/dem/source/{z}/{x}/{y}.png",
  encoding: "terrarium", // "mapbox" or "terrarium" default="terrarium"
  maxzoom: 13,
  worker: true, // offload isoline computation to a web worker to reduce jank
  cacheSize: 100, // number of most-recent tiles to cache
  timeoutMs: 10_000, // timeout on fetch requests
});
demSource.setupMaplibre(maplibregl);

Then configure a new contour source and add it to your map:

map.addSource("contour-source", {
  type: "vector",
  tiles: [
    demSource.contourProtocolUrl({
      // convert meters to feet, default=1 for meters
      multiplier: 3.28084,
      thresholds: {
        // zoom: [minor, major]
        11: [200, 1000],
        12: [100, 500],
        14: [50, 200],
        15: [20, 100],
      },
      // optional, override vector tile parameters:
      contourLayer: "contours",
      elevationKey: "ele",
      levelKey: "level",
      extent: 4096,
      buffer: 1,
    }),
  ],
  maxzoom: 15,
});

Then add contour line and label layers:

map.addLayer({
  id: "contour-lines",
  type: "line",
  source: "contour-source",
  "source-layer": "contours",
  paint: {
    "line-color": "rgba(0,0,0, 50%)",
    // level = highest index in thresholds array the elevation is a multiple of
    "line-width": ["match", ["get", "level"], 1, 1, 0.5],
  },
});
map.addLayer({
  id: "contour-labels",
  type: "symbol",
  source: "contour-source",
  "source-layer": "contours",
  filter: [">", ["get", "level"], 0],
  layout: {
    "symbol-placement": "line",
    "text-size": 10,
    "text-field": ["concat", ["number-format", ["get", "ele"], {}], "'"],
    "text-font": ["Noto Sans Bold"],
  },
  paint: {
    "text-halo-color": "white",
    "text-halo-width": 1,
  },
});

You can also share the cached tiles with other maplibre sources that need elevation data:

map.addSource("dem", {
  type: "raster-dem",
  encoding: "terrarium",
  tiles: [demSource.sharedDemProtocolUrl],
  maxzoom: 13,
  tileSize: 256,
});

How it works

DemSource.setupMaplibre uses MapLibre's addProtocol utility to register a callback to provide vector tile for the contours source. Each time maplibre requests a vector tile:

• DemManager fetches (and caches) the raster-dem image tile and its neighbors so that contours are continuous across tile boundaries.
  • When DemSource is configured with worker: true, it uses RemoteDemManager to spawn worker.ts in a web worker. The web worker runs LocalDemManager locally and uses the Actor utility to send cancelable requests and responses between the main and worker thread.
• decode-image.ts decodes the raster-dem image RGB values to meters above sea level for each pixel in the tile.
• HeightTile stitches those raw DEM tiles into a "virtual tile" that contains the border of neighboring tiles, aligns elevation measurements to the tile grid, and smooths the elevation measurements.
• isoline.ts generates contour isolines from a HeightTile using a marching-squares implementation derived from d3-contour.
• vtpbf.ts encodes the contour isolines as mapbox vector tile bytes.

MapLibre sends that vector tile to its own worker, decodes it, and renders as if it had been generated by a server.

Why?

There are a lot of parameters you can tweak when generating contour lines from elevation data like units, thresholds, and smoothing parameters. Pre-generated contour vector tiles require 100+gb of storage for each variation you want to generate and host. Generating them on-the-fly in the browser gives infinite control over the variations you can use on a map from the same source of raw elevation data that maplibre uses to render hillshade.

License

maplibre-contour is licensed under the BSD 3-Clause License. It includes code adapted from:

• d3-contour (ISC license)
• vt-pbf (MIT license)