topohelper

TopoJSON Helper is a javascript library that will help you manipulate topoJSON with ease. From a geoJSON or a topoJSON, apply precise operations to take full advantage of this great format.

See it in action on Observable

• Hello topohelper
• Interactive docs

Install

In browser

<script src="https://cdn.jsdelivr.net/npm/topohelper@latest/+esm"></script>

In ObservableHQ

topohelper = require("topohelper");

In Node.js or Application Bundles

After installing topohelper as a dependency.

CommonJS module

const topohelper = require("topohelper@latest");

ES module

import * as topohelper from "topohelper";

Build Instructions

To build and develop topohelper locally:

• git clone https://github.com/AtelierCartographie/topohelper.git
• Run pnpm install to install dependencies for all packages.
• Run pnpm build

Usage

Docs

Import geofile (topojson or geojson)

from(topojson) - source

Instantiate a topohelper class of a topojson.
A deep copy of the topojson can be made.

• options.deep {Boolean} - create a deep copy of the topojson. Default is false.

topohelper.from(topojson_file);

fromGeojson(geojson) - source

Instantiate a topohelper class from a geojson or an array of geojson.
Before inputs are convert to a single topojson.

• options.name {String|String[]} - name of the new layer

Operations

lines(options) - source

Convert to a single MultiLineString a topojson or a topojson geometry object.
Point and MultiPoint geometry object are ignore.

• options.layer {String|Number} - a single target layer (name or index), if "all", all layers are used. Default: last layer created or first layer on first operation
• options.name {String} - name of the new layer; default: operation name "lines"
• options.addLayer {Boolean} - if true add a layer to existing ones; default: true

topohelper.from(topojson_file).lines();

innerlines(options) - source

Keep share arcs of adjacent Polygon|MultiPolygon as a single MultiLineString.
A group by a property can be applied at the same time.
Point and MultiPoint geometry object are ignore.

• options.layer {String|Number} - a single target layer (name or index); default: last layer created or first layer on first operation
• options.groupby {String} - group by a data property before
• options.name {String} - name of the new layer; default: operation name "innerlines"
• options.addLayer {Boolean} - if true add a layer to existing ones; default: true

topohelper.from(topojson_file).innerlines({ groupby: "property_name" });

outerlines(options) - source

Remove share arcs of adjacent Polygon|MultiPolygon and transform the rest as a single MultiLineString.
Point and MultiPoint geometry object are ignore.

• options.layer {String|Number} - a single target layer (name or index); default: last layer created or first layer on first operation
• options.name {String} - name of the new layer; default: operation name "outerlines"
• options.addLayer {Boolean} - if true add a layer to existing ones; default: true

topohelper.from(topojson_file).outerlines();

merge(options) - source

Union all Polygon|MultiPolygon of a layer. Share arcs of adjacent Polygon|MultiPolygon are removed.
A group by a property can be applied at the same time.

• options.layer {String|Number} - a single target layer (name or index); default: last layer created or first layer on first operation
• options.groupby {String} - group by a data property before
• options.name {String} - name of the new layer; default: operation name "merge"
• options.addLayer {Boolean} - if true add a layer to existing ones; default: true

topohelper.from(topojson_file).merge({
  groupby: "property_name",
  rollup: { sum: aq.op.sum("property_name") },
});

filter(options) - source

Filter a topojson on a condition. The condition function expose geometry object (type and arcs) and his properties if any.

• options.layer {String|Number} - a single target layer (name or index); default: last layer created or first layer on first operation
• options.condition {Function} - function or arrow function.
• options.name {String} - name of the new layer; default: operation name "filter"
• options.addLayer {Boolean} - if true add a layer to existing ones; default: true

topohelper
  .from(topojson_file)
  .filter({ condition: (d) => d.properties.dep == "76" });

centroids(options) - source

Get centroids of each Polygon|MultiPolygon of a topojson layer.
Point|MultiPoint and LineString|MultiLineString geometry object are ignore.

• options.layer {String|Number} - a single target layer (name or index); default: last layer created or first layer on first operation
• options.name {String} - name of the new layer; default: operation name "centroids"
• options.addLayer {Boolean} - true add a layer to existing ones
• options.better {Boolean} - if true calcul a pole of inaccessibility instead of a centroid; default: false

topohelper.from(topojson_file).centroids({ layer: 0 });

simplify(options) - source

Simplify all layers of a topojson. Intensity of simplification is control by the level option.

• options.level {Number} - a level of simplification between 0 and 1. 1 = no simplification, 0 = maximum. Default 0.5

topohelper.from(topojson_file).simplify({ level: 0.3 });

properties(options) - source

Manipulate properties of a topojson layer.
Four types of operations are available: select, rename, derive and join.
Manipulation order of the verbs respect options object order. {select, rename} ≠ {rename, select}

• options.layer {String|Number} - a single target layer (name or index); default: last layer created or first layer on first operation
• options.select {String|String[]} - Select properties to keep.
• options.rename {Object} - Rename some or all properties. ex: {'oldName': 'newName'}.
• options.derive {Object} - Add or modify properties. ex: {newColumn: (d) => d.oldColumn + 1}
• options.join {Object} - Join left an external table (array of objects). ex: {'data': 'table', on: 'id'} or {'data': 'table', on: ['source_id', 'target_id']}.

topohelper.from(topojson_file).properties({
  select: ["ID", "POPULATION"],
  rename: { ID: "id", POPULATION: "pop" },
  derive: { pop1000: (d) => d.pop / 1000 },
});

Preview

view(options) - source

Preview a topojson in Canvas. Zoom and tooltip are built in as option.
Multiple layers can be shown at the same time.

• options.layer {String[]|Number[]} - one or several layer (name or index). If "last", render the last layer created. If omit or "all" render all layers. Default: "all".
• options.zoom {Boolean} - if true add a tooltip; default: false
• options.tooltip {Boolean} - if true add a tooltip; default: false
• options.size {Number[]} - canvas dimensions as [width, height]

topohelper.from(topojson_file).view({ zoom: true });

Export

toTopojson(options) - source

Convert a topohelper topojson to a classic topojson with option to select layers and quantize result (good size reduction).

• options.layer {String[]|Number[]} - targets layers (name or index). If omit or "all" use all layers.
• options.q {Boolean|Number} - level of quantization. if true quantization is applied with a value of 1e4. Default: true.

topohelper.from(topojson_file)
          .innerlines()
          .toTopojson({layer: "innerlines}) // export a single layer

toGeojson(options) - source

Convert a topohelper topojson to a geojson or an array of geojson.

• options.layer {String[]|Number[]} - targets layers (name or index). If omit or "all" use all layers.

topohelper.from(topojson_file).innerlines().centroids({ layer: 0 }).toGeojson(); // export all layers as an array of geojson

toObjects(options) - source

Export properties of a layer.

• options.layer {String|Number} - target layer (name or index).

topohelper.from(topojson_file).toObjects();