proj4

What is proj4?

The proj4 npm package is a JavaScript library for performing cartographic projections and coordinate transformations. It allows you to convert geographic coordinates from one coordinate system to another, which is essential for mapping and GIS applications.

What are proj4's main functionalities?

Coordinate Transformation

This feature allows you to transform coordinates from one projection system to another. In this example, coordinates in WGS84 (EPSG:4326) are transformed to Web Mercator (EPSG:3857).

const proj4 = require('proj4');

// Define two coordinate systems
const firstProjection = 'EPSG:4326'; // WGS84
const secondProjection = 'EPSG:3857'; // Web Mercator

// Transform coordinates from WGS84 to Web Mercator
const coordinates = [12.4924, 41.8902]; // Longitude, Latitude for Rome, Italy
const transformedCoordinates = proj4(firstProjection, secondProjection, coordinates);
console.log(transformedCoordinates); // Output: [1389919.142, 5146592.928]

Defining Custom Projections

This feature allows you to define custom projections using Proj4 strings. In this example, a UTM projection is defined and used to transform UTM coordinates to WGS84.

const proj4 = require('proj4');

// Define a custom projection
const customProjection = '+proj=utm +zone=33 +ellps=WGS84 +datum=WGS84 +units=m +no_defs';

// Transform coordinates using the custom projection
const coordinates = [500000, 4649776.22482]; // UTM coordinates
const transformedCoordinates = proj4(customProjection, 'EPSG:4326', coordinates);
console.log(transformedCoordinates); // Output: [9.0, 42.0]

Batch Transformations

This feature allows you to perform batch transformations on an array of coordinates. In this example, coordinates for Rome, London, and Tokyo are transformed from WGS84 to Web Mercator.

const proj4 = require('proj4');

// Define two coordinate systems
const firstProjection = 'EPSG:4326'; // WGS84
const secondProjection = 'EPSG:3857'; // Web Mercator

// Batch transform an array of coordinates
const coordinatesArray = [
  [12.4924, 41.8902], // Rome
  [-0.1276, 51.5074], // London
  [139.6917, 35.6895] // Tokyo
];
const transformedArray = coordinatesArray.map(coords => proj4(firstProjection, secondProjection, coords));
console.log(transformedArray); // Output: [[1389919.142, 5146592.928], [-14273.880, 6711545.653], [15550447.354, 4258423.671]]

Other packages similar to proj4

proj4js

Proj4js is another JavaScript library for performing cartographic projections and coordinate transformations. It is similar to proj4 but is specifically designed for use in web applications. Proj4js is often used in conjunction with mapping libraries like OpenLayers and Leaflet.

geodesy

The geodesy package provides tools for geodesic calculations, including coordinate transformations, distance calculations, and more. While it offers similar functionality to proj4, it also includes additional geodesic calculations that may be useful for more advanced GIS applications.

turf

Turf is a JavaScript library for spatial analysis. It includes a wide range of geospatial functions, including coordinate transformations, but also offers additional features like buffering, clustering, and spatial joins. Turf is more comprehensive than proj4 and is suitable for more complex geospatial analysis tasks.

README

PROJ4JS

Proj4js is a JavaScript library to transform point coordinates from one coordinate system to another, including datum transformations. Originally a port of PROJ (then known as PROJ.4) and GCTCP C (Archive) it is a part of the MetaCRS group of projects.

Installing

Depending on your preferences

npm install proj4
bower install proj4
component install proj4js/proj4js

or just manually grab the file proj4.js from the latest release's dist/ folder.

If you do not want to download anything, Proj4js is also hosted on cdnjs for direct use in your browser applications.

Using

The basic signature is:

proj4([fromProjection, ]toProjection[, coordinates])

Projections can be proj or wkt strings.

Coordinates may be an object of the form {x:x,y:y} or an array of the form [x,y].

When all 3 arguments are given, the result is that the coordinates are transformed from projection1 to projection 2. And returned in the same format that they were given in.

var firstProjection = 'PROJCS["NAD83 / Massachusetts Mainland",GEOGCS["NAD83",DATUM["North_American_Datum_1983",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],AUTHORITY["EPSG","6269"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4269"]],UNIT["metre",1,AUTHORITY["EPSG","9001"]],PROJECTION["Lambert_Conformal_Conic_2SP"],PARAMETER["standard_parallel_1",42.68333333333333],PARAMETER["standard_parallel_2",41.71666666666667],PARAMETER["latitude_of_origin",41],PARAMETER["central_meridian",-71.5],PARAMETER["false_easting",200000],PARAMETER["false_northing",750000],AUTHORITY["EPSG","26986"],AXIS["X",EAST],AXIS["Y",NORTH]]';
var secondProjection = "+proj=gnom +lat_0=90 +lon_0=0 +x_0=6300000 +y_0=6300000 +ellps=WGS84 +datum=WGS84 +units=m +no_defs";
//I'm not going to redefine those two in latter examples.
proj4(firstProjection,secondProjection,[-122.305887, 58.9465872]);
// [-2690575.447893817, 36622916.8071244564]

The library can also parse coordinates provided with an elevation and measure, again as an object of the form {x:x,y:y,z:z,m:m} or an array of the form [x,y,z,m].

proj4(firstProjection,secondProjection,[-122.305887, 58.9465872,10]);
// [-2690575.447893817, 36622916.8071244564, 10]

If only 1 projection is given then it is assumed that it is being projected from WGS84 (fromProjection is WGS84).

proj4(firstProjection,[-71,41]);
// [242075.00535055372, 750123.32090043]

If no coordinates are given an object with two methods is returned, its methods are forward which projects from the first projection to the second and inverse which projects from the second to the first.

proj4(firstProjection,secondProjection).forward([-122.305887, 58.9465872]);
// [-2690575.447893817, 36622916.8071244564]
proj4(secondProjection,firstProjection).inverse([-122.305887, 58.9465872]);
// [-2690575.447893817, 36622916.8071244564]

And as above if only one projection is given, it's assumed to be coming from wgs84:

proj4(firstProjection).forward([-71,41]);
// [242075.00535055372, 750123.32090043]
proj4(firstProjection).inverse([242075.00535055372, 750123.32090043]);
// [-71, 40.99999999999986]

Note: The generation of the floating point value 40.99999999999986 in this example represents the fact that some variance in precision is involved in any conversion between one coordinate reference system and another.

Named Projections

If you prefer to define a projection as a string and reference it that way, you may use the proj4.defs method which can be called 2 ways, with a name and projection:

proj4.defs('WGS84', "+title=WGS 84 (long/lat) +proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees");

or with an array

proj4.defs([
  [
    'EPSG:4326',
    '+title=WGS 84 (long/lat) +proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees'],
  [
    'EPSG:4269',
    '+title=NAD83 (long/lat) +proj=longlat +a=6378137.0 +b=6356752.31414036 +ellps=GRS80 +datum=NAD83 +units=degrees'
  ]
]);

you can then do

proj4('EPSG:4326');

instead of writing out the whole proj definition, by default proj4 has the following projections predefined:

• 'EPSG:4326', which has the following alias
  • 'WGS84'
• 'EPSG:4269'
• 'EPSG:3857', which has the following aliases
  • 'EPSG:3785'
  • 'GOOGLE'
  • 'EPSG:900913'
  • 'EPSG:102113'

Defined projections can also be accessed through the proj4.defs function (proj4.defs('EPSG:4326')).

proj4.defs can also be used to define a named alias:

proj4.defs('urn:x-ogc:def:crs:EPSG:4326', proj4.defs('EPSG:4326'));

Axis order

By default, proj4 uses [x,y] axis order for projected (cartesian) coordinate systems and [x=longitude,y=latitude] for geographic coordinates. To enforce the axis order of the provided proj or wkt string, use the

proj4(fromProjection, toProjection).forward(coordinate, enforceAxis);
proj4(fromProjection, toProjection).inverse(coordinate, enforceAxis);

signatures with enforceAxis set to true:

proj4('+proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees +axis=neu', firstProjection).forward([41, -71], true);
// [242075.00535055372, 750123.32090043]
proj4('+proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees +axis=neu', firstProjection).inverse([242075.00535055372, 750123.32090043], true);
//[40.99999999999986, -71]
//the floating points to answer your question

Grid Based Datum Adjustments

To use +nadgrids= in a proj definition, first read your NTv2 .gsb file (e.g. from https://github.com/OSGeo/proj-datumgrid) into an ArrayBuffer, then pass it to proj4.nadgrid. E.g:

const buffer = fs.readFileSync('ntv2.gsb').buffer
proj4.nadgrid('key', buffer);

then use the given key in your definition, e.g. +nadgrids=@key,null. See Grid Based Datum Adjustments.

TypeScript

TypeScript implementation was added to the DefinitelyTyped repository.

$ npm install --save @types/proj4

Developing

To set up build tools make sure you have node and grunt-cli installed and then run npm install.

To do the complete build and browser tests run

node_modules/.bin/grunt

To run node tests run

npm test

To run node tests with coverage run

npm test --coverage

To create a build with only default projections (latlon and Mercator) run

node_modules/.bin/grunt build

To create a build with only custom projections include a comma separated list of projections codes (the file name in 'lib/projections' without the '.js') after a colon, e.g.

node_modules/.bin/grunt build:tmerc
#includes transverse Mercator
node_modules/.bin/grunt build:lcc
#includes lambert conformal conic
node_modules/.bin/grunt build:omerc,moll
#includes oblique Mercator and Mollweide