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.
PROJ4JS
Proj4js is a JavaScript library to transform point coordinates from one coordinate system to another, including datum transformations.
Originally a port of PROJ.4 and GCTCP C 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 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";
proj4(firstProjection,secondProjection,[2,5]);
If only 1 projection is given then it is assumed that it is being projected from WGS84 (fromProjection is WGS84).
proj4(firstProjection,[-71,41]);
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([2,5]);
proj4(secondProjection,firstProjection).inverse([2,5]);
and as above if only one projection is given, it's assumed to be coming from wgs84
proj4(firstProjection).forward([-71,41]);
proj4(firstProjection).inverse([242075.00535055372, 750123.32090043]);
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
- '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'));
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
node_modules/.bin/grunt build:lcc
node_modules/.bin/grunt build:omerc,moll