This software is a JavaScript port of v4.9.3 of the Proj.4 map projection library. Although it was created to be used by mapshaper, all are welcome to use and improve it.

Why create another Proj.4 port, when there is already proj4js? Unlike proj4js, this port is a very literal translation to JavaScript, so staying up-to-date with future changes to Proj.4 should be relatively simple. Also, this software's output is more consistent with Proj.4's output.

• No support for datum transformations using grid files.

• About 1/5 of Proj.4's 130-odd projections have not been ported.

These are workalike versions of the proj and cs2cs programs from Proj.4.

To install the current versions system-wide, run npm install -g mproj.

The software can be used in either the Proj.4 style or the proj4js style.

var proj = require('mproj');
# create functions for translating between two coordinate systems
# returned object has forward() and inverse() functions
var obj = proj(<source definition>, <dest definition>);

# WGS84 is assumed if a source definition is not given
var obj = proj(<dest definition>);

# a shortcut for translating a single point
# <point> is an [x, y] array or an object with x and y properties
var xy = proj([<source definition>,] dest definition, <point>);

See the proj4js.org website for more detailed help.

var proj = require('mproj');
# create a projection object from a proj4 string
var P = proj.pj_init(<proj4 crs definition>);

# project and inverse-project geographical coordinates
var xy = proj.pj_fwd({lam: <longitude in radians>, phi: <latitude in radians>}, P);
var lp = proj.pj_inv({x: <easting in meters>, y: <northing in meters>}, P);

# transform arrays of coordinates from one coordinate system to another
proj.pj_transform(<source crs>, <dest crs>, <x array>, <y array>[, <z array>]);

The x and y coordinates of unprojectable points are set to Infinity. Other errors cause an Error to be thrown containing an appropriate message.

mapshaper-proj now uses Rollup to build from the legacy shared-scope source graph. This preserves the original C-ported structure while producing both CommonJS and ESM outputs.

Running node build creates a build containing all supported projections in the dist/ directory.

Running node build merc,lcc,aea creates a build containing only the listed projections. You can customize the comma-separated list to include only the projections that you want. Run mproj -l to see a list of all supported projections.

Running node build -m creates the minimal build.

Running node build -f watches files and rebuilds on change.

Build artifacts:

• dist/mproj.js (CommonJS)
• dist/mproj.mjs (ESM)
• dist/mproj-minimal.js and dist/mproj-minimal.mjs
• dist/mproj-custom.js and dist/mproj-custom.mjs (for custom projection lists)