proj4

Proj4js is a JavaScript library to transform point coordinates from one coordinate system to another, including datum transformations.

npm install proj4
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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
jam install proj4
component install proj4js/proj4js

or just manually grab the file dist/proj4.js

Using

the basic signature is:

proj4(fromProjection[, toProjection2, coordinates])

Projections can be proj or wkt strings, or a proj4.Proj object.

Coordinates may be proj4.Point objects, 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,[2,5]);
// [-2690666.2977344505, 3662659.885459918]

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([2,5]);
// [-2690666.2977344505, 3662659.885459918]
proj4(secondProjection,firstProjection).inverse([2,5]);
// [-2690666.2977344505, 3662659.885459918]

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]
//the floating points to answer your question

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

grunt

to run node tests run

npm test

to run node tests with coverage run

node test --coverage

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

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.

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